The Access Layer is the first layer inside the network. Hosts use the access layer in order to access other network devices and share files and printers. The Access Layer contains computers along with the first line of networking devices to which they are attached.
There are more than a few types of network appliances that can be used to connect computers at the Access Layer, such as Ethernet hubs and switches. Network switches allows us to connect many computers with each other. In the Access Layer, every single computer is connected to a hub or switch. Inside an Ethernet network, every computer is able to connect directly to an Access Layer network appliance with a point-to-point cable. These cables must meet specific Ethernet specifications. Every single cable is plugged into a computer's network card and then into a port on the hub or switch.
Hubs have several ports that are used to connect computers to the net. Hubs do not have complicated and advanced electronics that can decode the messages sent between computers on the network. Hubs do not know which computer should get any particular message. This network device accepts electronic messages from one port and repeats the same messages to all the other ports.
Network access cards accept messages addressed to the correct MAC address. Computers disregard IP packets that are not addressed to them. Only the computer specified in the destination address of the IP packet processes the data and responds to the sender.
The hub can sent only one message at a time. Sometimes 2 or more computers connected to a hub try to send a message at the same time. When the electronic signals collide with each other, the communication stops for a very short period.
Collisions are very bad because the messages become corrupted and unreadable. The hub cannot detect that the message is corrupted and repeats it out all the ports. The subnet of the network where a computer can receive a corrupted TCP/IP packet resulting from a collision is called collision domain.
When a computer gets a corrupted message, it detects that a collision has occurred. Every single sending computer waits for a very short period and then retransmits the same message. More computers inside the network simply mean more collisions. More collisions cause more retransmissions. Usually the networks that have many retransmissions are slow. Modern wired networks use anetwork switch to eliminate collisions.
