Friday, April 2, 2010


In 1972, Xerox (the printer company) created the Ethernet standard (3 Mbps). The Ethernet standardized in 1982, and the speed was bumped up to 10Mbps. In 1995, Fast Ethernet emerges, running at 100Mbps. Five years later in 2000, the Gigabit Ethernet is finalized, running at 1Gbps. In 2002, the 10Gbps Ethernet emerges. Finally, in 2007, the 100Gbps Ethernet appears.
Most of the time we deal with Bytes when working in a PC. A byte represents one character. A KiloByte is 1024 Bytes. 1024 Bytes is roughly a page full of characters. A MegaByte is a 1024 KiloBytes, and then on to GigaByte, TeraByte, PetaByte, ExaByte and all the way up to Yottabyte. A byte is made up of 8 bits, and a bit is a 0 or a 1. Speeds in terms of networking is usually defined in bits.

Ethernet technology exists in the Data Link layer. The Data Link layer can be divided into two sub-layers, the LLC (Logical Link Control) and the MAC (Media Access Control). LLC allows a packet to know what network layer protocol to use (e.g. IP, IPX, AppleTalk) during decapsulation.

In the physical layer, we have Cat 5 cables, Wireless, Fiber cables, and so on.

CSMA/CD is a set of rules that govern how a device talks on an Ethernet network. It stands for Carrier Sense Multiple Access / Collision Detection. If we break down the words:
-Carrier refers to the network signal, the medium in which data travels across.
-Sense is the ability to detect.
-Multiple Access means that all devices have equal access to the carrier.
-Collision is when two or more devices send at once in a collision domain, resulting in unreadable information.
-Detection is how a device knows that a collision has occurred and how it handles it.

In a Token Ring environment, you literally have no collision. This is known as CSMA/CA. This is because they make use of a token that flies around a ring. This is great because you can't have collisions, but this results in a speed bottleneck.

A unicast communication is when a host sends a message to another host. In a switched network, only the two hosts will see the message. A broadcast on the other hand, sends the message to all hosts in the switched network. You can think of broadcasts as a giant speaker blasting music all over the country.

A multicast on the other hand sends a message and only registered members will receive them. This helps with the bandwidth. You can think of multicast as a radio broadcast, where only those who tune in would receive the data.

The MAC address can be split into two parts: The OUI, and a vendor assigned portion. The MAC address has 12 hexadecimal characters of either groups of 4 or groups of 2 separated by colons. The first 24 bits (or first 6 characters) of a MAC address defines the OUI. The OUI is the organizational unique identifier, which is assigned organizations who register for them. You can look up the OUI of a MAC by checking online. You can't usually easily tell what type of equipment a MAC belongs to from the vendor assigned portion though.

No comments :

Post a Comment