Ethernet

Introduction

Ethernet History

Broadcast Network Operation

Ethernet Protocol

Ethernet at 10Mbps

Ethernet at 100Mbps

Ethernet at 1000Mbps and above

Broadcast Network Operation

Ethernet, by its very nature is a Broadcast Network. This means that the hosts are connected to a network through a single shared medium. This has the advantage that messages don't have to be routed to their destination, as all hosts are present on the shared medium, but it does incur another set of problems. The main problem which needs to be addressed is that of Media Access Control (MAC) or giving fair access to multiple nodes on a shared medium.

Collisions

When a number of nodes are connected to a single shared medium, one of the first things that need to be considered is what happens if two or more nodes try to broadcast at the same time. This is called a collision and prevents any information passing along the network because the multiple messages would corrupt each other, destroying both. There are two main methods for reducing the effect of collisions, Collision Avoidance or Collision Resolution. Collision Avoidance involves systems which prevent any collisions occurring in the first place, such as polling or token passing. Collision Resolution or Contention MAC Strategies rely on the fact that collisions will occur, and try to cope with them as well as possible. Ethernet uses Collision Resolution, so I shall focus on this strategy for the rest of the page.

ALOHA

The most basic form of Collision Resolution is to simply allow any station to send a message (or packet) whenever it is ready to send one. This form of transmission was first used in a prototype packet radio network, ALOHANET, commissioned in Hawaii in 1970, and has been known ever since as unslotted ALOHA. In Pure ALOHA, packets contain some form of error detection which is verified by the receiver. If the packet is received correctly, the destination returns an acknowledgment. If a collision occurs and the message is destroyed or corrupted, then no acknowledgment will be sent. If the sender does not receive an acknowledgment after a certain delay, it will re-send the message.

Carrier Sense Multiple Access

The next stage in Collision Resolution after ALOHA was to add the ability for devices to detect whether the shared medium is idle or not. This is called "Carrier Sense Multiple Access" or CSMA. This, however, does not completely eliminate collisions, since two devices could detect the medium as idle, then attempt to send at approximately the same time.

CSMA is actually a family of protocols which vary by the method which they wait for the medium to become idle, known as the persistence strategy. Below is an explanation of some of the major strategies:

• 1-Persistent CSMA - In this strategy, when a device wants to send a message, it first listens to the medium. If it is idle the message is sent immediately, however, if it is busy the device continues to listen to the medium until it becomes idle and then sends the message immediately. The problem with 1-Persistent CSMA is that if a number of devices attempt to send during a busy period, then they shall all send as soon as the medium becomes idle, guaranteeing a collision.
• nonpersistent CSMA - This strategy attempts to reduce the greediness of 1-Persistent CSMA. It again first listens to the medium to see if it is idle, if so it sends immediately. If the medium is busy, instead of continuing to listen for the medium to become idle and transmitting immediately, it waits a random period, then tries again. This means that in high load situations, there is less chance of collisions occurring.

Collision Window

We have talked about a collision occurring if two devices send at approximately the same time, but how long does a device have to wait until it knows that its message has not been corrupted by a collision. Messages take a certain amount of time to travel from the device to the end of the signaling medium, which is known as the propagation delay. It would seem that a device only needs to wait for one propagation delay, until the message reaches the last receiver, to know if a collision has occurred. This, however, is not the case. Take for example the following situation. A device sends a message, which takes 1 propagation delay to reach the last device on the medium. This last device on the medium could then send a message just before the original message reaches it (i.e. just before 1 propagation delay). This new message would take an additional propagation delay to reach the original device, which means that this device would not know that a collision had occurred until after 2 propagation delays.

Collision Detection

Now that we know how long we need to wait to discover if a collision has occurred, we can use this to increase the effectiveness of CSMA. CSMA behaves inefficiently when a collision occurs, since both stations continue to send their full packet, even though it will be corrupted. A simple enhancement to CSMA is the addition of Collision Detection (CSMA/CD). A simple check is made to make sure that the signal present on the medium is the same as the outgoing message. If it isn't then a collision is occurring, and the message can be aborted. This means that the time spent sending the doomed messages can be put to a more productive use.

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