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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 raceDevices in a network are arranged in a ring topologyThe token circulates across th
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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 devices 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