TOKEN BUS & TOKEN RING

IEEE 802 Subgroups and their Responsibilities 802.1 Internetworking 802.2 Logical Link Control (LLC) 802.3 CSMA/CD 802.4 Token Bus LAN

IEEE 802 Subgroups and their Responsibilities (Cont.) 802.5 Token Ring LAN 802.6 Metropolitan Area Network 802.7 Broadband Technical Advisory Group 802.8 Fiber-Optic Technical Advisory Group

IEEE 802 Subgroups and their Responsibilities (Cont.) 802.9 Integrated Voice/Data Networks 802.10 Network Security 802.11 Wireless Networks 802.12 Demand Priority Access LANs Ex: 100BaseVG-AnyLAN

Local Area Network Technology There are two types of token-passing architectures: Token Bus is similar to Ethernet because all clients are on a common bus and can pick up transmissions from all other stations Token Ring is different from Token Bus in that the clients are set up in a true physical ring structure

Token Passing Standards IEEE 802.5 For the token-ring LANs IEEE 802.4 For the token-bus LANs A FDDI protocol is used on large fiber-optic ring backbones

INTRODUCTION Token Ring defines a method for sending and receiving data between two network-connected devices To communicate in a token-passing environment, any client must wait until it receives an electronic token The token is a special frame that is transmitted from one device to the next

TOKEN RING Token ring LAN are logically organized in a ring topology with data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access. This token passing mechanism is shared by ARCNET, token bus, and FDDI, and has theoretical advantages over the stochastic CSMA/CD of Ethernet.

TOKEN RING Token ring local area network (LAN) technology is a 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 around the ring. Token Data packet that could carry data Circulates around the ring Offers an opportunity for each workstation and server to transmit data.

IEEE 802.5 and Token Ring Proposed in 1969 and initially referred to as a Newhall ring. Token ring :: a number of stations connected by transmission links in a ring topology. Information flows in one direction along the ring from source to destination and back to source. Medium access control is provided by a small frame, the token, that circulates around the ring when all stations are idle. Only the station possessing the token is allowed to transmit at any given time.

Token Ring Operation When a station wishes to transmit, it must wait for token to pass by and seize the token. One approach: change one bit in token which transforms it into a “start-of-frame sequence” and appends frame for transmission. Second approach: station claims token by removing it from the ring. Frame circles the ring and is removed by the transmitting station. Each station interrogates passing frame, if destined for station, it copies the frame into local buffer.

IEEE 802.5 Token Ring 4 and 16 Mbps using twisted-pair cabling with differential Manchester line encoding. Maximum number of stations is 250. Waits for last byte of frame to arrive before reinserting token on ring {new token after received}. 8 priority levels provided via two 3-bit fields (priority and reservation) in data and token frames. Permits 16-bit and 48-bit addresses (same as 802.3).

Token Ring Under light load – delay is added due to waiting for the token. Under heavy load – ring is “round-robin” The ring must be long enough to hold the complete token. Advantages – fair access Disadvantages – ring is single point of failure, added issues due to token maintenance.

Token Maintenance Issues What can go wrong? Loss of token (no token circulating) Duplication of token (forgeries or mistakes) The need to designate one station as the active ring monitor. Persistently circulating frame Deal with active monitor going down.

A Token Bus Network

Token Passing in a Token Bus Network

Token Passing in a Token Bus Network

Token Bus Server Token Client Client Client A token is distributed to each client in turn.

TOKEN FRAME When no station is transmitting a data frame, a special token frame circles the loop. This special token frame is repeated from station to station until arriving at a station that needs to transmit data.

TOKEN FRAME When a station needs to transmit data, it converts the token frame into a data frame for transmission. Once the sending station receives its own data frame, it converts the frame back into a token.

TOKEN FRAME PRIORITY Token ring specifies an optional medium access scheme allowing a station with a high-priority transmission to request priority access to the token. 8 priority levels, 0–7, are used. When the station wishing to transmit receives a token or data frame with a priority less than or equal to the station's requested priority, it sets the priority bits to its desired priority. The station does not immediately transmit; the token circulates around the medium until it returns to the station.

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

IEEE 802.5 Token and data frame structure Token Frame Format SD AC ED Data Frame Format 1 1 1 2 or 6 2 or 6 4 1 1 Destination Address Source Address SD AC FC Information FCS ED FS Starting delimiter J K 0 J K 0 J, K non-data symbols (line code) Access control PPP Priority; T Token bit M Monitor bit; RRR Reservation P P P T M R R R Frame control FF frame type ZZZZZZ control bit F F Z Z Z Z Z Z Ending delimiter I intermediate-frame bit E error-detection bit J K 1 J K 1 I E A address-recognized bit xx undefined C frame-copied bit Frame status A C x x A C x x

Token Bus Data Pickup A token is sent from one node to the other The client wanting to transmit grabs an empty token Data is attached Token leaves for the next node and its travel on the bus until it reaches the address to which the data is destined

Token Bus Data Delivery Token delivers the data to the addressee Acknowledgement is returned to the sender Token is passed to the next node The process continues If there is an error in delivering the information, a request for retransmission attached to the token and it is sent to the sender

Token Bus Standard and Applications IEEE 802.4 It can be used in both broadband and baseband transmission

Token Passing Protocol in Operation Chapter 23 8/7/2018 Token Passing Protocol in Operation Circulating Token Workstation Server No collisions Workstation (c) Copyright N Ganesan

Comparison with CSMA/CD Absence of collision Offers a systematic method of transmitting information In theory, it is superior to CSMA/CD More sophisticated to implement Protocols used in the newer and most popular networks are, however, based on CSMA/CD

The Transmitting Workstation Waits for a free token in order to be able to attach the data to be transmitted to the token On finding a free token, attach the following: Sender’s address Receiver’s address Data block to be transmitted Error checking details etc.

At the Receiving End Data is received and checked for errors Outcomes at the receiving end Data received without errors Date received with errors

Error-free Delivery of Data An acknowledgment is attached to the token Acknowledgment is passed to the sender Token is set free for other nodes to transmit information At this time, the next workstation on the ring will receive an opportunity

Correcting Errors in Delivery A request for retransmission is attached to the token Token carries the message for retransmission to the sender The data is thus retransmitted

Token Regeneration The token is regenerated at regular intervals to sustain the timing of circulation of the token

Usage of Token Passing Used extensively in ring LANs Especially in the IBM token-ring LAN A version of this protocol is also used on certain types of bus LANs Token-bus networks Used in large fiber-optics backbones Used for the construction of very large networks

Usage in Practice Used in backbones Uses in a number of IBM shops Overall, the usage of Ethernet surpasses the usage of Token-Ring networks that are based on the Token-Passing protocol