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
Standard | Speed in bps |
V.21 | 300 |
V.22 | 1200 |
V.22bis | 2400 |
V.32 | 9600 |
V.32bis | 14400 |
V.FC | 19200 |
V.34 | 28800 |
V.34+ | 33600 |
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.
Advantages | Disadvantages | Common Usage |
Widely available | Low speed | Remote access |
Low Cost | Error Prone | Low bandwidth requirements like email |
Most interoperate reliably | Technology changing rapidly | Roving users |
Portable |
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
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.
Advantages | Disadvantages | Common Usage |
Private and secure | Locked into Tele-communications pricing regime | Connecting large sites |
Cost effective for regular transfer of large amounts of data | High monthly rental | Establishment of a permanent internet presence |
Fixed costs easier to budget for than if you pay for data transferred |
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.
Advantages | Disadvantages | Common Usage |
Low fixed cost | Not available in all centers or countries | Periodic Internet Access (for email etc) |
Scalable (B circuits can be combined for greater speeds) | Not suited to mobile users (users dialing in via remote access) | LAN-LAN remote connections which are not permanent |
Fast call set up times |