Ethernet overview

- Ethernet is a land technology that pretty much every network runs today. The speeds are from 10 megabits all the way up to 100 gigabits a second. Ethernet is the system we use to connect computer systems to form a local area network. The Ethernet term, Collision Domain, refers to a networks scenario. Wherein, one device sends a frame out on a physical network, forcing every other device on the same segment to pay attention to it. This can cause a lot of extra traffic, slowing down every systems connection in the domain.

We can break up collision domains with a switch. Each port on a switch is a single collision domain. This decreases traffic, freeing up band width for users. Which is a great start! But switches don't break up broadcast domains by default, so this is still only one broadcast domain, which is not so good. Segmenting broadcast domains is one of the benefits routers offer. On a router, each port is its own broadcast domain. In this network, there are two router interfaces providing two broadcast domains.

I count 10 switch segments, meaning we've got 10 collision domains. Each switch port is capable of running in full duplex mode, meaning it can transmit and receive traffic simultaneously. When a host wants to transmit over the network, it first checks for the presence of a digital signal on the wire. If all is clear and no other host is transmitting, the host will then proceed with its transmission. If the host attacks another signal on the wire, it sends out an extended jam signal that causes all nodes on the segment to stop sending data, like a busy signal.

Back off algorithms determine when the colliding stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting to transmit will eventually time out. Half duplex means the connection is either transmitting or receiving at any one time. Defined in the original IEEE802.3 Ethernet specification. Half duplex also uses the Carrier Sense Multiple Access with Collision Detect protocol to help prevent collisions and to permit retransmitting if one occurs.

If a Hub is attached to a switch, it must operate in half duplex mode, because the end stations must be able to detect collisions.