Token ring

Token ring is a local area network protocol which resides at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a token that travels unidirectionally around a star-wired logical ring. Token ring frames travel completely around the ring in a counter-clockwise direction.

OSI model

Application layer	FTP	SMTP	HTTP	...
Transport layer	TCP		UDP
Network layer	IP ICMP			ARP
Data link layer	Ethernet	Token Ring	FDDI	...

Each station passes or repeats the special token frame around the ring to its nearest upstream neighbor. This token-passing process is used to arbitrate access to the shared ring media. Stations that have data frames to transmit must first acquire the token before they can transmit them. Token ring LANs normally use differential Manchester encoding of bits on the LAN media.

IBM popularized the use of token ring LANs in the mid 1980s when it released its IBM token ring architecture based on active multi-station access units (MSAUs or MAUs) and the IBM Structured Cabling System. The Institute of Electrical and Electronics Engineers or IEEE (http://www.ieee.org) later standardized a token ring LAN system as IEEE 802.5 (http://www.8025.org).

Token ring LAN speeds of 4Mbps, 16Mbps, 100Mbps and 1Gbps have been standardized by the IEEE 802.5 working group.

Token ring networks had significantly superior performance and reliability compared to early shared-media implementations of Ethernet (IEEE 802.3), and were widely adopted as a higher-performance alternative to shared-media Ethernet.

However, with the development of switched Ethernet, token ring architectures lagged badly behind Ethernet in both performance and reliability. The higher sales of Ethernet allowed economies of scale which drove down prices further, and added a compelling price advantage to its other advantages over token ring.

Token ring networks have since declined in usage and the standards activity has since come to a standstill as switched Ethernet has dominated the LAN/layer 2 networking market.

Table of contents

1 The Token Frame
2 Token Ring Frame Format
3 Active and Standby Monitors
4 Token Ring Insertion Process

The Token Frame

When no station is transmitting a data frame, a special token frame circles the ring. This special token frame is repeated from station to station until arriving at a station that needs to transmit data. When a station needs to transmit a data frame, it converts the token frame into a data frame for transmission. The special token frame consists of three bytes as follows:

Starting Delimiter - consists of a special bit pattern denoting the beginning of the frame. The bits from most significant to least significant are J,K,0,J,K,0,0,0. J and K are code violations. Since Manchester encoding is self clocking, and has a transition for every encoded bit 0 or 1, the J and K codings violate this, and will be detected by the hardware.

Access Control - this byte field consists of the following bits from most significant to least significant bit order: P,P,P,T,M,R,R,R. The P bits are priority bits, T is the token bit which when set specifies that this is a token frame, M is the monitor bit which is set by the Active Monitor (AM) station when it sees this frame, and R bits are reserved bits.

Ending Delimiter - The counterpart to the starting delimiter, this field marks the end of the frame and consists of the following bits from most significant to least significant: J,K,1,J,K,1,I,E. I is the intermediate frame bit and E is the error bit.

Token Ring Frame Format

A data token ring frame is an expanded version of the token frame that is used by stations to transmit medium access control (MAC) management frames or data frames from upper layer protocols and applications.

The token ring frame format is defined as follows:

Starting Delimiter - as described above.
Access Control - as described above.
Frame Control - a one byte field that contains bits describing the data portion of the frame contents.
Destination address - a six byte field used to specify the destination(s).
Source address - a six byte field that is either the locall assigned address (LAA) or universally assigned address (UAA) of the sending station adapter.
Data - a variable length field of 0 or more bytes, the maximum allowable size depending on ring speed containing MAC management data or upper layer information.
Frame Check Sequence - a four byte field used to store the calculation of a CRC for frame integrity verification by the receiver.
Ending Delimiter - as described above.
Frame Status - a one byte field used as a primitive acknowledgement scheme on whether the frame was recognized and copied by its intended receiver.

Active and Standby Monitors

Every station in a token ring network is either an active monitor (AM) or standby monitor (SM) station. However, there can be only one active monitor on a ring at a time. The active monitor is chosen through an election or monitor contention process.

The monitor contention process is initiated when

i) a loss of signal on the ring is detected,

ii) an active monitor station is not detected by other stations on the ring or

iii) when a particular timer on an end station expires such as the case when a station hasn't seen a token frame in the past 7 seconds.

The station with the highest MAC address will win the election process. Every other station becomes a standby monitor. All stations must be capable of becoming an active monitor station if necessary.

The active monitor performs a number of ring administration functions. The first function is to operate as the master clock for the ring in order to provide synchronization of the signal for stations on the wire. Another function of the AM is to insert a 24-bit delay into all frame transmissions. A third function for the AM is to ensure that a frames on the ring is present or to detect a broken ring. Lastly, the AM is responsible for removing circulating frames from the ring.

Token Ring Insertion Process

Token ring stations must go through a 5-phase ring insertion process before being allowed to participate in the ring network. If any of these phases fail, the token ring station will not insert into the ring and the token ring driver may report an error.

Phase 0 (Lobe Check) - A station first performs a lobe media check. A station is wrapped at the MSAU and is able to send 2000 test frames down its transmit pair which will loop back to its receive pair.

The station checks to ensure it can receive these frames without error.

Phase 1 (Physical Insertion) - A station then sends a 5 volt signal to the MSAU to open the relay.
Phase 2 (Address Verification) - A station then transmits MAC frames with its own MAC address in the destination address field of a token ring frame.

When the frame returns and if the address copied and frame recognized bits are set, the station knows there is another station using its MAC address on the ring and will then de-insert.

Phase 3 (Participation in Ring Poll) - Before transmitting any data, the station must participate in the periodic (every 7 seconds) ring poll process. This is where stations identify themselves on the network as part of the MAC management functions.
Phase 4 (Request Initialization) - Finally a station sends out a special request to a parameter server to obtain configuration information. This frame is sent to a special functional address, typically a token ring bridge, which may hold timer and ring number information with which to tell the new station about.