Lecture 9: Ethernet and Token Ring Networks

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

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

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

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”

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

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

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

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

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

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

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

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

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

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

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

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

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