Token Passing Protocols

1. Token Passing Protocols

2. Token Passing Protocols Overview The order each station gets to send a frame is predetermined Similar to a relay race: each runner (station) must wait for the baton (token) to be passed to complete his leg of race Devices in a network are arranged in a ring topology The token circulates across the network Each station must wait for the token to arrive at its location before it can send data on the network

3. Characteristics of Token Passing Protocols Access of the network is guaranteed Priority schemes can be deployed Timers are used to ensure proper operation No collisions occur Time-sensitive applications can be supported A high degree of reliability (backup cabling paths) More predictable than Ethernet Higher component costs (hubs, NICs)

4. Token Passing and Time-Sensitive Applications

5. IEEE 802.5 Token Ring Token ring operates at 4 Mbps or 16 Mbps (Speeds on the same ring cannot be mixed) Each device is physically star wired back to a hub or MAU (multi-station access unit) The hub or MAU logically connects the stations to form a ring A priority scheme for stations may be configured Access to the ring is guaranteed Built-in network management is provided

6. MAU

7. Token Ring Design and Implementation MAU (multistation access unit) � the term used by IBM Most token ring electronics (hubs) have evolved to include active retiming of the signals and network management features Maximum number of stations supported vary from 72-250 on a single ring A backup path exists on the main ring cabling

8. Token Ring Implementation

9. Token Ring Token Passing Protocol Once a station detects a token, it uses it to construct a frame that it then transmits onto the ring When a receiving station detects a frame with its address as the destination address, it copies the frame. The receiving station flips the recognized address and frame-copied bits in the frame status field in the frame, and sends the modified frame back out to the network When the frame arrives back at the sending station, it examines and removed frame from the ring The source station then transmits a new token

10. Token Ring Token Passing Protocol

11. Token Ring Active Monitor The active monitor is responsible for many ring management activities It is nominated through a procedure called the claim process (the first station turned on or the station with highest-vales MAC address) The active monitor: Sends out the first token Insures there is only one token on the ring at any time Prevents frame from circulating endlessly by flagging the data when it first passes A standby monitor takes over the role of the active monitor if the active monitor fails or leaves the network

12. Token Ring Active Monitor

13. Token Format

14. Token Format The SD (starting delimiter) � identify the beginning of a transmission and for synchronization purpose The Access Control field The priority field: Each station has an assigned priority, ranging from 0-7, corresponding to the three ppp bits in the field To use a token, a station must wait until it detects a token with a priority less than or equals to its own The token bit Set to 0 if it is a token Set to 1 if it is a data frame The monitor bit Set to 0 by the transmitting station Set to 1 by the active monitor station If the active monitor station sees the frame again, the frame is removed from the network The reservation bits The ED (ending delimiter) � end of the frame

15. Ring Frame Format

16. Token Ring Frame Format The access control field is the same as the token format (the token bit is set to 1) The frame control field specifies the type of data in the data field The route info field is used in the network with multiple token ring LANS The data field contains the NOS information, plus the data (generally, the data is between 0 -4,500 bytes) The FCS (frame check sequence) is used to detect an error The FS (frame status) field us to indicate to the transmitter if the frame was copied by the intended destination

17. Token Ring Frame example

18. ANSI X3T9.5 FDDI FDDI (Fiber Distributed Data Interface) A high speed LAN technology Serving as a campus backbone Operating as a building backbone, connecting Ethernet and token ring LANs via bridge and/or routers Connecting mainframes Uses a token passing protocol

19. FDDI

20. Characteristics of FDDI 100 Mbps transmission Up to 500 network attachments Dual ring topology (primary and secondary rings) for redundancy Up to 200 km of total path lengths Built-in network management (station management) Three cable options Multimode fiber optic cable (2000 m between stations) Single mode fiber optic cable (58 km between stations) Category 5 UTP (100 meters)

21. Campus Backbone Design Example

22. FDDI Dual Ring Topology Two Rings are used to provide redundancy In normal operation, the token and frames travel only on the primary ring in a single direction. The second ring transmits idle signals in the opposite direction (through state) If a cable or a device becomes disabled, the primary ring wraps back around onto the secondary ring (wrap condition)

23. FDDI Wrap Condition

24. FDDI Token Passing Protocol FDDI deploys several timers TTRT (target token rotation time): an agree-upon time between all devices on the network FDDI allows a station to send multiple frames based on each device�s percentage of the TTRT FDDI allows a station to use other stations� unused time on the ring TTRT is between 4 ms and 165 ms (.004 - .165 seconds)

25. FDDI Token Passing Protocol