1. Lecture 9: Ethernet and Token Ring Networks

2. Ethernet review
Network topology with two stars connected by a point-to-point link

3. Ethernet Frame MAX=1500 bytes Min=46 bytes
Type—demux key to identify application
CRC-32

4. Addresses Media Access Control (MAC) address Broadcast address
48 bits/6 bytes-X1X2:X3X4:X5X6:X7X8:X9X10:X11X12
Unique worldwide for all time
Belongs to adapter
Broadcast address
FF:FF:FF:FF:FF:FF
Multicast address
1xxxxxxx:XX:XX:XX:XX:XX

5. Receiver Algorithm Adaptor Receives all frames on the ethernet
Only Accepts Frames Addressed to:
Its unicast address
the broadcast address
A mult-cast address it has been programmed to accept
Accepts all frames in “promiscuous (eavesdropping) mode”

6. Transmitter Algorithm
If adapter has a frame to send:
Line is idle-sends its frame-up to 1500 bytes
Line is busy-wait til line goes idle and send the frame
Collisions
These are detected as overvoltage AT AN ADAPTER
When collision is detected, receiver transmits 32 bit jamming sequence and shuts down-purpose of jamming sequence is to cause a collision at the sender
Frame must be long enough for this collision to occur—512 bits in classic ethernet

7. The ethernet advantage
It’s simple—no routing tables to maintain
It’s flexible—just plug in your PC
It’s cheap
Cable—$ 0.12/ft
Adaptors—$ 20
Hubs--$ 30/port
Switch--$ 50/port

8. Token Ring Shared medium similar to ethernet
Distributed algorithm controls transmission
All nodes see all frames
Token=small (24 bit) circulating frame
To send: Remove token and send frame
Sender: removes message when it comes around and reinserts token

9. Multi-station access unit (MSAU) create a star-like topology

10. Token Ring Media Access Control MAC
Network adaptor contains-receiver, transmitter, buffer >1 bit
Need enough storage within the ring to store a 24 bit token
If you have less than 24 stations—one has to provide the storage—this will be the monitor

11. MAC cont.
To seize the ring-change 1 bit in the second byte-becomes preamble to the data packet.
A host copies those packets addressed to it and retransmits it around the ring
Sender removes the packet as it comes around—may still be sending information

12. Key Concept Token Holding Time (THT)>transmission time of a packet
Token Rotation Time (TRT)< ActiveNodes x THT+RingLatency Note: TRT is load dependent

13. Early release versus delayed release
THT=Frame transmission time + Ring Latency
THT=Frame transmission time

14. Reliable Transmission
A-bit set to 1 by destination host when message is detected
C-bit set to 1 if successfully copied

15. Priority Priority (3-bits)—8 levels Token priority, packet priority
To seize the ring: Packet priority > Token priority
Reservations: Use reservation bits (3-bits) on a circulating packet to lock out low priority users.
Increase rb’s to your packet priority.
Current token holder sets token priority to the reservation
All users with packet priority > reservation may seize the ring.
New user must reset token priority to the original value

16. Token Ring Maintenance
Monitor roles
Ensure adequate delay
Check for token circulation— Max TRT=Number of users*THT+RingLatency
Removes orphaned and corrupted frames
Detects dead stations-beacon frame

17. Death of a Monitor Monitor sends control message periodically
Hosts watch for this signal-if timeout, they try to become the monitor by sending a claim token
If claim token returns-sender is the monitor
If another claim token returns first-arbitration determines the new monitor

18. Token ring frame Manchester encoding
Illegal Manchester codes are used as delimiters
Frame status bits-A & C