6. LIMITATIONS OF LANS

Whilst LANs are extremely wide-ranging in their applications, as with most technologies they do have some limitations. Principally these centre on two areas: capacity and compatibility, most of these can be overcome by good design and careful product selection.

Capacity

Many people familiar with earlier forms of data communications may consider a network operating at 10 or 100 Megabits a second a great luxury. However there can still be throughput problems because the data volumes that are being considered are huge in comparision to the data rates used by terminals with async serial ports.

Consider that a standard VDU (Visual Display Unit) connected to a computer would receive a normal screen full of characters to display. This is typically about 800 characters or 800 bytes which on a 9,600 bps line would take about 0.8 of a second to transfer (800 x 10 bits =8000 bits)/9600). A PC on a LAN would usually transfer a file across from a server, rather than transfer a screen. Files vary greatly in size, but assume that a five page document is being transferred which also has a small graphic included. This may be as much as 20,000 bytes long, 25 times as large as the screen of information. On the LAN even with all the protocol overheads, this may take one quarter of a second to transfer. If the original 9,600 bps line were used the transmission would take 21 seconds.

On looking at the above example it would appear that while volumes of data are much greater this does not cause a problem. However, it should be remembered that there is more than one device on the LAN and therefore, although there is more capacity, there are also more users contending for it.

Capacity problems will become evident during the heaviest loading or “peak loading” of the network. This may be first thing in the morning when every user connects to the network and collects the information for the day or, even more commonly, towards the end of the day, possibly as people are contending for the printers to produce letters for the final post.

To the network user capacity problems become apparent in two ways.

Firstly – when trying to connect to a resource on the network they will not be able to gain access because other users are already connected. This can usually be resolved by adding another resource, such as an extra printer, or adding extra access, such as an additional host server for the computer.

Secondly – response across the network begins to degrade. Files take longer to transfer, connections take longer to set up or even in some cases transfers fail to complete due to the protocol reaching a timeout. (Timeouts occur when one device waits for a preset period to receive something, but does not receive it.)

This type of problem can be more difficult to correct. The easiest method is usually to split the LAN into two parts and use an Ethernet switch or bridge between them. However unless the connection patterns are known accurately, allowing the correct devices to be placed on each LAN to spread the loading, little may be achieved.

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RMON

RMON stands for Remote Monitoring. It is a standard used in network equipment which implement a MIB (Management Information Base) which allows for remote monitoring and management of that equipment. RMON uses an agent running on the device being monitored to supply information over SNMP to a management workstation (or some other system).

The current RMON standard is RFC 2819. It is extended by several RFCs, including RFC 2021 which extends it towards the application layer.

The standard describes functions, messages and data structures to support the nine RMON groups of:

1. Statistics
2. History
3. Alarms
4. Hosts
5. Host Top N
6. Traffic Matrix
7. Filters
8. Packet Capture
9. Events

Each of these groups provides specific sets of data to meet common network-monitoring requirements. Each group is optional so that vendors do not need to support all the groups within the Management Information Base (MIB). Some RMON groups require support of other RMON groups to function properly.

The use of RMON aids the network manager with not only fault finding but it allows the manager to predict future trends and identify potential problems.

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