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Ethernet: Distributed Packet Switching for Local Computer Networks

Ethernet: Distributed Packet Switching for Local Computer Networks. Authors: Robert M. Metcalfe and David R. Boggs Presentation: Christopher Peery. Outline. Quick Overview Design Choices Ethernet’s Behavior Limitations The Good Side of the Ether. Quick Overview.

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Ethernet: Distributed Packet Switching for Local Computer Networks

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  1. Ethernet: Distributed Packet Switching for Local Computer Networks Authors: Robert M. Metcalfe and David R. Boggs Presentation: Christopher Peery

  2. Outline • Quick Overview • Design Choices • Ethernet’s Behavior • Limitations • The Good Side of the Ether

  3. Quick Overview • So we probably all heard about it one time or another, so what is it? • In brief, it’s a broadcast medium with no centralized control, that is used to interconnect computers in a localized area. • Was greatly influenced by the Aloha Network used by the University of Hawaii. This was a satellite based communication system • Exists as 10 Mbs, 100 Mbs, and 1 Gbps • Uses 1-Persistent CSMA-CD (Carrier Sense Multiple Access – Collision Detection)

  4. Design Choices • Requirements: • Was intended for allow localized communication among workstations • Should be easily extensible • Control should be completely decentralized to avoid creating a bottleneck • Should be relatively inexpensive • Ethernet will not make any absolute guarantees with respect to packet deliver. It is “best-effort”. • Communication medium should be as simple as possible

  5. Identification • Each Ethernet packet contains a source and destination address. • All packets reach every workstation on the segment • Packets are filtered at the network interfaces • There is a reserved address for broadcast

  6. Ethernet Behaviour • Ethernet can be examined in the following areas: • Topology • Control • Collision detection

  7. Topology • Ethernet is a broadcast medium • Workstations single attach themselves to the shared medium • Imagine a straight line with workstations attached to it on either side. • Several Ethernet segments can be connect in sequence using a device known as a repeater or a bridge (a little smarter then a repeater).

  8. Control • Control in Ethernet is completely decentralized. • The medium has no control over who uses it. Its only task to to move information from point A to B. • Is this good? • Every time a workstation needs to transmit, it tries to take control of its Ethernet segment. • This is done by listening to the shared medium to determined if it can be used and then actually using it. • How does this work?

  9. When to transmit… • If a node want to transmit, it will first listen on the segment. • If the node hears a transmission, it will wait for one transmission period for the sending node to finish. Then it will immediately attempt to send its data. This is known as deference. • If the node hears no one sending, it will send immediately. • The idea then once a node is transmitting, it effectively owns the segment is known as acquisition. • This is 1-Persistent CSMA. Meaning when you detect a chance to transmit… do so with probability 1. • What about collisions?

  10. Collisions Handling • If a collision is detected, the two sending nodes stop transmitting. • Each node will then chose a new random slot from 2k slots, where k is the number of collisions already suffered. • This is done 16 times… then a failure is reported • This call Binary Exponential Back-off • Note for this to work… everyone has to play nicely • The duration of the retransmission interval is computed using collision history. This allows it to change with the network load.

  11. Detecting a Collision • When a node sends a packet, it also listens to the segment to ensure the information is not damaged. This is collision detection. • When a collision is detected the sending node will jam the Ether. • This is done to guarantee that all sending nodes are made aware of the collision. • Ethernet also specifies that a checksum be computer for each packet to detect errors from noise… this kind of an error does not result in a back-off.

  12. Limitations • Ethernet has been very successful but also has some limitations • The Ethernet Standard specifies that up to most 5 segments may be connected with repeaters. Each segment at most 500m. • 256 workstations can be attached to a given segment (this is never done in practice… why?) • Most sys admins try to keep Ethernet segments at 60% utilization. Else they become unusable. • The minimum packet size is determined by the segment bandwidth and length (RTT). Why is this?

  13. The Good side of the Ether • Good example of the End-to-End argument • Completely distributed control • Simple • Easy • Cheap

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