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 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)

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

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

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

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).

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?

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?

Collisions Handling If a collision is detected, the two sending nodes stop transmitting. Each node will then chose a new random slot from 2 k 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.

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.

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?

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