Token Bus (IEEE 802.4) : Definition Token bus is a physical bus that operates as a logical ring using tokens. (From: Local Area Networks - Fourzan)
Introduction to Token Bus Token Bus (IEEE 802.4) combines features of Ethernet (a bus topology) and Token ring. Stations are logically organized into a ring. A token is passed among the stations. If a station wants to send data, it must wait and capture the token. Token Bus is limited to factory automation and process control and has no commercial applications in data communication.
Token Bus: Introduction The logical ring is formed based on the MAC address of the station in descending order. Each station considers the immediate lower address as next station and the station with immediate higher as the previous station. Eg: 20 44 65 90
Token Ring (IEEE 802.5) Token ring requires that stations take turns sending data. Each station may transmit only during its turn and may send only one frame during each turn. The mechanism that coordinates this rotation is called token passing.
Token Bus Layers The Token Bus protocol operates in the :- a) Data Link Layer: 1. LLC Sublayer 2. MAC Sublayer b) Physical Layer The logical link control (LLC) is the upper part sublayer of the data link layer (logical addresses, control information, and data) Media Access Control (MAC) sublayer resolves the contention for the shared media.
Token Bus Layers LLC Data Link MAC Layer Physical LayerTransmission Medium MAC SublayerIt uses the token passing access method over a physical bus topology.
Mac Sublayer Frame Format – general format of fields in the frame at the MAC layer 1. Preamble: Consists of one or more bytes of a pre-defined pattern to synchronize the sender and receiver. 2. Start delimiter: Its 1 byte long and is used to alert the receiving station when a frame is arriving. 3. Frame control: Its 1 byte long and defines the type of the frame: if the 1st 2 bits are 00 - MAC control frame if 01 – data frame from LLC layer. if 10 – Data from other protocols.
Mac Sublayer (Frame Control Field) A MAC control frame can be: 000000 – Claim token 000001 – Solicit successor- 1 000010 – Solicit successor- 2 000011 – Who- follows FC for control 000100 – Resolve contention frames 001000 – Token 001100 – Set-successor 000 – request – with -no- response FC for 001 – request – with response Data frames 010 - response
Mac Sublayer4. Destination address: Contains the physical address of the frame’s next destination. The DA may or may not be valid based on the type of frame. FRAME Destination Address Claim token IGNORED Solicit successor- 1 ADDRESS OF SUCCESSOR Solicit successor- 2 ADDRESS OF SUCCESSOR Who- follows IGNORED Resolve contention IGNORED Token ADDRESS OF SUCCESSOR Set-successor ADDRESS OF STATION THAT SENT SOLICIT-SUCCESSOR FRAME Data ADDRESS OF RECEPIENT OF DATA
5. Source Address: Its 2 to 6 bytes long and contains the physical address of the sending station. Its PRESENT in all types of frames6. Data: It contains data coming from the LLC layer for a data frame or extra information required for some control frames7. FCS: Its 4 bytes long and contains a CRC-32 error detection sequence8. End delimiter: Its 1 byte long and is used to alert the receiving station to the termination of a frame. It serves as a flag. The field may be 1 or N (non-data) bit.
DATA FIELD CONTENTS: FRAME DATAClaim token Length of frameSolicit successor- 1 MissingSolicit successor- 2 MissingWho- follows MissingResolve contention MissingToken MissingSet-successor New address of successorData Data coming from LLC level ED FIELD FORMAT: N N 1 N 1 1 1 1