DATA COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.

Part 19: WIDE AREA NETWORKING - 1

Analog Modems | Dedicated or Leased Lines | Packet Switching
Integrated Services Digital Network | Frame Relay
Asynchronous Transfer Mode | Digital Subscriber Line

The following is a brief discussion on the merits of the technologies used to implement Wide Area Networks.

Analog Modems
Analog modems use the existing telephone infrastructure to link sites together. The telephone cabling supports analogue frequencies in the range 300Hz to 3400KHz, and is primarily designed for speech. The available bandwidth of the speech circuits provided by telecommunication companies imposes limits on the available speed in bits per second that can be transmitted.

The modems implement a dial-up connection. A connection is made between the two modems by dialing the number assigned to the other modem, using the existing dial up telephone network. Generally, connections are established for limited duration's. This suits remote access users who might want to dial into their network after hours, or small offices which dial into their internet service provider at regular intervals during the day to exchange (upload and download) email.

Current Modem standards are

The speeds stated above are maximum speeds, and often, modems fail to achieve this. Errors caused by noise on the telecommunication lines often cause modems to fall back to a much lower speed, in order to reduce the number of errors. Thus a high speed modem rated at 33600bps often achieves a throughput of 9600bps due to the existing phone lines being too error prone to support the higher rate.

Another problem that occurs is with modems that utilize compression techniques. Often, compression is measured on the transmission of uncompressed files like text files. When these same compression modems are asked to deal with the transfer of compressed files like .ZIP files, they do not perform well, and effectively either transfer at a much reduced rate or no compression at all. Some typical compression type modems are MNP4 and MNP5. In addition, modems utilizing the different compression schemes often fail to communicate properly with compression enabled. This is due to variances in manufacturers implementations of compression algorithms.

Dedicated Lines (Leased Line)
Dedicated lines are fixed connections which do not involve dialing. They are permanent end to end connections. The telecommunications company provides a dedicated high speed connection between the two desired locations, at speeds ranging from as low as 9600bps to as high as 45Mbps. The higher the speed, the greater the cost, which is usually a fixed monthly rental charge (does not include data charges, only rental charges).

The connection is available 24 hours a day, seven days a week, and is thus suited to companies who want permanent connections between their office branches, or perhaps to a company who wants a permanent connection to the Internet (they are providing a WWW server for people to access).

The basic unit of measurement for dedicated lines is a T1 connection, which supports 1.544Mbps. A T3 connection supports 45Mbps. Fractional T1 circuits are available in units of 64Kbps, with connections of 384Kbps, 512Kbps and 768Kbps being common.

The connection is implemented with two units

• Channel Service Unit (CSU)
  This provides the interface to the dedicated line
• Data Service Unit (DSU)
  This interfaces between the CSU and the customers equipment, using RS232 for low speeds up to 56Kbps, and V.35 (RS-422/499) for higher speeds

It is common to have the units as a single component. The CSU/DSU is normally the demarcation zone which defines where the customers responsibility ends the the telecommunications company begins. Most telecommunication companies provide the ability to perform real-time monitoring of the connection via the CSU/DSU.

Packet Switching (X.25)
Packet switching has been around for some time now. It is an established technology which sends data across a packet switched network in small parcels called packets. If the data packets travel the same path to the destination, this is called virtual circuit, if packets can travel any path, not necessarily the same as each other, this is called datagram.

Packet switched connections are normally in the speed of 19.2Kbps to 64Kbps, though some higher speed connections may be available in certain countries. It is a dial-up switched connection, in that the user pays connection charges, traffic charges and time charges. As such, its not suitable for permanent connections.

X.25 was designed to be implemented over noisy analogue phone lines, thus has a lot of built in error control. With today's relatively low error links, this can result in an unnecessary overhead.

An X.25 connection supports a number of virtual circuits which are each numbered. These represent a time division of the available bandwidth of the connection. This division into virtual circuits allows each VC to support a single device. X.25 uses the lower 3 levels of the OSI model. The virtual circuit is a full duplex connection which is established for the duration of the call.

Devices which do not have built in packet switched support can be interfaced to a packet switched network using a Packet Assembly/Disassembly (PAD) unit. This allows existing computers or terminals to be connected.

Integrated Services Digital Network (ISDN)
ISDN was developed in order to provide the user with a single interface which supported a range of different devices simultaneously. The basic ISDN connection is a 2B + D connection, that is, 2 B channels each of 64Kbps, and a single D channel of 16Kbps. The B channels are designed to carry user data, whilst the D channel is meant to carry control and signaling information. This format is known as the Basic Rate Interface (BRI), which also provides for frame control and other heads, which gives an overall capacity of 192Kbps per BRI ISDN connection.

Higher capacity circuits are available. ISDN uses the existing telecommunications dial-up infrastructure, though special ISDN connection interface boxes are required at the users premises. Each B channel can be used separately or combined with other B channels to achieve higher speeds.

The Primary Rate Interface (PRI) offers 23B channels and one D channel at 64Kbps (North America and Japan) giving a total of 1.544Mbps. The PRI for Europe, Australia and some other parts of the world is 30B channels and one D channel at 64Kbps giving a total of 2.048Mbps.

© Copyright Brian Brown, 1995-2000. All rights reserved.