Token Ring - IEEE What the IEEE standard covers History Differences between and "Physical layer standard (gives link layer format)" Essentially an IBM standard 'given' to the industry" "Guaranteed response Priorities Controlled delays"

Token Ring History Presented by IBM in 1982 to IEEE 802 committee. First prototype developed in 1983 in Geneva, Switzerland. Cabling System was announced in Officially announced in Standardized by IEEE in Only one adopted by the IEEE committee.

Token Ring Technology Summary Access method by which network attachments gain access to the cable plant by acquiring a special frame called the token. {Token is a special 24-bit pattern that continuously circulates the ring. } Token Ring is a broadcast medium. {To receive data, a destination station performs an address match.} The destination station merely copies the frame as it repeats it back to the ring. When the frame arrives back to the source station, it strips the frame from the ring and then releases the token (4 megabit operation only). The token is allowed to be released prior to frame reception on 16- megabit rings. Token Ring originally ran at 4 Mbps. Upgraded in 1989 to 16 Mbps Maximum frame size for 4 Mbps is –This is based only on the fact a station cannot hold the token longer than milliseconds. Maximum frame size for 16 Mbps is 17,800.

TRN Features Traffic usually (always in 802.5) unidirectional RAR (802.5) vs RAT (FDDI) for Token Passing Recovery from lost token Priorities Frame Structure "data rate of 4 or 16Mbps" "one frame on the net at a time..."

Controller Attachment to a MAU The IBM 8228 MAU Shielded or UTP cable Lobe cables

Cable Connectors Token Ring controller DB-9 connector RJ-11 or RJ-45 connector Hermaphroditic or RJ-45 connectors on MAU Media filter for UTP only MAU Media filter can be on-board

Multiple MAU Connection Ring inRing out MAU Ring inRing out MAU Ring inRing outMAU Type 6 patch cables

MAU Operation MAU top view Relays Lobe cables All stations are active Ring in Ring out MAU bus Closed

MAU Operation (Inactive Station) MAU top view Relays Lobe cables Ring in Ring out MAU bus Closed Inactive station

Token Ring Cable Types Type 1 –A shielded data grade cable with two solid wire twisted pairs. –Available in indoor and outdoor versions. Type 2 –A Type 1 indoor cable with four solid twisted pairs of 24 AWG wire. –Contains four voice grade wires along with four data grade wires. Type 3 –Unused existing telephone wire or EIA category 3 wire (4 Mbps operation). –Category 4 is needed for 16 Mbps (speed of the Token Ring) operation. –Must use a special media filter. Type 5 –100/140 micron fiber cable used for fiber optic repeater links. Type 6 – Often used for patch cables. Patch cables can be used for MAU-to-MAU connection or from a wall outlet to a network attachment.

Type 3 Media Filter Type 3 cable requires a device known as a media filter. Its purpose is to filter out any unwanted signals. It is a small rectangular device that is usually part of the UTP cable itself. It can be a separate device that attaches to the UTP cable at the end of the cable that attaches to the controller card. It can be used on 16- or 4-mb Token Rings. It is only used with Type 3 (UTP) cable.

802.5 Framing IEEE uses special characters, but does not use bit stuffing! “1” bit“0” bit Manchester Violations!

Token Ring Frames 1 byte 1 byte Physical header Physical trailer MAC or LLC Frame Abort frame SDAC FCDASA Routing Information Fields DataFCSEDFS IEEE no preset size 1 byte 1 byte 1 byte Token frame SDACED SD

Token Ring Frame Field Definitions SDAC FC DA SA Routing Information Fields DataFCSEDFS SD - Starting Delimiter AC - Access Control FC - Frame Control DA - Destination Address SA - Source Address FCS - Frame Control Sequence ED - Ending Delimiter FS - Frame Status Legend IEEE no preset size 1 byte 6 bytes <= 18 bytes 1 byte 1 or 2 bytes 4 bytes1 byte DSAPSSAPControl

The SD and the AC Fields P P P T M R R R AC Bit 0Bit 7 Field SD J K 0 J K PPP - priority bits T - Token bit M - Monitor bit RRR - Reservation bits

The FC, ED, and FS Fields Bit 0Bit 7 Field FC FS F F r r Z Z Z Z A C r r A - Address recognized bits C - Frame copied bits ED J K 1 J K 1 I E I - Intermediate bit E - Error bit FF - indicates a MAC or LLC frame. ZZZZ - indicates the type of MAC frame.

Bit Order Transmission for Token Ring Bit 0 is the first bit transmitted. –Bit 0 is the left most bit of the byte. Unlike Ethernet, the bits in the bytes are not reversed as they are transmitted. Example: – are the first three bytes of a MAC address. Translated to binary: As transmitted on a Token Ring: Compared to Ethernet transmission:

Token Passing Policies (Defn) Multiple Token –RAT (FDDI): free token is appended to tail of last packet Single Token –?: Token is released upon receipt of leading edge of own packet Single Packet –RAR (802.5):Token is released upon receipt of trailing edge of own packet

Token Passing Policies (Usage) Multiple Token –Allows multiple packets on the segment at one time. Good when packet length is less than ring latency Single Token –More efficient than RAR; when packet length is about the same as ring latency Single Packet –Least efficient, but allows controlling station knowledge of (un)successful transfer before the token is released (see pg. 224, 1st paragraph)

Token Passing Policies (Perf.) Multiple Token –Always the best performer, but more complex Single Token –Closer to RAR than RAT Single Packet –‘Worst’ performance KEY POINT: Ratio of ring latency to packet length, a, is real determiner of performance. For a << 1, RAR is OK.

Controller Operation - Phases 0 and 1 Five-phase initialization –Phase 0 - Lobe test The controller transmits frames between the controller card and the cable attached between the controller card and the MAU. The controller tests to ensure that the lobe cable can successfully transmit and receive frames. –Phase 1 - Monitor Check Station inserts into the ring (flips the relay in the MAU) and looks for special frames that are transmitted by the monitors. Sets a timer to wait for these frames. If the station does not receive any of the frames, the controller assumes: –it is the first ring station on the network, –there is not an Active Monitor present, or –inserting into the ring disrupted the ring. –The controller may initiate the token claim process.

Controller Initialization - Phases 2, 3, and 4 Phase 2 - Duplicate address check. –Checks to ensure that it can successfully transmit and receive a frame and to detect other stations that might have the same MAC address. The controller transmits a frame to itself. If the frame returns with the address recognized bit set, it notifies one of the monitors and removes itself from the ring. Phase 3 - Participation in neighbor notification. –The station transmits a special frame that will identify itself to its downstream neighbor. –The station should receive a similar frame for its upstream neighbor. Phase 4 - Lan Network Manager Notification –Notifies LAN Network Manager about its presence on the ring

Claim Token Process A ring cannot operate without a token circulating on the ring. –There is only one token per ring. The token-claiming process allows one station to insert the token onto the ring. –This station will be elected as the AM. It will purge the ring (ability to transmit a frame to itself). After purging the ring, it will insert a new token on the ring. The Token-Claim process can be started when the AM –detects a loss of signal, –a timer expires and it has not yet received its AM frame back, or the AM –cannot receive enough of its own Purge Ring MAC frames. It can be started when the SM –detects loss of signal or –detects expiration of its timer for receiving SM frames.

Details of the Claim Token Process If there is no token on the ring, all activity will cease on the ring. –The Active Monitor should be able to recover by purging the ring and issuing a new Token. –If the Active Monitor cannot recover, the token-claim process will begin. Any station will insert its master clock, a 24-bit delay, and start to transmit Token-Claim frames. –These frames are received by all stations on the ring. –The station will follow these frames with idle (clock) signals. –After transmitting the Token Claim frames, the station starts a timer. If it does not receive its frames or someone else’s claim frames, it will beacon the ring. Once the process is started other stations may participate. –Stations bid for the right to become the AM. –The station with the highest priority (MAC address) wins. –That station becomes the AM. It will purge the ring and insert a new token.

Claim Token Process Example A B C D Detected condition Token Claim frames Not participating Repeat frame Higher priority than C. Does not repeat C’s A transmits its own Token Claim frames B has higher priority than A B transmits its own Token Claim frames A B C D Continues transmitting its own Stops transmitting its own Claim frames and repeats B’s Repeats B's Token Claim frame Stops transmitting its own and repeats B’s claim frames 8

Token Ring Transmit Mode A station that needs to transmit receives the SD of approaching frame. This station quits transmitting idles (clock signals). Checks for priority. –If the priority in the frame is greater than the station's priority, then the station sets reservation bits and awaits new token. If the priority in the frame is less than or equal to the station’s priority then –the station changes the T bit in the AC field from a 0 to a 1, –appends its information to the rest of the frame and transmits the frame. –If the end of its transmission is reached and it has not received its current transmission back, the station transmits idle characters and awaits current transmission. When the station receives its frame back it will strip the frame and release the token. The station enters normal repeat mode.

Token Ring Copy Mode The destination Token Ring controller recognizes its address in the destination field of a received frame and copies the frame into its buffer. If at any time an error is detected, the copy phase ends and the controller sets the A and E bits and repeats the frame back to the ring. If no errors are found, the destination sets the A and C bits and repeats the frame back to the ring. The destination station enters Normal Repeat mode. The frame travels on the ring until it reaches the originator and that station strips the frame off of the ring and submits the token to the ring.

Normal Repeat Mode A station in normal repeat mode checks current frames and token for signalling errors. –If any errors are found the station sets the E bit and repeats the frame back to the ring. A station in this mode also checks every frame for its address. –A duplicate address could be found. –If a duplicate address is found, the station will transmit a soft error MAC frame to one of the monitors.

The Active Monitor (AM) Functional address is C It must be present in order for the ring to function properly. The AM is the kingpin of the ring. The AM: –tracks lost tokens and ensures that only one token exists on a single ring. –monitors frames and priority tokens that circulate the ring more than once. –initiates neighbor notification, –provides a latency buffer to recover the clock signal and so that at least 24 bits (the size of the token) can be transmitted on the ring, and –supplies the master clocking.

Token Recovery Monitor Station –1 station becomes responsible for monitoring the token for token loss or token busy Time Outs –Token time out (‘Beaconing’) –No monitor (Claim frames (highest addr wins)

Options for Token Ring For 16 megabit rings, early token release allows a ring station to release the token before receiving its original frame back. –It is based on the ring length A station will not release the token when it is still transmitting its frame and it has started to receive its frame back. –Allows greater use of Token Ring bandwidth. Token Ring operates at 4 and 16 Mbps. –4 and 16 Mbps controllers are not allowed on the same ring. Ring will beacon when this condition occurs. –To have 4 and 16 Mbps ring interoperate, you must use a data forwarding device such as a bridge or a router. IBM is currently experimental with a new Token Ring controller which allow it to operate between Mbps.