清华大学：Local Area Network and Ethernet（PPT课件讲稿）

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