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

Part 20: WIDE AREA NETWORKING - 2

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

Frame Relay
Sometimes referred to as Fast packet, it is designed for modern networks which do not need lots of error recovery (unlike packet switching). Typical Frame relay connections range from 56Kbps to 2Mbps. Frame relay is similar to packet switching X.25, but is more streamlined giving higher performance and greater efficiency.

Frame relay, like X.25, implements multiple virtual circuits over a single connection, but does so using statistical multiplexing techniques which yields a much more flexible and efficient use of the available bandwidth. FR includes a cyclic redundancy check (CRC) for detecting corrupted data, but does not include any mechanism for corrected corrupted data.

In addition, because many higher level protocols include their own flow control algorithms, FR implements a simple congestion notification mechanism to notify the user when the network is nearing saturation.

Frame Format
The format of FR frames is shown in the diagram below. Flags define a frames start and end. The address field is 16 bytes long, 10 of which comprise the actual circuit ID (Data Link Connection Identifier). The DLCI identifies the logical connection that is multiplexed into the physical channel. Three bits of the address field are allocated to congestion control.

FR also supports multi-casting, the ability to send to more than one destination simultaneously. Four reserved DLCI values (1019 to 1022) are designated as multicast groups.

Additional References
Frame Relay Resources
Basics of Frame Relay

Asynchronous Transfer Mode (ATM)
ATM breaks data into small chunks of fixed size cells (48 bytes of data plus a 5 byte overhead). ATM is designed for handling large amounts of data across long distances using a high speed backbone approach. Rather than allocating a dedicated virtual circuit for the duration of each call, data is assembled into small packets and statistically multiplexed according to their traffic characteristics.

One problem with other protocols which implement virtual connections is that some time slots are wasted if no data is being transmitted. ATM avoids this by dynamically allocating bandwidth for traffic on demand. This means greater utilization of bandwidth and better capacity to handle heavy load situations.

When an ATM connection is requested, details concerning the connection are specified which allow decisions to be made concerning the route and handling of the data to be made. Typical details are the type of traffic [video requires higher priority], destination, peak and average bandwidth requirements [which the network can use to estimate resources and cost structures], a cost factor [which allows the network to chose a route which fits within the cost structure] and other parameters.

UNDER SONSTRUCTION

155Mbps

622Mbps

Additional References
ATM Frequently Asked Questions (FAQ)
A Very Cool Java-based ATM Signalling Simulator (requires Java-capable browser)
IETF IP over ATM Working Group

Digital Subscriber Line (xDSL)
xDSL is a high speed solution that allows megabit bandwidth from tele-communications to customers over existing copper cable, namely, the installed telephone pair to the customers premises (called the local loop). With the high penetration and existing infrastructure of copper cable to virtually everyone's home (for providing a voice telephone connection), xDSL offers significant increases in connection speed and data transfers for access to information.

In many cases, the cost of relaying fiber optic cable to subscriber premises is prohibitive. As access to the Internet and associated applications like multi-media, tele-conferencing and on demand video become pervasive, the speed of the local loop (from the subscriber to the telephone company) is now a limiting factor. Current technology during the 1980's and most of the 1990's has relied on the use of the analog modem with connection rates up to 56Kbps, which is too slow for most applications except simple email.

xDSL is a number of different technologies that provide megabit speeds over the local loop, without the use of amplifiers or repeaters. This technology works over non-loaded local loops (loaded coils were added by telephone companies on some copper cable pairs to improve voice quality). xDSL coexists with existing voice over the same cable pair, the subscriber is still able to use their telephone, at the same time. This technology is referred to seamless.

To implement xDSL, a terminating device is required at each end of the cable, which accepts the digital data and converts it to analogue signals for transmission over the copper cable. In this respect, it is very similar to modem technology.

xDSL provides for both symmetric and asymmetric configurations.

Variations of xDSL
There are currently six variations of xDSL.

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