Connecting to a Public Network

If you want to connect to a public network, for example one managed by a PTT, you need to provide the network administering body with information about the kind of connection and configuration you would like. Typically, when you contact the administering body, you are sent a network subscription form. Use this to specify what you want. Public networks vary in the services they offer and in the information they require. You may need to answer some or all of the following questions:

• Do you want a dial-up line or a leased line? Consider the tariff structure of the public network you are connecting to and how you want to use your network connection.

• Do you want a single connection or an MLP connection? You can only use MLP across a public network if you have subscribed to an MLP type line.

• What mix of Permanent and Switched Virtual Circuits do you need? A Permanent Virtual Circuit (PVC) is a permanent association between two specified end-points. A Switched Virtual Circuit (SVC) is a temporary association between two endpoints, established for the duration of a call. Compare the tariff structures for PVCs and SVCs before deciding how many of each you need. Some network providers charge a flat rate per month for PVCs and a charge per minute for SVCs, as well as a charge per unit of data on either type of virtual circuit. In general, SVCs tend to be cheaper unless two end-points need to be in permanent communication, but this may not be true for all networks.

• What line speed would you like? This is worth investigating carefully, as line speed can have a significant impact on network performance and on the price you will pay for your connection. Consider factors such as the volume of data you expect to transfer, the level of interactive traffic, the type of virtual circuits you plan to have, the expected duration and frequency of connections, the types of modems you have available, and so on.

• What is the maximum size of Information Frame (I-Frame) you need? This must be larger than the maximum possible packet size on your network.

• How many times should a frame be retransmitted before an error is reported? A large value increases the chance of data getting through correctly. A small value allows fast detection of errors. The value you choose may also depend on whether or not the public network charges for each failure.

• What is the maximum packet size you require? In most cases, use of the default value of 128 is appropriate.

• What is the maximum window size you require? In most cases, use the default value of 2. A larger window size increases throughput but will probably also increase the cost.

• Which throughput class do you require? The throughput class determines the amount of resources allocated to a virtual circuit. It is determined by the line speed. The default value is 2.

• Do you need non-standard flow control? If you have chosen non-default packet and window sizes, you may need to use non-standard flow control.

• Do you want local or remote packet acknowledgment? Local packet acknowledgment is simpler, so use this, unless a particular application requires remote packet acknowledgment.