ocal Area Network and Ethernet Dan li CS Department, Tsinghua University 2021/2/9 1
Local Area Network and Ethernet 2021/2/9 1 Dan LI CS Department, Tsinghua University
Today's Lecture MAC (Medium Access Control) Ethernet Bridges and Lan switches Reading list 2021/2/9
Today’s Lecture MAC (Medium Access Control) Ethernet Bridges and LAN switches Reading list 2021/2/9 2
Problem 1-Sharing a Wire two hosts share a wire one to send The other to receive and wait to send s But what if we want more hosts? 2021/2/9
Problem 1 – Sharing a Wire Two hosts share a wire One to send The other to receive and wait to send … But what if we want more hosts? 2021/2/9 3
Problem 1-Sharing a Wire Wires for everybody Switches Expensive! Can we share a wire? 2021/2/9
Problem 1 – Sharing a Wire 2021/2/9 4 Expensive! Can we share a wire? Switches Wires for everybody!
Problem 2-Listen and talk yak yak w Natural scheme- listen before you talk k Works well in practice 2021/2/9
Problem 2 – Listen and Talk Natural scheme – listen before you talk… Works well in practice yak yak… 2021/2/9 5
Problem 2-Listen and talk ada yada Natural scheme listen before you talk k Works well in practice 2021/2/9
Problem 2 – Listen and Talk Natural scheme – listen before you talk… Works well in practice yada yada… 2021/2/9 6
Problem 2-Listen and talk yakyak Natural scheme - listen before you talk k Works well in practice s But sometimes breaks down when two want to speak at the same time k Why? How do we fix/ prevent this? 2021/2/9
Problem 2 – Listen and Talk Natural scheme – listen before you talk… Works well in practice But sometimes breaks down when two want to speak at the same time Why? How do we fix/prevent this? yada yada… yak yak… 2021/2/9 7
Problem 3- Who is the packet for? Need to put an address on the packet What should it look like? How do you know what address you want to send to? 2021/2/9
Problem 3 – Who is the Packet for? Need to put an address on the packet What should it look like? How do you know what address you want to send to? 2021/2/9 8
Multiple Access Methods FiXed assignment k Partition channel so each node gets a slice of the bandwidth Essentially circuit switching -thus inefficient Examples: TDMA, FDMA, CDMA Random access nodes contends equally for bandwidth and recover from collisions Examples: Aloha, Ethernet Token-based or reservation -based k Take turns using the channel EXamples: Token ring 2021/2/9
Multiple Access Methods Fixed assignment Partition channel so each node gets a slice of the bandwidth Essentially circuit switching – thus inefficient Examples: TDMA, FDMA, CDMA Random access Nodes contends equally for bandwidth and recover from collisions Examples: Aloha, Ethernet Token-based or reservation-based Take turns using the channel Examples: Token ring 2021/2/9 9
Random Access protocols w When node has packet to send k transmit at full channel data rate No a priori coordination among nodes * Two or more transmitting nodes“ collision” Random access MAC protocol specifies k How to detect collisions How to recover from collisions(e.g. via de layed retransmissions Examples of random access MAc protocols slotted aloha and aloha CSMA and CSMACD 2021/2/9 10
Random Access Protocols When node has packet to send Transmit at full channel data rate No a priori coordination among nodes Two or more transmitting nodes → “collision” Random access MAC protocol specifies How to detect collisions How to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: Slotted ALOHA and ALOHA CSMA and CSMA/CD 2021/2/9 10
