Specifications

7 LAN -- The Networked Home

The Local Area Network (LAN) allows computers to be used anywhere in the house. Each computer has

access to shared resources such as printer, files, and the Internet.

The LAN is 100 megabit per second Ethernet over Cat5 unshielded twisted pair wire. Most rooms are

wired with two network outlets. Cable from each receptacle is run to a central wiring closet. In the wiring

closet a 16-port hub connects everything together. Ethernet over Cat 5 is the most prevalent LAN

technology by far. It is well suited for residential use components are readily available and easy to install.

The only protocol used on the LAN is TCP/IP. This is the same protocol used on the global Internet

7.1 Ethernet

Ethernet IEEE 802.3 is the most common local network technology used today. It is based on CDMA/CA

(Collision Detection Multiple Access Collision Avoidance) scheme. Think of Ethernet as a telephone party

line. Before speaking you listen to see if anyone else is talking. If no one is talking then you start. It is

possible that several people may start talking at the same time. That is a collision; no one can understand

what is being said. When this occurs everyone stops talking for a while. When the line is idle they try

again. Each party waits a different length of time to minimize the chance of colliding again. CDMA/CD

imposes a number of design constraints on the network. The minimum packet size must be longer then the

end-to-end propagation delay of the system. This insures the transmitter is still transmitting when the

collision occurs allowing retries to be done by the network layer. Power levels must be set to allow

collision detection.

When Ethernet was developed it used a fat coax cable with taps clamped on at prescribed intervals. Today

the most common type of Ethernet wiring is unshielded twisted pair (UTP) copper cable consisting of 4

pairs of wire terminated with 8 conductor jacks similar to those used for telephone wiring. This has

dramatically reduced the cost of implementing a LAN.

7.1.1 Media Access Controller (MAC) Address

Each Ethernet interface has a unique address called the MAC address. This allows each interface to be

uniquely addressed. This is not the same as the IP address that will be discussed later.

Excerpt from Assigned Ethernet numbers:

Ethernet hardware addresses are 48 bits, expressed as 12

hexadecimal digits (0-9, plus A-F, capitalized). These 12 hex

digits consist of the first/left 6 digits (which should match the

vendor of the Ethernet interface within the station) and the

last/right 6 digits which specify the interface serial number for

that interface vendor.

These high-order 3 octets (6 hex digits) are also known as the

Organizationally Unique Identifier or OUI.

Ethernet addresses might be written unhyphenated (e.g.,

123456789ABC), or with one hyphen (e.g., 123456-789ABC), but

should be written hyphenated by octets (e.g., 12-34-56-78-9A-BC).

These addresses are physical station addresses, not multicast nor

broadcast, so the second hex digit (reading from the left) will

be even, not odd.