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Token-Ring Technical Summary

Table of Contents

1.0 Token-Ring Overview

2.0 Token-Ring Formats

3.0 Token-Ring Protocols

4.0 Token-Ring Cabling and Connectors

5.0 Token-Ring Glossary


1.0 Token-Ring Overview

1.1 What is Token-Ring?

Token-Ring is the second most widely used local area network (LAN) technology after Ethernet. Stations on a Token-Ring LAN are organized in a ring topology with data being transmitted sequentially from one ring station to the next. The ring is initialized by circulating a token. A station must capture the token to gain the right to transmit information onto the ring. A transmitting station replaces the token with a frame which carries the information to be transferred. The frame circulates the ring and may be copied by one or more destination stations. When the frame returns to the transmitting station, it is removed from the ring and a new token is transmitted.


1.2 The History of Token-Ring

Token-Ring was initially defined by IBM at its research facility in Zurich Switzerland in the early 1980s. IBM pursued standardization of Token-Ring under the 802.5 Working Group of the Institute of Electrical and Electronic Engineers (IEEE).

IBM introduced its first Token-Ring product, an adapter for the original IBM Personal Computer, in October 1985. The initial Token-Ring products operated at 4 Mbit/sec. IBM collaborated with Texas Instruments to develop a chipset that would allow non-IBM companies to develop their own Token-Ring compatible devices.

In 1989, IBM improved the speed of Token-Ring by a factor of four when it introduced the first 16 Mbit/sec Token-Ring products. The 802.5 standard was extended to support 16 Mbit/sec operation.

In 1994, the leading Token-Ring suppliers formed the Alliance for Strategic Token-Ring Advancement and Leadership (ASTRAL). ASTRAL's mission was to promote Token-Ring technology in the face of increasing popularity of Ethernet technology. The initial members of ASTRAL were 3Com, ACE/North Hills, Bay Networks (SynOptics and Wellfleet), Bytex, Cabletron, Centillion, Chipcom, Hewlett-Packard, IBM, Intel, Madge, Olicom, Proteon, Racore, SMC, Texas Instruments, Xircom, XPoint and UB Networks.

In 1997, the draft 802.5r standard became available which defined Dedicated Token-Ring (DTR) operation. Dedicated, or full duplex, Token-Ring bypasses the normal token passing protocol to allow two stations to communicate over a point to point link. It effectively doubles the transfer rate by allowing each station to concurrently transmit and receive separate data streams. For example, a 16 Mbit/sec dedicated Token-Ring station can transmit one 16 Mbit/sec stream at the same time it receives a separate 16 Mbit/sec stream. This provides an overall data transfer rate of 32 Mbit/sec. The DTR protocol extends to other Token-Ring data rates as well.

In 1997, the High Speed Token-Ring Alliance (HSTRA) was formed to pursue "an IEEE 802.5 standard for dedicated high-speed Token Ring that scales from 100 Mbit/sec to at least 1 gigabit". The primary HSTRA members were 3Com, Bay Networks, IBM, Madge Networks, Olicom, the UNH Interoperability Lab, and Xylan.

In 1998, the draft 802.5t standard became available which defines 100 Mbit/sec operation for Token-Ring. The 100 Mbit/sec standard is restricted to "dedicated" operation. No "shared media" 100 Mbit/sec standard is planned. The first 100 Mbit/sec Token-Ring products were introduced by Olicom and IBM in 1998.


1.3 IEEE Token-Ring Standards

The current IEEE 802.5 standards may be ordered from the following web address: http://standards.ieee.org/catalog/IEEE802.5.html. The following standards are available as of March 1999:

The IEEE 802.5 Committee has initiated two working groups to develop higher speed Token-Ring standards. The 802.5t Working Group is responsible for 100 Mbit/sec Dedicated Token-Ring. The 802.5t standard is available in draft form to committee members, but has not yet been published as an official standard that can be ordered from IEEE (as of March 1999). The 802.5v Working Group has been chartered to develop a Gigabit Token-Ring standard.

Refer to the IEEE 802.5 Web Site at http://www.8025.org for the latest information on Token-Ring standards.


1.4 IBM Token-Ring Documents

The following Token-Ring related documents may be ordered from IBM at http://www.ibmlink.ibm.com. Search IBM's PubCatalog for the document numbers listed below:


1.5 Recommended Book

The following book is an excellent source of information on the Token-Ring standard: Understanding Token Ring Protocols and Standards by James Carlo, Robert Love, Michael Siegel, and Kenneth Wilson. October 1998. 450p. ISBN 089006458X.


2.0 Token-Ring Formats

2.1 Frame Format

The basic transmission unit on Token-Ring is a frame. The frame format is used for transmitting both LLC and MAC frames. It may or may not include an information field. It may or may not include a routing information field. Frames are composed of the following fields:

Starting Delimiter (1-byte) Access Control (1-byte) Frame Control (1-byte) Dest. MAC Address (6-bytes) Source MAC Address (6-bytes) Routing Information
(0-30 bytes)
Information
(0-n bytes)
Frame Check Sequence (4-bytes) Ending Delimiter (1-byte) Frame Status (1-byte)


2.2 Token Format

The token is the means by which the right to transmit a frame is passed from station to station. Stations wishing to transmit capture the token and convert it into a frame. As the frame circles the ring and returns to its origin, the originating station removes it from the ring and transmits a new token for another station to capture and carry on the process.

Starting Delimiter (1-byte) Access Control (1-byte) Ending Delimiter (1-byte)


2.3 Abort Sequence Format

A Token-Ring station may abort a frame it is transmitting at any time by transmitting an abort sequence. It cause the stations receiving the frame to recognize that it is not a valid frame.

Starting Delimiter (1-byte) Ending Delimiter (1-byte)


2.4 Field Definitions

2.4.1 Starting Delimiter

A frame, token, or abort sequence always starts with a starting delimiter. It is one byte in length and consists of a unique sequence of symbols that includes "code violations" in the Manchester encoded data known as "J" and "K" symbols.

Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7
JK0JK000

J = Non Data "J" Symbol
K = Non Data "K" Symbol
0 = Data "0" Symbol

2.4.2 Access Control

The Access Control field is found in frames and tokens and is one byte in length. It includes several parameters as described below.

Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7
PPPTMRRR

PPP = Priority Bits - In tokens, these bits indicate the priority of the token, and therefore which stations are allowed to use the token. In frames, these bits indicate the access priority level that was used to capture the token. Eight priority levels are defined with b'000' being the lowest and b'111' being the highest.
T = Token Bit - Set to b'0' in tokens and b'1' in frames.
M = Monitor Bit - Used to prevent a token with priority greater than b'000' or any frame from continuously circulating the ring. Frames and tokens are initially transmitted with this bit set to b'0'. As a token with priority greater than b'000' or any frame is repeated by the active monitor, this bit is set to b'1'. All other stations repeat this bit unmodified. If the active monitor detects a frame with this bit set, it purges the ring and releases a new token.
RRR = Reservation Bits - Stations with high priority access will modify these bits when they repeat frames or tokens to request that a token be issued at the specified priority level. b'000' is the lowest priority level and b'111' is the highest.

2.4.3 Frame Control

The Frame Control field is one byte in length and indicates the type of frame.

Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7
FFZZZZZZ

FF = Frame Type Bits - Used to indicate the type of frame. b'00' indicates a "MAC Frame". b'01' indicates an "LLC Frame". The other values are reserved for future use.
ZZZZZZ = Control Bits - Used for MAC Frames to indicate whether the frame should be "normal" or "express" buffered. If these bits are all zeros, the frame should be normal buffered. If non-zero, the frame should be express buffered.

2.4.4 Destination Address

The Destination Address field is six bytes in length and identifies the station or stations that are to copy the frame. The two most significant bits have special meaning as described below.

Bit0Bit1Bit2 - Bit47
I/GU/L 

I/G = Individual/Group - This bit indicates whether the address is ad individual address (b'0') or group address (b'1).
U/L = Universal/Local - This bit indicates whether the address is a universally administered (b'0') or locally administered (b'1').

2.4.5 Source Address

The Source Address field is six bytes in length and identifies the station that originated the frame. The two most significant bits have special meaning as described below.

Bit0Bit1Bit2 - Bit47
RIIU/L 

RII = Routing Information Indicator - Since the source address field will always carry and individual address, there is no need for this bit to indicate individual vs. group as in the Destination Address field. Instead, it is used to indicate whether the Routing Information (RI) field is present in the frame. If b'0', no RI field is present. If b'1', an RI field is included in the frame.
U/L = Universal/Local - This bit serves the same function as in the Destination Address field. It indicates whether the address is a universally administered (b'0') or locally administered (b'1').

2.4.6 Routing Information

The Routing Information (RI) field is used as part of the Token-Ring source routing protocol for routing frames between rings in multiple-ring networks. If a frame is not going to leave the source ring, then the RI field is omitted. If a frame is destined for a station on another ring, then the RI field is used by bridge devices to route the frame across one or more additional rings until it reaches its final destination. The RI field is present only if the routing information indicator (RII) bit in the Source Address field is set to b'1'.

The Token-Ring standard permits the length of the RI field to be any even value from 0 to 30 bytes. However existing implementations typically support an RI field of only up to 18 bytes in length. When present, the RI field consists of one 2-byte Routing Control field, followed by 0, 1, or more 2-byte Route Designator fields as illustrated below:

Routing Control (2-bytes) Route Designator (2-bytes) Route Designator (2-bytes) ... Route Designator (2-bytes)

2.4.6.1 Routing Control

The Routing Control field has the following bit definitions:

Byte0
Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7
BBBLLLLL
Byte1
Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7
DFFFrrrr

BBB = Broadcast Indicators - These 3 bits indicate if and how bridges are to broadcast a frame across multiple ring segments. A value of b'0XX' is a "Non-Broadcast" frame with route designator fields that contain a specific route for the frame to travel through the network. b'10X' is an "All-routes-broadcast" frame that will be transmitted along every route in the network. This may result in multiple copies of the frame arriving at the destination station. b'11X' is a "Single-route-broadcast" that indicates only certain designated bridges should forward the frame from one segment to another. This results in exactly one copy of the frame appearing on every segment in the network.
LLLLL = Length Bits - These 5 bits indicate the length in bytes of the RI field. They permit ring stations to parse the rest of the frame correctly. For all-routes and single route broadcast frames the value in the length field is modified by bridges as they insert route descriptor fields into the frame. Non-broadcast frames destined for another ring segment are transmitted with a complete RI field and this field remains the same as the frame traverses the network.
D = Direction Bits - This bit is used by bridges to interpret the order of the route descriptor fields. If this bit is b'0', a bridge will interpret the routing field from left to right. If it is a b'1', the routing field is interpreted from right to left. This bit allows the routing descriptor entries to appear in the same order for frames traveling in either direction along the route.
FFF = Largest Frame Bits - This field specifies the largest information field that can be transmitted between two stations communicating over a specific route. Broadcast frames are transmitted with this field at the maximum value (b'111'). Bridges will reduce the value of these bits as necessary to indicate the largest frame size supported over that route. The largest value returned in responses to the broadcast indicates the largest possible frame the route can handle. The defined values for this field are: b'000' = 516 bytes, b'001' = 1500 bytes (Ethernet max), b'010' = 2052 bytes, b'011' = 4472 bytes (FDDI max), b'100' = 8144 bytes, b'101' = 11407 bytes, b'110' = 17800 bytes (Token-Ring max), b'111' = used in all-routes broadcast frames.
rrrr = Reserved Bits. These bits shall be transmitted as zeros.

2.4.6.2 Route Designator

The Route Designator field includes a 12-bit "ring number" field and a 4-bit "bridge number" field as shown below. When an all-routes or single routes broadcast frame is transmitted, each bridge that forwards the frame to another ring adds its bridge number and that ring's number to the frame's routing information field. Therefore, when the frame reaches is destination it contains a record of the route taken. The route designators created during the broadcast process are then included when transmitting subsequent non-broadcast frames to specify the route the frame should take.

Byte0Byte1
Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7 Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7
RN (12bits) BN (4bits)

RN = Ring Number - Each ring in a multiple ring networking is assigned a unique 12-bit ring number.
BN = Bridge Number - Each bridge between two rings is assigned a unique 4-bit bridge number. Since the end of a route is a ring and not a bridge, the bridge number in the last route designator is reserved and transmitted as all zeros.

2.4.7 Information Field

This field contains the data transferred from the source station to the destination station or stations. The size of this field may be anywhere from 0 to 4472 for 4 Mbit/sec Token-Ring, and 0 to 17,800 bytes for 16 Mbit/sec Token-Ring. This field may contain "MAC" frame data or "LLC" frame data as indicated by the frame type bits in the Frame Control field.

Information Field (0 to n bytes)

2.4.8 Frame Check Sequence

This field contains a 4-byte cyclical redundancy check (CRC) value used for error checking. When a source station assembles a frame, it performs a CRC calculation on all the bits in the frame from the Frame Control field through the Information Field. The source station stores the value in this field and transmits it as part of the frame. When the frame is received by the destination station, it performs an identical check. If the calculated value does not match the value in this field, the destination station assumes an error has occurred during transmission and discards the frame.

Byte0Byte1Byte2Byte3
FCS

FCS = Frame Check Sequence

2.4.9 Ending Delimiter

The Ending Delimiter is one byte in length and consists of a unique sequence of symbols that includes "code violations" in the Manchester encoded data known as "J" and "K" symbols. Receiving stations shall consider the ED valid if the first six symbols (J K 1 J K 1) are received correctly.

Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7
JK1JK1IE

J = Non Data "J" Symbol
K = Non Data "K" Symbol
1 = Data "1" Symbol
I = Intermediate Frame Bit - This bit is transmitted as a b'1' in intermediate (or first) frames of a multiple frame transmission using a single token. The "I" bit in the last (or only) frame of the transmission shall be set to b'0'. The ability to transmit multiple frames per token is not widely implemented in existing products.
E = Error Detected Bit - This bit shall be transmitted as b'0' by a station when it originates a token, frame, or abort sequence. Stations that repeat a token or frame set this bit to b'1' when detecting an error such as a "code violation", non-integral number of bytes, or CRC error. In a frame, the "E" bit protects all bytes from the Access Control field through the Frame Check Sequence.

2.4.10 Frame Status

The Frame Status field has the following format:

Bit0Bit1Bit2Bit3Bit4Bit5Bit6Bit7
ACrrACrr

A & C = Address Recognized (A) and Frame Copied (C) Bits - These bits are transmitted as zero by the originating station. If a ring station recognizes the frame's Destination Address and copies the frame, it sets the A & C bits to b'11'. If a ring station recognizes the frame's Destination Address but is unable to copy the frame, it sets the A bit to b'1' but leaves the C bit at b'0' When the frame returns to the originating station the A & C bits provide and indication of whether the frame was recognized and/or copied. Two copies of the A & C bits are included in this byte because the Frame Status field is not covered by the Frame Check Sequence. The bits should be considered valid only when both copies of the A & C bits match. Use of these bits should be restricted as some bridge and switch implementations do not consistently set these bits when a frame is forwarded to another ring.
r = Reserved Bits - These bits shall be transmitted as zeros.


2.5 Interframe Gap

An idle period, known as the "interframe gap" (IFG), is placed between frames to provide a brief recovery time for stations on the ring. For 4 Mbit/sec transmission the interframe gap must be a minimum of 1 byte (2 bytes are recommended). For 16 Mbit/sec transmission it must be a minimum of 5 bytes. The data transmitted during the interframe gap may consist of any sequence of "0" and "1" bits. The size and content of the interframe gap may be adjusted slightly as it flows through the station on the ring that is operating as the "active monitor".


2.6 Address Formats

This section describes the various types of addresses that may be specified in the Destination Address (DA) field.

2.6.1 Individual Address

When the first bit of the DA field is b'0', the address is an individual address. It identifies a particular station in the network, and must be distinct from all other individual addresses on the same LAN.

2.6.2 Group Address

When the first bit of the DA field is b'1', the address is a group address. It is used to address a frame to multiple stations in the network. A group may consist of zero or more stations. Group addresses permit a frame to be addressed to a set of stations that are logically related via a common function or protocol.

2.6.3 Broadcast Address

x'FF FF FF FF FF FF' and x'C0 00 FF FF FF FF' are "all-stations broadcast addresses". They are addressed to all stations on a given ring or set of interconnected rings. The x'C0 00 FF FF FF FF' address is intended for use in MAC frames only. Broadcast Addresses are a specific type of Group Address.

2.6.4 Null Address

The Individual Address x'00 00 00 00 00 00' is known as a "Null Address". No station shall be assigned the Null Address and frames addressed to the Null Address are not expected to be copied.

2.6.5 Functional Address

Functional Addresses are a type of locally administered Group Addresses that are assigned for use by well known functions or protocols. The most significant 17 bits of Functional Addresses are fixed as illustrated below. The least significant 31 bits provide 31 bit significant addresses. Each station maintains a Functional Address "mask" that is used to determine which Functional Addresses will be copied. For example, a station with a Functional Address mask set to x'C0 00 00 00 00 12' will copy frames with Functional Addresses of x'C0 00 00 00 00 10' and x'C0 00 00 00 00 02'. Or, a frame may be addressed to multiple functions by setting more than one bit in the 31 bit field.

Byte0Byte1Byte2Byte3Byte4Byte5
Bit
01234567
Bit
01234567
Bit
01234567
Bit
01234567
Bit
01234567
Bit
01234567
11000000000000000xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

The following is a list of some of the Functional Addresses assigned by IBM:

Active Monitorx'C00000000001'
Ring Parameter Serverx'C00000000002'
Ring Error Monitorx'C00000000008'
Configuration Report Server x'C00000000010'
NETBIOSx'C00000000080'
Bridgex'C00000000100'
LAN Managerx'C00000002000'


2.7 Order of Transmission

All the fields described above are transmitted serially onto the Token-Ring media with the left most, or most significant, bit being transmitted first. Note this is different from Ethernet which transmits the right most, or least significant, bit of each byte first when transmitting the destination and source address fields. The Ethernet method of transmitting addresses is called "canonical" or "lsb" format. The Token-Ring method is called "non-canonical" or "msb" format. Both methods transmit the most significant byte of an address first. They differ only in whether the most or least significant bit of each byte is transmitted first.

The IEEE 802.1 standard specifies that addresses should be represented in canonical format. The following figure illustrates how a canonical destination or source address of hex "12 34 56 78 9A BC" is converted into non-canonical form:

canonical Hex:123456789ABC
Binary: 0001 00100011 01000101 01100111 10001001 10101011 1100
non-canonical Binary:0100 10000010 11000110 10100001 11100101 10010011 1101
Hex: 482C6A1E593D


3.0 Token-Ring Protocols

3.1 Classic Token Ring

Classic Token Ring refers to Token Ring as it was originally defined in the early 1980s. It uses "shared bandwidth" to connect multiple stations with a single Token Ring. It may also be referred to as "shared Token Ring" or "half-duplex Token Ring". It exists in contrast with "dedicated Token Ring" which was defined in the late 1990s.

3.1.1 The Ring

In a Token-Ring Network a ring consists of the ring stations and the transmission medium, or cabling, used to interconnect them. Token-Ring uses a star-wired ring topology. Stations include a Token Ring circuit board, or adapter, that connects to a concentrator via a lobe cable. Concentrators may be serially interconnected through patch or trunk cables using the concentrator ring in and ring out ports. Token-Ring concentrators are often called Multistation Access Units, or MSAUs.

Each station receives data through a connection from its nearest upstream neighbor, and transmits data through a connection to its nearest downstream neighbor. Data transmitted by a station travels sequentially, bit by bit, through each station. Each station repeats the data, while checking it for errors. The addressed destination station(s) copy the information as it passes. When the data returns to the originating station, it is stripped, or removed from the ring.

A station gains the right to transmit data, or frames, onto the medium when it detects a token passing on the medium. The token consists of a unique signaling sequence that circulates on the medium following each frame transfer. Any station, upon detecting a valid token, may capture the token by modifying it to start of frame sequence and appending appropriate control and status fields, address fields, routing information field, information field, check sum, and the ending frame sequence. After completion of the frame transfer, the station transmits a new token, allowing other stations the opportunity to gain access to the ring.

Token Ring concentrators include an "insertion/bypass" mechanism that allows stations to enter or leave the network. When in bypass mode, the lobe cable is wrapped back to the attaching station, allowing the station to perform a self-test of its Token Ring circuitry and lobe cable by operating as a one-node network. When the concentrator receives a DC signal, called phantom drive, from the station via the lobe cable, it switches from bypass mode to inserted mode, allowing the inserted station to receive an input data stream from its upstream neighbor, and transmit an output data stream to its downstream neighbor.

Classic Token Ring supports data transmission rates of 4 Mbit/sec and 16 Mbit/sec (100 Mbit/sec "High Speed Token Ring" is supported only by the "Dedicated Token Ring" (DTR) protocol). Current Token-Ring adapters are typically capable of operating at either rate and include ring speed listen circuitry that allows them detect and adapt to the current ring speed when inserting into the network. It is possible for a single ring to consist of as many as 260 stations. However, the total station count may be limited to a smaller number depending on factors such as media type, data rate, use of repeaters and converters, concentrator type, etc. Multiple rings may be interconnected through the use of bridges.


3.1.2 Active Monitor

One station on each ring, called the active monitor, provides token monitoring and other functions. Any operating ring station can be assigned the active monitor responsibility. Other ring stations act as standby monitors, prepared to take over if the active monitor fails. The active monitor has the following duties:


3.1.3 Standby Monitor

Standby monitors continuously check for failures in the active monitor.


3.1.4 Claim Token Process

The Claim Token process determines which station becomes the active monitor when 1) the Beacon process is resolved, 2) the Ring Purge process fails, or 3) a station detects that the active monitor functions are not being performed properly.

The Claim Token process is started when a station enters the Transmit Claim Token state and begins transmitting Claim Token MAC frames. Stations not in the Claim Token state enter the Repeat Claim Token state when they receive a Claim Token MAC frame. If a station in the Transmit Claim Token state receives a Claim Token MAC frame with a source address that has a higher numerical value than its own station address, then it reverts to Repeat Claim Token state. This ensures that if multiple stations enter the Transmit Claim Token state, then stations will be eliminated until a single "winner" remains. That last station remaining in the Transmit Claim Token state wins the Claim Token process when it receives its own Claim Token MAC frame.

The station winning the Claim Token process becomes the active monitor and purges the ring by transmitting Ring Purge MAC frames. The other stations, upon receiving the Ring Purge MAC frame, transition from Repeat Claim Token state to the normal repeat state. If a station identifies that the Claim Token process has failed to complete within the time specified by the "claim token" timer, it starts the Beacon process by entering the Transmit Beacon state and transmitting Beacon MAC frames.


3.1.5 Ring Purge Process

The purpose of the Ring Purge process is to clean up the ring and release a new token. The Ring Purge process is started 1) when a station wins the Claim Token process and becomes the active monitor, or 2) when the active monitor detects a failure in the normal token protocol. The Ring Purge process starts when the active monitor enters the Transmit Ring Purge state and transmits Ring Purge MAC frames. When the active monitor receives its own Ring Purge MAC frame, it transitions to the normal repeat state and transmits a token to start normal ring operation. If the active monitor fails to receive its own Ring Purge MAC frame before the "ring purge" timer expires, it disables its active monitor function and starts the Claim Token process.


3.1.6 Beacon Process

When a station detects a failure in the Claim Token process, it enters the Transmit Beacon state and transmits Beacon MAC frames until its input signal is restored, or until it removes itself from the ring. This is called beaconing. All other stations that receive the Beacon MAC frame enter the Repeat Beacon mode.

The Beacon MAC frame identifies the reason for the beaconing (Beacon Type) and the address of its last known upstream neighbor. When the beaconing station's nearest upstream neighbor has received eight of these Beacon MAC frames, it removes itself from the ring and tests itself to verify it is not the source of the fault.

If a station in the Transmit Beacon state receives its own beacon frame, then the Beacon process is resolved and the station enters the Claim Token process. If a station in the Transmit Beacon state receives a beacon frame from another station with an equal or lower Beacon Type, then the station enters the Repeat Beacon state. If a station remains in the Transmit Beacon state until its "beacon transmit" timer expires, it removes itself from the ring and performs a self test to verify it is not the source of the fault.

If the ring does not recover after both the nearest upstream neighbor and the beaconing station have tested themselves, the error cannot be automatically repaired and manual intervention is required. A "Ring Error Monitor" tool may be used to identify the address of the beaconing station to isolate the location of the wire fault.


3.1.7 Neighbor Notification Process

To support accurate problem determination, all Token Ring stations are required to know the identity of their upstream neighbor station. The Neighbor Notification process provides each station with the identity of its upstream neighbor.

The neighbor notification process depends on the capability provided by the "A" and "C" bits of the Frame Status field. The "A" and "C" bits are always transmitted as zeros. If a station recognizes the frame's destination address as a frame it should copy, it sets the "A" bit to one. If the station copies the frame, it sets the "C" bit to one.

When a frame is broadcast to all stations, the first station downstream from the broadcaster will be the only station that receives the frame with the "A" and "C" bits as zeros. All other stations will see the bits as ones since the first station sets them.

This feature is used by the neighbor notification process. The neighbor notification process begins when the active monitor sends an Active Monitor Present MAC frames to the broadcast address. If a station receives the frame with the "A" and "C" bits as zeros, then the frame's source address identifies this station's nearest upstream neighbor. If the station is not the active monitor, then it broadcasts a Standby Monitor Present MAC frame to notify its downstream neighbor of its address. The process continues in a circular, daisy-chained fashion to let every station know the identity of its upstream neighbor.


3.1.8 Early Token Release

Early Token Release (ETR) is an optional feature that may be implemented by Token Ring stations to increase the available bandwidth on the ring in cases where a frame is shorter than the ring latency. ETR allows a transmitting station to release a token as soon as it completes frame transmission, whether or not the frame header has returned to the station. A station that does not implement, or does not have ETR enabled, will release a token only after the entire frame has been stripped from the ring.

Stations which implement ETR are compatible and interoperable with stations that do not. ETR was not defined at the time the 4 Mbit/sec Token Ring standard was released, so it is supported only for 16 Mbit/sec operation.


3.1.9 Inserting into the Ring

A station goes through a five phase process when inserting into the ring. The phases are as follows:
Phase 0: Lobe Test
A station performs a lobe test before entering the ring. A series of Lobe Test MAC frames are wrapped over the station's lobe cable to make sure there are no faults in the cable. The lobe cable is the cable that extends from the attaching station to the concentrator, or Multistation Access Unit. If the lobe test is successfully completed, the station enters the ring by asserting its phantom drive signal to the concentrator, and continues with Phase 1. If the lobe test fails, the insertion process is terminated.
Phase 1: Monitor Check
A station starts an "attachment timer" and waits for an Active Monitor Present, Standby Monitor Present, or Ring Purge MAC frame. If one of these frames is received before the timer expires, the station assumes an active monitor is present and proceeds to Phase 2. If the timer expires without any of the frames being received, the station initiates the Claim Token process.
Phase 2: Duplicate Address Check
The ring station checks for the presence on its ring of another stations with the same address using the Duplicate Address Test MAC frame. If a duplicate address is found, the stations removes itself from the ring. If not, it proceeds with Phase 3.
Phase 3: Neighbor Notification
During this phase the station participates in the neighbor notification process. This allows the station to learn its nearest active upstream neighbor address and identify itself to its nearest downstream neighbor.
Phase 4: Request Initialization
In this state the station transmits a Request Initialization MAC frame to the Ring Parameter Server (RPS) functional address. If the RPS is present, as determined by the "A" bits on the frame, the station expects to receive a valid Initialize Station or Change Parameters MAC frame. These MAC frames may set parameters in the station such as physical location, soft error report timer value, or ring number.


3.1.10 Priority Operation

Token Ring supports a priority mechanism that allows stations to transmit data on eight different priority levels. Each station is assigned an allowed access priority that indicates the maximum token priority the station can use to transmit data. A station can use a token only if it is at a priority less than or equal to the station's allowed access priority.

The priority of a token or frame is indicated in the first 3 bits (the priority bits) of the Access Control field. Any reservation for a different priority is indicated in the last 3 bits (the reservation bits) of the same field. A station uses the reservation bits to request that a token be originated on the ring at the requested priority level.

If another station has already reserved an equal or higher priority, the station cannot make a reservation in the frame or token. If the reservation bits have not been set, or if they have been set to a lower priority than that required by the station, then a station may set the reservation bits to the required priority. When a station removes one of its frames from the ring and finds a non-zero value in the reservation bits, it must originate a non-zero priority token.

To prevent a station from continually transmitting priority frames, fairness is provided within each priority level. A station that originates a token of increased priority must eventually replace it with a token of the original priority.


3.1.11 Error Detection and Reporting

Soft errors are intermittent faults that temporarily disrupt operation of the ring. They are normally tolerated by error recovery procedures. Examples of soft errors include lost tokens, frame check sequence error, abort delimiter transmitted, or receiver congestion. Each station maintains a set of counters to measure the frequency of occurrence of each type of soft errors. When detecting soft errors, a station will periodically transmit a Report Soft Error MAC frame to report the errors to a ring error monitor. After successfully transmitting each Report Soft Error MAC frame, the station resets the soft error counters.

Hard errors are permanent faults, usually in equipment or cabling, that cause the ring to stop operating within the normal protocols. The token ring protocol uses the Beacon process in an attempt to recover from hard errors and restore normal token operation. If recovery is not possible, the beacon MAC frame identifies the fault domain for analysis by network management functions.


3.1.12 Bridges

Bridges are devices used to interconnect two or more separate rings to create a larger Token Ring network. A bridge acts as a station on each ring to which it is attached. It copies frames destined for other rings, and transmits frames from other rings destined for the local ring, or for rings beyond it. A bridge typically performs a filtering function by passing frames from one ring to another only when necessary. A bridge uses the routing information or destination address field of a frame to determine whether the frame should be copied.


3.1.13 Source Routing

Token Ring networks use a form of bridging called Source Routing. Source Routing is an alternate to transparent bridging technique used with Ethernet LANs. The term routing used in this context is not to be confused with the function performed by routers. Routers operate at Layer-3 (Network Layer) of the OSI model, where the source routing function takes place at Layer-2 (Data Link Layer).

When source routing bridges are used, the source station is expected to know the route over which to send each frame it transmits. If a source station does not know the route, or if the source station determines that a previously known route is no longer active, the station broadcasts a route discovery frame. A route discovery frame, also known as a TEST or XID frame, contains the MAC address of the destination station the source station is attempting to reach. The route discovery frame is propagated by the bridges throughout the entire network. As a route discovery frame travels across the interconnected rings, each bridge that receives the frame adds routing information to the routing information field of the frame. When the route discovery frame reaches its final destination, the destination station sends a response back to the source station. The response contains the routing information field that describes the route the original route discovery frame used to reach the destination station. The original source station then uses the routing information field that is contained in the response to create the routing information field in data frames it sends to the destination station.

An advantage of using source routing over transparent bridging is that multiple bridges may be installed to create parallel, active paths between individual rings. Multiple active paths allow for higher throughput and load balancing through the various bridges. A disadvantage of the source routing technique is that source routing bridges cannot be used to interconnect Token Ring with other LANs like Ethernet.


3.2 Dedicated Token Ring

The release of the IEEE 802.5r standard in 1998 defined a second mode of operation for Token Ring, called Dedicated Token Ring (DTR) that bypasses the classic token passing protocol. The classic Token Ring protocol is "half-duplex". This implies that a station may either transmit data, or receive data, but never both at the same time. DTR is a "full duplex" protocol that allows two stations to simultaneously exchange data over a point to point link that provides independent transmit and receive paths. Since each station can simultaneously transmit and receive data, the aggregate throughput of the link is effectively doubled. For example, A 16 Mbit/sec station operating in DTR mode provides a maximum bandwidth of 32 Mbit/sec.

DTR operation is restricted to point to point links connecting exactly two stations. Since there is no contention for a shared medium, the token becomes unnecessary. Frames may be transmitted at will, limited only by the required separation of the minimum interframe gap. To support DTR both stations on the link must be capable of, and be configured for full-duplex operation. Token-Ring products capable of supporting DTR first became available about 1997.

Concentrators that are capable of DTR operation are typically called switches. DTR connections may exist between two switches, or between a switch and a workstation adapter. A Token Ring switch may use transparent bridging or source route bridging techniques to route frames between its various ports.


4.0 Token-Ring Cabling and Connectors

Token Ring was originally implemented on 150-ohm shielded twisted pair (STP) cable using a unique hermaphroditic connector, commonly called the IBM Data Connector. This cable type was originally specified as part of the IBM Cabling System and was later adopted as part of the TIA-568-A standard. Later, Token Ring was adapted to use conventional 100-ohm unshielded twisted pair (UTP) cable.

Token Ring uses a star-wired ring topology that allows an electrically contiguous ring to be implemented with wiring that extends in lobes from the wiring hub to the workstation adapters. The wiring hub is called a concentrator, or Multistation Access Unit (MSAU or MAU).

Token Ring uses two pairs of wires to connect each workstation to the concentrator. One pair of wires is used for transmitting data, and the other for receiving data. STP cabling contains two wire pairs for Token Ring (data) and may include additional pairs for carrying telephone (voice) transmission. UTP cabling typically includes four wire pairs of which only two are used for Token Ring.

STP Token Ring installations typically connect to the workstation adapter through a 9-pin D-Shell connector, and connect to the concentrator or wall outlet (faceplate) through an IBM Data Connector. UTP Token Ring installations typically uses an 8-pin RJ-45 connector on both the workstation adapter and concentrator/wall outlet ends. Newer Token Ring adapters include both a 9-pin D-Shell and RJ-45 connector to permit attachment to either STP or UTP cabling.

Older Token Ring adapters may have only a 9-pin D-Shell connector for attachment to STP cabling. Attaching older adapters to UTP requires a "media filter" device to translate from the 150-ohm impedance of the 9-pin D-shell connector to the 100-ohm impedance of the UTP cabling. The media filter contains an impedance-transformer (balun) that compensates for the impedance difference. Operating without a media filter in applications where it is required can cause serious problems that will limit lobe distances and cause unwanted signal reflections.


5.0 Token-Ring Glossary

abort sequence
A sequence transmitted by an originating ring station that terminates the transmission of a frame prematurely. It also causes the ring station receiving the frame to terminate the frame's reception.
access unit
A wiring concentrator.
accumulated jitter
The jitter at a PHY entity in the ring measured against the transmit clock of the active monitor. It is the total jitter accumulated by all the stations from the active monitor to the measurement point. It is typically used to determine the required size of the elastic buffer.
active monitor
A station on the ring that is performing certain functions to ensure proper operation of the ring. These functions include 1) establishing clock reference for the ring, 2) assuring that a usable token is available, 3) initiating the neighbor notification cycle, 4) preventing frames and priority tokens from repeatedly circulating the ring. In normal operation only one station on a ring may be the active monitor at any instance in time.
active retimed concentrator
A type of token ring concentrator that performs an embedded repeater function in the lobe port's data path, thereby providing ring segment boundaries at the concentrator lobe port connector.
adapter
A circuit board, or network interface card (NIC), that plugs into a computer station and provides the MAC and PHY layer functions required to attach the station to a Token Ring network.
all-rings broadcast frame
A frame that has the B-bit in the routing information field set to a 1. In a network of interconnected rings, bridges forward all such frames to all rings. The destination address is not examined and plays no role in bridge routing.
all-stations broadcast frame
A frame whose destination address is set to all ones. All stations on any ring on which the frame appears will copy it. Which rings it appears on is determined by the routing information, not the destination address. All stations broadcasting is independent of all-rings broadcasting; the two can be done simultaneously or one at a time.
beaconing
A ring state that occurs when a station on the ring has detected a serious failure, such as a broken cable. The frame transmitted by the state to alert the other stations on the ring of the failure is called a beacon frame.
bit error rate (BER)
A measurement of error rate stated as a ratio of the number of bits with an error to the total number of bits passing a given point on the ring. A BER of "10 to the -6" indicates that an average of one bit per million is in error.
bridge
A device that links two networks that use the same protocol (i.e. Token Ring).
broadcast
The act of sending a frame addressed to all stations.
burst error
Multiple consecutive signal elements with the same polarity. A "Burst4 error" would have exactly four consecutive signal elements with the same polarity.
channel
The data path from any transmitting MIC to the next downstream receiving MIC.
claiming
A ring state that occurs when a station detects that the active monitor functions are not being performed and at least one station is contending to become active monitor.
classic token ring
The original Token Ring defined in the early 1980s that provides shared bandwidth to multiple nodes on a single token ring. Also called "half-duplex Token Ring". Contrast with "Dedicated Token Ring".
code violation
In differential Manchester encoding, a bit that does not have a state transition at the bit mid-point.
configuration report server (CRS)
A function that monitors and controls the stations of the ring. It receives configuration information from the stations on the ring and either forwards it to the network manager or uses it to maintain a configuration of the ring. It can also, when requested by a network manager, check the status of stations on the ring, change operational parameters of stations on the ring, and request that a station remove itself from the ring.
correlated jitter
The portion of the total jitter that is related to the data pattern. Since every PHY receives the same pattern, this is correlated among all similarly configured PHYs receiving the same data pattern and therefore may grow in a systematic way along the ring. Also referred to as "pattern jitter" or "systematic jitter".
converter
A type of repeater that converts the data signal from one media to another.
crosstalk
Crosstalk is undesired energy appearing in one signal path as a result of coupling from other signal paths.
cumulative latency
The time it takes for a signal element to travel from the active monitor's transmitter output to its receiver input.
cyclic redundancy check (CRC)
An error checking technique used to ensure the accuracy of data transmitted over a communications channel. The transmitted messages are divided into predetermined lengths (frames) which, used as dividends, are divided by a fixed divisor. The remainder of the calculation is appended onto and sent with the message. At the receiving end, the computer recalculates the remainder. If it does not match the transmitted remainder, and error is detected.
data connector
A four position connector for 150-ohm STP used primarily with Token-Ring networks.
data grade
A term used for twisted-pair cable used in networks to carry data signals. Data grade media has a higher frequency rating than voice grade media used in telephone wiring.
dedicated token ring (DTR)
Token ring architecture extension defined in the late 1990s that provides shared bandwidth to multiple node count classic token rings or single classic stations, and enhanced full-duplex dedicated bandwidth to single stations attached to a DTR concentrator. Contrast with "classic Token Ring".
delimiter
A bit pattern that defines the limits of a frame or token on a ring.
differential Manchester encoding
A signaling method used in Token-Ring and other networks to encode clock and data bit information into bit symbols. Each bit symbol is split into two halves, or signal elements, where the second half is the inverse of the first half. A 0 bit is represented by a polarity change at the start of the bit time. A 1 bit is represented by no polarity change at the start of the bit time. Differential Manchester encoding is polarity independent.
downstream
On a ring, the direction of data flow. Contrast with upstream.
early token release
A station operating in early token release mode will release (transmit) a token immediately after transmitting a frame and the subsequent interframe gap. In contrast, a station operating in normal token release mode does not release a token until the frame it transmitted has returned to the transmitting station and been completely stripped from the ring. Early token release achieves higher efficiency by allowing frames to be transmitted back to back.
elastic buffer
A variable delay element inserted in the ring by the active monitor to ensure that ring latency remains constant when the cumulative latency changes.
express buffering
A method of improving the likelihood that a station will copy a MAC frame immediately, when the stations' normal receive buffers are full.
faceplate
A plate for connecting data and voice connectors to a cabling system. It may be wall mounted or surface mounted.
fill
A sequence of data symbols of any combination of 0 and 1 data bits (as opposed to non-data-J and non-data-K bits) whose primary purpose is to maintain timing and spacing between frames and tokens.
frame
A transmission unit that carries a protocol data unit (PDU) on the Token-Ring.
full-duplex Token Ring
see dedicated Token Ring
functional address
A subset of group addresses that is encoded in bit-significant format, thereby allowing multiple individual groups to be designated by a single address.
group address
An address assigned to a collection of stations.
half-duplex Token Ring
see classic Token Ring
hard error
An error condition on a ring that requires that the ring be reconfigured or that the source of the error be removed before the ring can resume reliable operation.
IEEE 802.5
The IEEE standards committee defining Token-Ring standards.
insertion loss
The signal loss that results when a channel is inserted between a transmitter and a receiver, which is the ratio of the signal level delivered to a receiver before a channel is inserted, to the signal level after the channel is inserted.
jitter
The time varying phase of a pulse train relative to the phase of a reference pulse train. For Token-Ring, jitter is usually measured as the difference in edge times of the receiver's recovered clock or transmitter data output to a reference clock or data signal, typically the preceding station's transmitter clock or data input. The specifications are measured in nanoseconds.
latency
The time, expressed in number of symbols, it takes for a signal to pass through a ring component.
LLC frame
A token ring frame containing an LLC PDU exchanged between peer entities using the MAC services.
lobe cabling
The cabling used to interconnect the Multistation Access Unit (MSAU) to the workstation adapter. This cabling includes all work area cabling, horizontal cabling, and patch cables. The lobe cable only carries ring signals when the station is actively connected to the ring (inserted). When the station is not inserted in the ring, the lobe cable may contain local test (wrap) signals.
local area network (LAN)
A network in which communications are limited to a moderate-sized geographic area such as a single office building, warehouse, or campus, and which do not generally extend across public rights-of-way.
Logical Link Control (sublayer) (LLC)
That part of the Data Link layer that supports media-independent data link functions and uses the services of the MAC to provide services to the network layer.
MAC frame
A token ring frame containing a MAC PDU exchanged between MAC entities used to convey information that is used by the MAC protocol or management of the MAC sublayer.
Manchester encoding
see differential Manchester encoding.
media filter
An impedance matching component used in Token-Ring networks to transform the 100 ohm impedance of UTP cabling to the 150 ohm impedance of media interface connections.
medium
The material on which the data may be transmitted. STP, UTP, and optical fibers are examples of media.
Medium Access Control (sublayer) (MAC)
The portion of the data station that controls and mediates access to the ring.
medium interface connector (MIC)
A connector interface at which signal transmit and receive characteristics are specified for attaching stations and concentrators. One class of MICs is the connection between the attaching stations and the lobe cabling. A second set is the attachment interface between the concentrator and its lobes. A third set is the interface between the concentrator and the trunk cabling. Two types of connectors are specified: one for connecting to STP media and one for connecting to UTP media.
monitor functions
The functions that recover from various error situations and are contained in each ring station. In normal operation only one of the stations on a ring is the active monitor at any point in time. The monitor functions on all other stations on the ring ensures that the active monitor function is being performed.
MSAU
Multi-station Access Unit. Device used to interconnect lobe cables from stations on a Token-Ring network. Also called "MAU".
multiple frame transmission
A frame transmission where more than one frame is transmitted when a token is captured (not widely supported by existing Token Ring products).
NAUN
see Nearest Active Upstream Neighbor.
Nearest Active Upstream Neighbor
For any given station on a ring, the station directly upstream that is participating in the ring protocols.
passive concentrator
A type of token ring concentrator that contains no active elements in the signal path of any lobe port. Embedded repeater functions may be provided by the ring in and ring out ports.
phantom drive
A technique where a dc power source is superimposed on the transmit and receive signal pairs in a transparent or "phantom" fashion such that its application does not affect the data bearing signals on either pair. For classic token ring, this dc power source is normally applied to request a concentrator to insert a station into the ring. For dedicated token ring, this dc power source is used as part of the registration process.
Physical Layer (PHY)
The layer responsible for interfacing with the transmission medium. This includes conditioning signals received from the MAC for transmitting to the medium and processing signals received from the medium for sending to the MAC.
port
A signal interface provided by token ring stations, passive concentrator lobes, active concentrator lobes, or concentrator trunks that is generally terminated at a media interface connector (MIC). Ports may or may not provide physical containment of channels.
protocol data unit (PDU)
Information delivered as a unit between peer entities that contains control information and, optionally, data.
purging
A ring state that occurs when the active monitor has detected a ring error and is returning the ring to an operational state by transmitting purge frames.
recovery
The process of restoring a ring to normal operation. When the ring is beaconing, claiming, or purging, the ring is in a state of recovery.
repeat
The action of receiving a bit stream (for example, frame, token, or fill) and placing it on the medium. Stations repeating the bit stream may copy it into a buffer or modify control bits as appropriate.
repeater
Physical layer coupler of ring segments. Provides for physical containment of channels, dividing the ring into segments. A repeater can receive any valid token ring signal and retransmit it with the same characteristics and levels as a transmitting station.
ring error monitor (REM)
A function that collects ring error data from ring stations. The REM may log the received errors, or it may analyze this data and record statistics on the errors.
ring in
A port that receives signals from the main ring path on the trunk cable and transmits signals to the backup path on the trunk cable, and provides connectivity to the immediate upstream ring out port.
ring latency
In token ring, the time (measured in bit times) it takes for a signal to propagate once around the ring. The ring latency time includes the signal propagation delay through the ring medium plus the sum of the propagation delays through each station or other element in the data path connected to the token ring.
ring network
A network configuration where a series of attaching devices are connected by unidirectional transmission links to form a closed path.
ring out
A port that transmits the output signals to the main ring path on the trunk cable and receives from the backup ring path on the trunk cable, and provides connectivity to the immediate downstream ring in port.
ring parameter server (RPS)
A function that is responsible for initializing a set of operational parameters in ring stations on a particular ring.
ring segment
Section of a transmission path bounded by repeaters or converters. Ring segments boundaries are critical for determining the transmission limits that apply to the devices within the segment.
ring speed listen
The ability of a Token Ring adapter to detect and adapt to the current data rate, or speed, of the ring.
ring status
The condition of the ring.
ring topology
A logically circular, unidirectional transmission path without defined ends.
routing information
A field, carried in a frame, used by source routing bridges that provides source routing operation in a bridged LAN.
shielded twisted pair (STP)
A type of twisted pair cable in which the pairs are enclosed in an outer braided shield, although individual pairs may also be shielded. STP most often refers to the 150 ohm IBM Type 1, 2, 6, 8, & 9 cables used with Token Ring networks.
signal element
The logical signal during one half of a bit time which may take on the values of Logic_1 or Logic_0.
soft error
An intermittent error on a network that causes data to have to be transmitted more than once to be received. A soft error does not, by itself, affect the network's overall reliability. If the number of soft errors reaches the ring error limit, reliability is affected.
source routing
A mechanism to route frames through a bridged LAN. Within the source routed frame, the source station specifies the route that the frame will traverse.
standby monitor
A station on the ring that is not in active monitor mode. The function of the standby monitor in normal ring operation is to assure that an active monitor is operating.
station
A physical device that may be attached to a shared medium LAN for the purpose of transmitting and receiving information on that shared medium. A station is identified by a destination address (DA).
STP
see shielded twisted pair
STP-A
Refers to the enhanced IBM Cabling System specifications with the Type "A" suffix. The original IBM Type 1, 2, 6, & 9 specifications were designed to support operation of 4 and 16 Mbps Token-Ring. The enhanced Type 1A, 2A, 6A, & 9A cable specifications were designed to support operation of 100 Mbps FDDI signals over copper.
stripping
The action of a station removing the frames it has transmitted from the ring.
symbol
With respect to Token Ring, a symbol consists of two signal elements. Four symbols are defined: data_zero, data_one, non-data_J, and non-data_K.
token
A signal sequence passed from station to station that is used to control access to the medium.
transferred jitter
The amount of jitter in the recovered clock of the upstream PHY which is subsequently transferred to the downstream PHY which is transferred to the next downstream PHY. Transferred jitter is important because each PHY must both limit the amount of jitter it generates and track the jitter delivered by the upstream PHY.
transmission medium
see medium.
transmit
The action of a station generating a frame, token, abort sequence, or fill and placing it on the medium. Contrast with "repeat".
transparent bridging
A bridging mechanism in a bridged LAN that is transparent to the end stations.
trunk cable
The transmission medium for interconnection of concentrators providing a main signal path and a backup signal path, exclusive of the lobe cabling.
Type 1
150 ohm shielded twisted pair (STP) cabling conforming to the IBM Cabling System Specifications. Two twisted pairs of 22 AWG solid conductors for data communications are enclosed in a braided shield covered with a sheath. Tested for operation up to 16 MHz. Available in plenum, non-plenum, riser, and outdoor versions.
Type 1A
Enhanced version of IBM Type 1 cable rated for operation up to 300 Mhz. Meets electrical specifications for 150 ohm STP-A Cable as documented in the TIA/EIA 568-A standard.
Type 2
150 ohm shielded twisted pair (STP) cabling conforming to the IBM Cabling System specifications. Two twisted pairs of 22 AWG solid conductors for data communications are enclosed in a braided shield. Four additional pairs of 22 AWG solid conductors for telephones are also included in the cable jacket but outside the braided shield. Tested for operation up to 16 MHz. Available in plenum and non-plenum versions.
Type 2A
Enhanced version of IBM Type 2 cable rated for operation up to 300 Mhz. Meets electrical specifications for 150 ohm STP-A Cable as documented in the TIA/EIA 568-A standard.
Type 3
IBM Cabling System designation for 100 ohm unshielded twisted pair (UTP) cabling similar to TIA/EIA Category 3 cabling. 22 AWG or 24 AWG conductors with a minimum of two twists per linear foot. Typically four twisted pairs enclosed within cable jacket.
Type 5
100/140 micron optical fiber cable conforming to the IBM Cabling System specifications. Two optical fibers are surrounded by strength members and a polyurethane jacket. Type 5J is a 50/125 micron version defined for use in Japan.
Type 6
150 ohm shielded twisted pair (STP) cabling conforming to the IBM Cabling System specifications. Two twisted pairs of 26 AWG stranded conductors for data communications. Flexible for use in making patch cables. Tested for operation up to 16 MHz. Available in non-plenum version only.
Type 6A
Enhanced version of IBM Type 6 cable rated for operation up to 300 Mhz. Meets electrical specifications for 150 ohm STP-A Cable as documented in the TIA/EIA 568-A standard.
Type 8
150 ohm under-carpet cable conforming to the IBM Cabling System Specifications. Two individually shielded parallel pairs of 26 AWG solid conductors for data communications. The cable includes "ramped wings" to minimize visibility when installed under carpeting. Tested for operation up to 16 MHz.
Type 9
150 ohm shielded twisted pair (STP) cabling conforming to the IBM Cabling System Specifications. A plenum rated cable with two twisted pairs of 26 AWG solid or stranded conductors for data communications enclosed in a braided shield covered with a sheath. Tested for operation up to 16 MHz.
Type 9A
Enhanced version of IBM Type 9 cable rated for operation up to 300 Mhz. Meets electrical specifications for 150 ohm STP-A Cable as documented in the TIA/EIA 568-A standard.
uncorrelated jitter
The portion of total jitter that is independent of the data pattern. This jitter is generally caused by noise that is uncorrelated among stations and therefore grows in a non-systematic way along the ring. Uncorrelated jitter is also called "noise jitter" or "non-systematic jitter".
upstream neighbor's address (UNA)
The address of the station functioning upstream from a specific station.
unshielded twisted pair
Twisted pair cabling that includes no shielding. UTP most often refers to the 100 ohm Category 3, 4, & 5 cables specified in the TIA/EIA 568-A standard.
upstream
On a ring network, the direction opposite to that of data flow. Contrast with downstream.
UTP
see unshielded twisted pair.
wire fault
An error condition caused by a break in the wires or a short between the wires (or shield) in a segment of cable.

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