Data Communication

Data Communication

This lesson is organized into four parts. The first part deals with the most fundamental

aspects of the communications function, focusing on the transmission of signals

in a reliable and efficient manner. For want of a better name, we have given

Part I the title "Data Communications," although that term arguably encompasses

some or even all of the topics of Parts 11, 111, and IV.

To get some flavor for the focus of Part I, Figure 1.2 provides a new perspective

on the communications model of Figure l.la. Let us trace through the details

of this figure using electronic mail as an example.

Consider that the input device and transmitter are components of a personal

computer. The user of the PC wishes to send a message to another user-for example,

"The meeting scheduled for March 25 is canceled" (m). The user activates the

electronic mail package on the PC and enters the message via the keyboard (input

device). The character string is briefly buffered in main memory. We can view it as

a sequence of bits (g) in memory. The personal computer is connected to some

transmission medium, such as a local network or a telephone line, by an I10 device

(transmitter), such as a local network transceiver or a modem. The input data are

transferred to the transmitter as a sequence of voltage shifts [g(t)] representing bits

on some communications bus or cable. The transmitter is connected directly to the

medium and converts the incoming stream [g(t)] into a signal [s(t)] suitable for

transmission. Specific alternatives to this procedure will be described in Lesson 4.

The transmitted signal s(t) presented to the medium is subject to a number of

impairments, discussed in Lesson 2, before it reaches the receiver. Thus, the

received signal r(t) may differ to some degree from s(t). The receiver will attempt

to estimate the original s(t), based on r(t) and its knowledge of the medium, producing

a sequence of bits gl(t). These bits are sent to the output personal computer,

where they are briefly buffered in memory as a block of bits (g). In many cases, the

destination system will attempt to determine if an error has occurred and, if so, will

cooperate with the source system to eventually obtain a complete, error-free block

of data. These data are then presented to the user via an output device, such as a

printer or a screen. The message (m'), as viewed by the user, will usually be an exact

copy of the original message (m).

Now consider a telephone conversation. In this case, the input to the telephone

is a message (m) in the form of sound waves. The sound waves are converted

by the telephone into electrical signals of the same frequency. These signals are

transmitted without modification over the telephone line. Hence, the input signal

g(t) and the transmitted signal s(t) are identical. The signal s(t) will suffer some distortion

over the medium, so that r(t) will not be identical to s(t). Nevertheless, the

signal r(t) is converted back into a sound wave with no attempt at correction or improvement of signal quality.

 Thus m ' is not an exact replica of m. However, the

received sound message is generally comprehensible to the listener.

The discussion so far does not touch on other key aspects of data communications,

including data-link control techniques for controlling the flow of data and

detecting and correcting errors, and multiplexing techniques for transmission efficiency.

All of these topics are explored in Part I.

Data Communication Networking

In its simplest form, data communication takes place between two devices that are

directly connected by some form of point-to-point transmission medium. Often,

however, it is impractical for two devices to be directly, point-to-point connected.

This is so for one (or both) of the following contingencies:

The devices are very far apart. It would be inordinately expensive, for example,

to string a dedicated link between two devices thousands of miles apart.

There is a set of devices, each of which may require a link to many of the

others at various times. Examples are all of the telephones in the world and

all of the terminals and computers owned by a single organization. Except

for the case of a very few devices, it is impractical to provide a dedicated wire

between each pair of devices.

The solution to this problem is to attach each device to a communications network.

Figure 1.3 relates this area to the communications model of Figure l.la and

also suggests the two major categories into which communications networks are traditionally

classified: wide-area networks (WANs) and local-area networks (LANs).

The distinction between the two, both in terms of technology and application, has

become somewhat blurred in recent years, but it remains a useful way of organizing

the discussion.

Wide-Area Networks

Wide-area networks have been traditionally been considered to be those that cover

a large geographical area, require the crossing of public right-of-ways, and rely at

least in part on circuits provided by a common carrier. Typically, a WAN consists

of a number of interconnected switching nodes. A transmission from any one device

is routed through these internal nodes to the specified destination device. These

nodes (including the boundary nodes) are not concerned with the content of the

data; rather, their purpose is to provide a switching facility that will move the data

from node to node until they reach their destination.

Traditionally, WANs have been implemented using one of two technologies:

circuit switching and packet switching. More recently, frame relay and ATM networks

have assumed major roles.

 

Circuit Switching

In a circuit-switched network, a dedicated communications path is established

between two stations through the nodes of the network. That path is a connected

sequence of physical links between nodes. On each link, a logical channel is dedicated

to the connection. Data generated by the source station are transmitted along

the dedicated path as rapidly as possible. At each node, incoming data are routed

or switched to the appropriate outgoing channel without delay. The most common

example of circuit switching is the telephone network.

Packet Switching

A quite different approach is used in a packet-switched network. In this case, it is

not necessary to dedicate transmission capacity along a path through the network.

Rather, data are sent out in a sequence of small chunks, called packets. Each packet

is passed through the network from node to node along some path leading from

source to destination. At each node, the entire packet is received, stored briefly, and

then transmitted to the next node. Packet-switched networks are commonly used

for terminal-to-computer and computer-to-computer communications.

Frame Relay

Packet switching was developed at a time when digital long-distance transmission

facilities exhibited a relatively high error rate compared to today's facilities. As a

result, there is a considerable amount of overhead built into packet-switched

schemes to compensate for errors. The overhead includes additional bits added to

each packet to introduce redundancy and additional processing at the end stations

and the intermediate switching nodes to detect and recover from errors.

With modern high-speed telecommunications systems, this overhead is unnecessary

and counterproductive. It is unnecessary because the rate of errors has

been dramatically lowered and any remaining errors can easily be caught in the end

systems by logic that operates above the level of the packet-switching logic; it is

counterproductive because the overhead involved soaks up a significant fraction of

the high capacity provided by the network.

Frame relay was developed to take advantage of these high data rates and low

error rates. Whereas the original packet-switching networks were designed with a

data rate to the end user of about 64 kbps, frame relay networks are designed to

operate efficiently at user data rates of up to 2 Mbps. The key to achieving these

high data rates is to strip out most of the overhead involved with error control.

ATM

Asynchronous transfer mode (ATM), sometimes referred to as cell relay, is a culmination

of all of the developments in circuit switching and packet switching over

the past 25 years.

ATM can be viewed as an evolution from frame relay. The most obvious difference

between frame relay and ATM is that frame relay uses variable-length

packets, called frames, and ATM uses fixed-length packets, called cells. As with

frame relay, ATM provides little overhead for error control, depending on the

inherent reliability of the transmission system and on higher layers of logic in the

end systems to catch and correct errors. By using a fixed-packet length, the processing

overhead is reduced even further for ATM compared to frame relay. The

result is that ATM is designed to work in the range of 10s and 100s of Mbps, compared

to the 2-Mbps target of frame relay.

ATM can also be viewed as an evolution from circuit switching. With circuitswitching,

only fixed-data-rate circuits are available to the end system. ATM allows

the definition of multiple virtual channels with data rates that are dynamically

defined at the time the virtual channel is created. By using full, fixed-size cells,

ATM is so efficient that it can offer a constant-data-rate channel even though it is

using a packet-switching technique. Thus, ATM extends circuit switching to allow

multiple channels with the data rate on each channel dynamically set on demand.

ISDN and Broadband ISDN

Merging and evolving communications and computing technologies, coupled with

increasing demands for efficient and timely collection, processing, and dissemination

of information, are leading to the development of integrated systems that transmit and process all types of data. A significant outgrowth of these trends is the

integrated services digital network (ISDN).

The ISDN is intended to be a worldwide public telecommunications network

to replace existing public telecommunications networks and deliver a wide variety

of services. The ISDN is defined by the standardization of user interfaces and

implemented as a set of digital switches and paths supporting a broad range of traffic

types and providing value-added processing services. In practice, there are multiple

networks, implemented within national boundaries, but, from the user's point

of view, there is intended to be a single, uniformly accessible, worldwide network.

Despite the fact that ISDN has yet to achieve the universal deployment hoped

for, it is already in its second generation. The first generation, sometimes referred

to as narrowband ISDN, is based on the use of a 64-kbps channel as the basic unit

of switching and has a circuit-switching orientation. The major technical contribution

of the narrowband ISDN effort has been frame relay. The second generation,

referred to as broadband ISDN, supports very high data rates (100s of Mbps) and

has a packet-switching orientation. The major technical contribution of the broadband

ISDN effort has been asynchronous transfer mode (ATM), also known as cell

relay.

Local Area Networks

As with wide-area networks, a local-area network is a communications network that

interconnects a variety of devices and provides a means for information exchange

among those devices. There are several key distinctions between LANs and WANs:

1. The scope of the LAN is small, typically a single building or a cluster of buildings.

This difference in geographic scope leads to different technical solutions,

as we shall see.

2. It is usually the case that the LAN is owned by the same organization that

owns the attached devices. For WANs, this is less often the case, or at least a

significant fraction of the network assets are not owned. This has two implications.

First, care must be taken in the choice of LAN, as there may be a substantial

capital investment (compared to dial-up or leased charges for widearea

networks) for both purchase and maintenance. Second, the network

management responsibility for a local network falls solely on the user.

3. The internal data rates of LANs are typically much greater than those of widearea

networks.

Traditionally, LANs make use of a broadcast network approach rather than a

switching approach. With a broadcast communication network, there are no intermediate

switching nodes. At each station, there is a transmitterlreceiver that communicates

over a medium shared by other stations. A transmission from any one

station is broadcast to and received by all other stations. A simple example of this

is a CB radio system, in which all users tuned to the same channel may communicate.

We will be concerned with networks used to link computers, workstations, and other digital devices. In the latter case, data are usually transmitted in packets.

Because the medium is shared, only one station at a time can transmit a packet.

More recently, examples of switched LANs have appeared. The two most

prominent examples are ATM LANs, which simply use an ATM network in a local

area, and Fibre Channel.