Business Library

A Local Area Network (LAN), as its name implies, is a communications network which covers the local area - the same department; the same office; the same building; or, sometimes, the same campus. It is a network which transfers data at high speeds over short distances (compared to a Wide Area Network (WAN) which runs at relatively low speeds over long distances). This article considers one of the two main types of LAN in common use, known as Ethernet - a standard published jointly by Digital Equipment Corporation, Intel Corporation, and Xerox Corporation in 1982.

This schematic illustrates an Ethernet LAN with five connected devices (which might be workstations, PCs, printers, or network devices). They share a common bus (ie. the same logical cable), shown by the bold black line, which means that only one device can talk at a time. A technique known as Carrier Sense Multiple Access with Collision Detection (CSMA-CD) controls who can talk when. That's a bit of a mouthful, but this is how it works:

1. A device has a message to send to someone else on the LAN
2. The device checks to see if the LAN is already busy. If yes, it waits a random period then retries [Carrier Sense]
3. If the LAN is not busy, the device starts transmitting the message across the shared bus [Multiple Access] and monitors the bus to ensure that no-one else attempts to transmit
4. If a collision is detected - what goes out is not the same as what comes in [with Collision Detection]:
   • Both devices briefly load the LAN with a jamming signal - which tells everyone on the bus that any incomplete messages are invalid and should be discarded.
   • The two devices now wait for a random period, then the process starts all over again.

If you consider this carefully, you will see that there is a finite time in which collisions might occur - ie. the period between a device starting to send and the time at which all devices can see the traffic. This is a factor of how long the media is and how fast the data propagates across it. The Ethernet standard limits the maximum distance between any two devices to 2,500 metres which [take my word for it!] makes a maximum collision window of 23 microseconds.

The second detail I will mention here is what happens if the recovery from a collision also results in a collision. The random periods described above are doubled each time such a consecutive collision occurs.

So, as the media becomes busier:

• the time waiting for the media to become free increases;
• the likelihood of collisions increases; and
• the time to recover from collisions might also increase.

An Ethernet LAN operates at a data rate of 10 Mbits/sec - which means that 1 million bits of information per second can be sent across the media - a bit being a binary digit (0 or 1). A byte [or octet], which is normally the smallest unit of useful information, consists of 8 bits - so an Ethernet LAN can transport 1,250,000 bytes per second.

However, life is not quite that simple:

1. There are overheads associated with the communication. The data you wish to transfer has to include the sender identity, the receiver's identity, the information itself - and some form of check so that the receiver can verify that the information has not been corrupted in transit. This parcel of information is known as a frame, and will be examined further in a future article.
2. In order to prevent a single device hogging the media, the maximum amount of user data transported in each frame is limited to 1,500 bytes. Long messages may therefore need to be split into more than one frame - each with their own overheads.
3. There is also a minimum amount of data of 46 bytes which ensures that frames take longer to cross the LAN than the collision window discussed earlier.
4. Other delays occur while sensing the media and recovering from collisions.

Business Implications So Far?

• The length of an Ethernet LAN is restricted.
• The performance of an Ethernet LAN is constrained by the number of active users - fewer users per LAN will yield better performance but at higher cost and complexity.
• The nominal capacity of the LAN can never be fully exploited.
• Applications which pass data in small messages make inefficient use of an Ethernet LAN.