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“Datapro Communications Analyst”Data Communications: Basic

Concepts

1001 DNW

Data Communications:

Basic Concepts

Datapro Summary

Data communications is an integral part of business. Whether a data network accommodates 10 personal

computers on a LAN or 100 nodes in a global network, data comm unications is the link for greater productivity,

efficiency. and cost savings. This report offers an overview of data comm unications principles, the products and

services comprising a data network, and the issues facing this dynamic industry.

43~ Thomas No//e

Pwsid& C/MI Corp.

Lipdated by Datapro staff.

Basic Principles of Data CommunicationsData comm unications is the set of products. concepts, and services that enable the connection of compu ting

systems. In this context, a “ computing system” can be a source of information or information processing or a

medium allowing an information user to access such a source. The “connection” ma’t’ be one that explicitlv

make s one system the “ client” and the other the “server,” as is the case with terminai-to-computer relationships.

It may also be “peer” in nature, imposing no specific master/slave relationship. A collection of such connections,

supported over a common circuit structure an d using shared technological compon ents. is called a data network.

The elements of data comm unications are all slaves to the relationship between the business applications for

compu ting, the carrier service an d standards infrastructure (which includes regulatory issues), and the

technology available. In the past. these factors tended to make data comm unications and data networks slaves of

data processing planning. While it is not yet true that networks drive applications more than computers, it is

certainly true that networks must be considered in designing huma n-system interactions. In the past, it was

possible to develop compu ting applications without anv form of data network. That is uncomm on today and willbe impossible in the near future.

Technical Basis of Data Communications

Comp uter systems and their associated devices store and use information using a binary coding. The numbe r of

binary bits that make up a character of information has varied, but the majority of systems today use eight binary

bits to a character, or “byte.” This allows 256 different combinations of value. which can be mapp ed to represent

letters, nu mbers, and special symbols. Any combination of bits can represent an ything, as long as the systems

using the data agree on the value. But to simplify information storage and retrieval, a standard code se t is

normallv employed. There are two such code sets in comm on use todav: the Extended Binary Coded Decima l

lntercha~~ge Code (EBCDIC), developed by IBM and used on its large*systems, and the American Standard

Code for Information Interchange (ASCII). a formal stand ard used by most midrange and personal computers.

Code sets describe OI I I V how textual or “ character” data is stored; there are other standards for the storage of

binary num eric data, both fixed point and floating point. A standard representation is necessary whenever

information generated by! one system must be read by another. This can occur if the systems exchange a

transportable media like magn etic tape. It can also occur if the systems are linked over a comm unications

channel. One computer system or device co uld com municate data to another over a wire or circuit if two basic

sets of conditions were true:

l The information channel was capable of transporting binary information with no errors or amb iguities

sufficient to interfere with the application.

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“Datapro Communications Analyst”Data Communications: Basic IOOlDNW

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l Both agreed on the strategy to be used to transfer the information and the structure of the information itself.

Data comm unications can be viewed a s the set of strategies needed to ensure these conditions.

Analog and Digital Channels

An easy wav to transmit binary da ta is to simply impress a two-value signal (+5 volts and -5 volts, +8 volts and

0 volts, etc.) on the channel, with one of the values used to represent the binary I and the other the binary 0. This

is called digital enc oding, and it requires a circuit capable of transmitting the kind of “square” pulses shown in

Figure “Anulog and Digital Coding Techniques.”The most pervasive comm unications system in the world is the voice telephone system, so it is logical to

assum e tha t any practical data comm unications network would have to rely heavily on this pervasive system for

support. Unfortunately, hum an voice is not binary data, and the voice phon e system was designed to pass audio,

or analog, data. In fact. it was designed to pass the relatively narrow range of frequencies in which hum an voice

carries most of its intelligence--roughly 300 Hz to 4000 Hz.

To make analog circuits suitable for digitally encoded information, it was necessary to develop a system to

modify or modulate an analog signal or carrier in such a way that changes generated in response to a binary I

could be distinguished from those generated in response to a binary 0. This might be done, for example, by

transmitting a tone of I kHz for a “1” and 2kHz for a “0.” A device at the other end could then, by separating the

tones, recover the digital signal. This modulation system, called frequency shifi keying, is still in use in

telegraphy and is illustrated in Figure “Analog and Digital Coding Techniques.” ?r c

A digital chan nel can. therefore, be created by a pair of modulator/dem odulator devices linked by an analog

channel. The devices became known by the acronym modern. Today’s modems employ enhanced techniques of

information coding. increasing reliability and information capacity. However, all modem s operate by impressing

multiple values onto an analog carrier sign al to represent digital d ata.

As the public telephone system advanced, the advantag es of integrated circuitry in processing phone signals

became clear. Digital signals can be regenerated more easily in the presence of noise, because they can only

have one of two possible values. Thus, the phone system moved to a strategy of digital coding of voice data

through pulse code modulation, or PCM .

Digital samp ling of voice information at a rate of 8,000 times per second, with 256 possible values per sample ,

requires 8,000 x 8 bits or 64K bps capacity. This type of digital channel. called a “ DSO,” is the foundation of

today’s digital carrier system or “ T-carrier” system. Tl, w hich consists of 24 DSO channels plus a framing bit, is

a I S44M bps channel often used in integrated voice/data networks.

Digital channels can be used to carry data directly; all that is needed is to connect the data device to the

channel in some way. In North A merica, this is done with a two-step device called a channel service unit/data

service unit (CSU /DSU ).

Protocols: Asynchronous and Synchronous

CSU/DS Us, in conjunction with digital channels or modems and analog channels. can transport binary data.

satisfying the first of the two requirements for data comm unications. The second requ irement is an agreement on

the format and rules for the exchange, called the “protocol.” The most b asic element of a protocol is the

definition of how data stored in a computer will be transferred to the line, and in what bit order. Another issue is

just ho w the receiving device will divide up the bytes into bits. This is more complex than it sounds; information

moving at a rate of 9600 bits per second (bps) would gene rate a new bit about once every 100 microseconds.

There are two major strategies for synchronizing the bit/character timing of comm unications: asynchronous andsynchronous. Figure “S~wchrmous and Asynchr*onozts Transmission Blocking Techniyltes” shows the difference

between the two.

Asynchronous comm unications places the bits on the line by framing them in a “start” and “stop” bit, with a

predictable value. Th is allows the receiver to distinguish the start of a character from a condition of an idle

channel. Within the character, the sender and receiver m ust “ clock,” or time the bit intervals accurately. This is

not a problem for a single eight-bit character. Because of the start and stop bits, however, the asynchronous

strategy requires an average of IO bits to be transmitted to send the S-bit character.

Synchronous comm unications is designed to eliminate this waste by grouping all of th e char acters of a

messag e into a block and sending them together, The block is started with a “sync character,” or “flog,” and ends

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with an error-checking sequence designed to help the receiver detect block errors. Bu t because blocks are likely

to be long, the transmitter and receiver cannot track accurately and might lose count of where ch aracters begin

and end (“lose svnc”). To prevent this, synchronous channels provide both sender and receiver a standard clockdsignal.

Asynchronous comm unications is inefficient when used with devices that can coHect and configure

information into blocks, but it is still very comm on for simple connections where the characters sent on a line

are keyed by, or displayed to, a hum an operator. Synchronous data Iinks are best implem ented where direct

hum an intervention is not possible because of the speed. The ability of a hum an to view the result of aconnection mea ns that little must be done by the computer!terminaI to protect data; the “protocol” is that there is

no special procedure. Thus. the term “asynchronous protocol” mean s that characters are sent as they are keyed or

as they are to be displayed, with little or no control dialog.

Synchronous blocks. hav ing a potential strategy for block error detection and correction, can justify a more

complex set of rules for information exchange. “ Synchronous protocols” are therefore more complicated.

employing strategies for detecting and correcting errors, controlling the rate of flow, and setting other

characteristics of connections.

Protocols-The OSI Model

From the titne of the first practical data networks in the late 19 150s~ protocols” have been an area of concern,

Cotnm unications is not possible betw een two systems that disagree on the procedures--that have different

protocols. Com puter vendors all invented their own (IBM’s Binary Synchronous Com munica tions or Bisync was

an early, popular examp le). Because each was incompa tible, equipme nt from different vendors could not

communicate.

In the early 197O s, a group of international comm unications experts devised a model for the connection of

generalized data systems through communications networks. The tnodel was called the “ Open Systems

Interconnection Basic Reference Model” and became known as the OS1 model, the structure of which is shown

in Figure “ The CXI Model Networ~k.”

The model divides all of the functions of data comtnunica tions into seven layers. each of which provides a

cohesive set of services. International standard s for each of the layers were developed in succeeding years, and

even vendor-proprietary protocols took on the basic structure. The OS1 model is the basis for higher-level

protocols.

Communications Standards

The need for both parties in a connection to agree on data presentation and dialog controt rules, the protocol, has

already been noted. Stan dards to define these rules are as old as data commun ications and arise from three major

sources:

l Vendors thetnsetves whose proprietary protocols may be “ open” to support by other vendors because the

originating vendor publishes the specifications. IBM’s Systems Network Architecture (SNA ) is an example of a

proprietary, but “ open” protocol.

l Trade groups or consortiums, which represent sp ecial interests in a given comm unications market. The

Institute of Electrical and Electronic Engineers (IEEE) is a trade group that has protnoted the basic standard s for

local area networking, IEEE 802.

l National or international standard s bod ies, which formally debate rules and publish standard s. In the U.S., the

Am erican National Standa rds Institute (ANS I) and the National Institute for Standa rds and Technology (NIST,formerly the National Bureau of Standa rds) are the principal standard s bodies. The International Organization

for Standardiza tion (ISO) and the International TeIecotnm unications Union-Telecom munications

Standardiza tion Sector (KU-TSS. formerly known as the CCITT) are the two international standard s group s

most involved in data comtnun ications.

The OSI model is not itself a standard , but a framework wlhich describes the relationship of standard s. There are

standa rds for each of the seven OSI model layers, often several at the same layer.

Applications for Data Communications

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nications: Basic 100’lDNW

Data comm unications and data networks serve the information processing systems used, or contemplated, by a

business. All information technology planning is based on estabt ishing a user-to-source relationship and

identifying the technology elements needed to support it. Data networks are such an element, and data

cotnmun ications is the foundation of data networks.

Early data comm unications applications were developed when computer facilities were so expensive th at

access to a cotnputer had to be given to numerou s users via relatively primitive entry/display terminals. The

computer processed and fortnatted all information at its central location. But as technology advanced, it created

the microprocessor and enabled the development of inexpensive desktop compu ter system s. These allowedcotnputing power an d information storage to be dispersed. In this new environment, four distinct types of data

cotntnunications relationships developed: host/terminal, client/server, peer or distributed processing, and

internetworking. Figure “ C(~nlnlz(17icatior~s etwork Relationships and Topologies” shows an example of the way that

each type of relationship affects the structure of the network that must support it.

HoWTerminal Relationships

Norma llv, host/terminal applications occur when the process of information entry and display is the major

element bf the application, and the goal is to support the fastest rate of acquisition or output of data. In almost all

cases, the system interaction is to a huma n operator, either directly (via keyboard and display) or indirectly (via

printout).

In host/terminal applications, the speed of the human /mecha nical component of the connection is often low

enough to limit the information flow rate to a level well below that of channel capacity. Because of this,

host/terminal networks often include facilities to share the information channel among multiple terminals to

reduce overall cost. These de vices are called “terminal servers” or “ cluster controllers.” IBM’s popular 3270

family of devices includes the 3 174 cluster controller.

Wh en a desktop computer is used to “ emulate” a terminal, the interaction between the personal or other

desktop com puter and its partner system is still considered a host/terminal interaction. Any processing

capabilities of the desktop system are “hidden” by the fact that the PC is emula ting a dumb terminal.

Host/tertninaI applications are forgiving of channel limitations. Their relatively limited speed has already been

noted: high-capacity channels can be justified only by sharing them among multiple termin als. Host/terminal

applications are also generally imm une to delays in the data path, since the hum an reaction titne is normally tong

enough to hide any network transit delay.

Networks for support of host/tertninal relationships take on a “tree” structure, as shown in Figure

“ Comntrnicatiom Netwwk Relationships a nd Topologies. ” The tertninals are often concentrated via cluster controllers

or servers onto a shared trunk, which may be further c oncentrated to a higher-speed facility. All infortnation

paths lea d to the computer systetn at the heart of the structure, and there is no connection between users except

through that computer.

Client/Server Relationships

Client/server applications utilize a small computer at the point of huma n/system interaction and a larger one as a

central repository for infortnation and/or information processing power. The client system can provide local

services to its user, but it may from time to time require access to information stored a t the server or to the

server’s specialized processing resources. Wh en this happen s, the client reties on a data comm unications

connection to the server. The file sharing and printer sharing done in PC-bas ed local area networks (LANs) is a

common example of a pritnitive fortn of client/server computing.Because client/server applications are between computing systems and not between a system an d a huma n, the

speed of the exchange of infortnation is not limited to huma n rates. Thus, the applications utilize a much higher

channel capacity for the brief period of the interaction, though the capacity might be wasted during periods wh en

the client svstem wa s involved in a local user dialog only, Client/server applications, therefore, benefit from

strategies for sharing infortnation channels as well.

w

The extent to which the client and server systetns interact in satisfying a user need varies con siderable. Some

systems, such as electronic tnail systems, simply deliver a messa ge to a client to be read by its user at ati

convenient titne. In this case, the client/server interaction is relatively infrequent and not highly constrained by

performance. But if the client system is processing a remote da tabase, each record may be sent over the network.

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In the latter case, network performance will have a major imp act on the application. Client/server relationships

often place information sources farther out toward the network user? often at the points of concentration. To

provide a ccess to these resources without loa ding the host, intermediate cross-connections are often provided,

creating a meshin g of these concentration points.

Peer Relationships

A client/server application requires two smart system devices, for example, a desktop computer and a data

center system. However, despite the fact that both devices are computers, there is a master/slave relationshipinherent in the fact that the server is a source of information--often for many clients.

Peer applications have no such inherent master. Peer systems are those that have relatively little difference in

information storage or processing capacitv and are capable of adopting virtually any sort of relationship with

one another according to the mom entary ieeds of the application or user.

Peer connectivity is the most challenging of all types of connectivity to provide, since there is no preferred

information focus among the systems comm unicating. Without such a focus, any numbe r of connections and

flow volumes could b e possible, and the capacity and number of channels needed to support them make design

of a total network difficult. The unpredictability also make s it difficult to concentrate traffic for efficient use of

circuits; there are no consistent partners to create consistent patterns of flow. A peer network, therefore, tends to

connect users at all levels.

There are few/ true peer applications today, becau se most compa nies have central data center resources or other

departmen tal information storage points. Peer networking is most likely to be found in compa nies that rely on

personal com puters or desktop UNIX systems.

Internetworking Relationships

All of the relationships described so far have been between information systems an d have been explained in an

application context. The last relationship, internetworking, is not a system relationship at all. but a network

relationship affecting all users on the network.

Internetworking is most likely to occur when a business that has previouslv planned d ata comm unications on a

per-application basis begins to consider it as a kev part of its strategic planning. A large part of creating a

strategic network is mak ing information access M iithin the firm more universal, something that is often called

“building an enterprise network.” reflecting the breakdow n of internal network barriers.

In a technical sense, inter-networking is the task of building a single, large network by combining existing ones

while retaining the application support charac teristics of each of the networks. Figure “ Communications Net-work

Relationships and Topologies” shows an internetwork structure created by linking a series of LAN s. It is an area

supported by specialized products discussed in the Switching Devices section later in this report.

Data Transmission Services

Given an application, the goal of data communica tions is to identify a set of transmission services that can be

made to effectively support it and the equipmen t necessary to provide whatever adaptation is required.

There are four major options for data transmission available to users:

I. Public carrier services provide raw analog or digital transport capacity, often suitable for other forms of

information transfer as well. D ial-up and leased analog an d digital lines are examples of this.

2. Public value-added networks, also called “packet switched n etworks,” are designed to transport data only, andto do so at an attractive price relative to the more general analog and digital lines.

3. Private tran smission systems on a single prem ises, based on local copper w ire or other technology, are

supported by a central switching device, such as a PBX , They may be called “local data switched services.” If

thev are provided bv a shared high-capacity channel, thev are called LAN s.

4. Private tran smiss4ion systems can be based on radio or’optical technology, which can operate over distances of

50 miles or more, Private microwave is the most common example of this type of system.

A “private network” is a collection of transmission services design ed to provide user-to-user connectivity at a

lower cost than could be achieved through the use of public switched services. Most “private networks” stilt relv

on leased carrier services.

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Local Area Networks (LANs)

In the late 1970~~ a Ph.D. candidate named Robert Metcalfe suggested that a coaxial cable might be routed

around a facility and tapped into by each user connection. The information capacity of such a cable. IO tnillion

bits per second, would be high enough to reduce loading and contention frotn this shared use to tolerable levels.

This was the first practical definition of a local area network and its subseque nt comm ercial exploitation by

Xerox, Intel. and Digital Equipme nt--Ethernet.

Ethernet was originally designed to support terminal acce ss, but the LAN quickly becam e associated withpersonal compu ters. A cotnmittee sponsored by the IEEE was charged with developing standa rds for LAN s, and

most LA Ns today ad here to the IEEE 802 standard set. LAN s can be classified according to their topology and

their access control. Topology refers to the physical configuration: access control to the way in which users are

granted access to the LAN to send data. The most popular topologies are the bus, the star, and the ring. The most

popular access control strategies are carrier sense multiple access with collision detection (CSM A/CD ) and

token passing. Ethernet is a CSM A/CD bus LAN; IBM’s Token-Ring is a token-passing ring.

Today, LAN s offer inexpensive and easy-to-implement comm unications between PCs, engineering

workstations. midrang e, and mainfram e compu ters, and allow the delivery of many shared applications and

services that were not possible before the advent of the LAN . Early LAN s allowed the sharing of printers and

disk drives; today’s systetns offer electronic mail, fax and imagin g services, access to pools of modem s and

comm unications gateway s, and the increasingly popular groupware,

Wide Area Bandwidth Services

Most data comtnunications today is based on carrier circuits designe d for use by data. voice, fax, and other

information forms because they supply only “band width” or unstructured information capacity. These circuits

can be classified according to their capacity, as follows:

l Voice grade circuits, analog lines with a data capacity of up to approximately 19.2K bps. Telephone dial lines

and leased analog lines are examp les of this type of service.

l Nar=l~oll~hanclcil~czrits,nalog or digital lines with a capacity range of 56K or 64K b ps to about I .5M bps (North

Ame rica’s TI ) or 2M bps (Europe’s El ). Datapho ne Digital Service (DDS ) is a narrowba nd service.

l Widehand cimri~s, digital lines with capacities from T I /El to 34M b ps (Europe’s E3) or 45M bps (North

America’s T3).

l Broadband c’ircwits, digital lines with capacity in excess of 45M bps .

Services can also be characterized as being leased or switched. Leased services are provided to the user

continually without the need to dial and link two fixed points. Pricing is based on circuit capacity and distance.

Switched services provide connections on request, between points selected from a list of available subscribers.

Pricing is based on the type of service, the length of the call, and the distance between the points.

A final service classification is terrestrial or satellite. A service bu ilt on ground-base d facilities is terrestrial.

Satellite services em ploy a geostationary satellite, orbiting approximately 22,500 miles above the equator as a

relay between sender and receiver. The cost of satellite service may be lower in some application s, since it

requires no intermediary relay points to serve remote areas and is adapta ble to applications where one message

is broadcast to many users. Satellite service introduce s a transit delay, owin g to the great distances involved in

the relay path. and may impact performance for some applications.

Today. voice grade, n arrowban d, and wideba nd services are based on the carriers’ own internal structure of

digital trun ks, called the Tl/El-carrier s)stem. This structure is based on the 64K bps DSO channel discussedearlier. Twenty-four or thirty-two DS Os are combined, with a framing bit, to form the I S44M bps Tl or 2.048M

bps E I trunk, respectively. W hen 28 of these trunks are combined , the result is a T3 trunk, offering a bandw idth

of 45M bps. The European equivalent, E3, carries 34M bps of bandwidth.

A fiber optic carrier transport architecture, called the Synchronous Optical Network (Sonet), increases capacity

beyond T3/E3. Sonet’s basic b uilding block is a 50M bps channel called an OC 1. Sonet d efines a hierarchy of

channel comb inations up to OC4 8, which would ha ve a capacity of 2.4 billion bits per second. Sonet deployment

is moving ahead quickly in carrier networks and services.

There are three m odern service concepts that may be of special interest to users:

l Fractional TI/EI is a carrier service allowing users to lease capacity less than I SM /2M bps--usually 256K bps

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or higher. They p rovide the benefits of wideba nd digital service at a lower cost than full Tl/El.

l Switched digital carrier services provide the bandw idth of a fractional or full TlfEI W AN link through

multiple, independent 56K/64K b ps dial-up connections. Multiplexing equipme nt owned by the user establishes

these dial-up links through a switched digital n etwork a nd evenly distributes the data, voice, or video

transmission across those links. Identical equipment at the destination, receiving those multiple transm issions,

resynchronizes and recombines them to restore the original messag e.

l The Integrated Services Digital Network (ISDN) is an ambitious plan to create a fully integrated switched

digital netw ork. Basic R ate ISDN , the most economical form, offers two 64K bps user channels and one 16Kbps “signaling ” channel. Primary Rate ISDN. a higher-capacity service, provides 23 (in the US.) or 3 I (in

Europe) user channels and I signaling channel, all operating at 64K bps.

A more detailed description of carrier services assoc iated with data transmissio n, including ISDN and fractional

Tl, can be found in other D atapro reports.

Value-Added Services

All of the services described so far provide the user with unformatted transmission capacitv and can be used

with any type of information source/user, including fax, voice. or video. w hose de mand is Within the capacity of

the service. This generality of capabilitv limits the capability of the service to offer specialized benefits to the

data user. A network designed onlv for data can optimize the transmission of data and reduce overall network’

costs.

A Ran d Corp. study of the 1960 s showed that data can be broken into small “ packets” of information and

moved through a network of shared trunks and “nodes” to its destination. The sharing of circuits and equipme nt

can result in a lower per-character cost than could be achieved through the use of “ban dwidth” services like

fractional Tl /E I. Th is study became the basis of v alue-added, packet switched data ne tworks.

Public packet network services differ from the dial-up or leased bandw idth services described earlier in several

important ways:

l There is a specific access protocol requ ired to attach to the network and transfer information. Traditional

carrier services are protocol independent.

l Network services are priced based on usage, meaning the numbe r of characters transmitted from source to

destination. The distance between users and the duration of the connection are not normally bit1 ing factors.

l The introduction of multiple shared trunks and nodes generates an appreciable delay in information transport,

often greater than that generated by a satellite data path. This can impact the performance of some applica tions.

Public pack et data services are most useful when an application involves the support of a widely dispersed

population of “ occasional” users.

Packet technology can also be employed in private networks. Because packet netwo rk interfaces are based on

international standard s, such networks are excellent for interconnecting compu ters from different vendors.

Packet standard s also form the foundation of the OS1 protocols, discussed briefly earlier in this report.

A recent industry developm ent is fast-packet switching--a streamlined approach to packet processing

providing greater efficiency and lower transit delay for interactive data, voice/video, and multim edia

comm unications. Fast-packe t switching assum es that the wide area connection. utilizing fiber optics rather than

copper, is virtual ly error-free. It elimi nates error ch ecking, th erefore, at all but the destination node of a

transmissio n. On e type of fast -packet service, frame relay, is ideal for interactive LAN /WA N applications that

cannot tolerate delay. It propagates data in variable-length frames across star and mesh networks of any size.

Fractional and full Tl /E I frame relay products and services are widely available.Asynchronous transfer mode (ATM ) products and services, operating at T3/E3 speeds and higher, process

videoconferencing, and multime dia network transm issions in small fixed-length (53.byte) cells. thereby

minim izing delay and congestion. ATM switches and carrier service offerings have been emerging rapidly since

1994.

Data Networking Equipment

The minim um amoun t of data comm unications equipme nt needed to support’s con nection is the comm unicating

device, a transmission facility. and the interface eq uipment needed to connect to it (a modem or CSU /DSU , for

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example).

Most data comm unications environments are much more complex. The need to utilize concentration to spread

the cost of carrier services amon g m ultiple users has already been noted. W ideband and broadband services not

only have capacity in excess of what most single application s can justify. they often have service interfaces that

most computers and terminals cannot directly support.

For these reasons, data comm unications equipme nt is most often really data networking equipme nt. Its

purpose is to establish a shared set of facilities that users and information sources can use to make connections

and to mainta in those facilities in correct operation.

Interface Devices

Most transmissio n services, and all public carrier services, require some form of interface through which

compu ters and terminals can attach. The interface provides w hatever transformation is required between the

digital interface on the computer or terminal and the carrier service itself. It also generally provides some status

indicators that enable the computer or terminal to determine if the service (and the interface device) is operating

properly. These are called “ control signals.” in that they provide for the local control of the interface.

Mod ems are the most common service interface, designed to link digital com puters and terminals to each other

via analog carrier services. Mode ms can be classified as either synchronous (supporting the generation of the

synchronous clock signal) or asynchronous, and by the data transfer rates they support. Mode ms normally

transmit data at 2400, 9600, 14.4K, l9.2K, or 28K bps.

CSU s/DSU s are devices that interface commu nicating equipm ent to digital carrier services. Lower-speed

CSU/D SUs, designed to support narrowband digital services, are much like modems in appearance. The

higher-speed CSU /DSU devices, designed for wideba nd Tl, for example, are normally built into networking

devices that use the wideba nd interface.

Both modem s and CSU /DSU devices can provide special services, going beyond simple interfacing. Data

compression uses one of several algorithms to contract the data stream generated by a data device, thereby

decreasing the numb er of characters being transmitted and increasing the effective throughput. Com pression

rates of 2: I or 3: I are typical, and some information can be compressed even more. N etwork mana geme nt

support o n interface devices allows a user to monitor the quality of the circuit connec ting the devices and to run

basic tests as well. Encryption prevents interception of information by encoding it. Backup features allow a

leased-line modem or CSU /DSU to dial a backup connection should the leased service fail.

Concentration Devices

Hum an-operated terminals are rarely utilized at a rate that taxes even a low-speed transmission facility, yet the

minim um dial or leased carrier circuit has a theoretical capacity of 2400 bps or more. Even for voice grade and

narrowban d services, some form of concentration of multiple terminals onto a single circuit will improve

economy. Terminal servers an d cluster controllers are devices that are provided by the computer vendor to

accom plish this concentration, but there are other devices as well.

Multiplexers are the most comm on form of third-party concentration device. There are several types of

multiplexers:

l Frequency-division multiplexers--separate multiple conversations by allocating each a different carrier

frequency to be modu lated by the digital d ata. This type of multiplexing is old and extremely rare outside ca rrier

applications.

l

Time-division multiplexer+-separate multiple c onversations by allocating each a reserved am ount of space in adigital “ frame” of data, which is transmitted at a regular interval. A TI or El frame consists of 24 or 32 eight-bit

bytes p lus a framing bit, so Tl/E I is a form of time-division multiplexing.

l Statistical time-division multiplexers--identify information generated by each conversation through a “heade

code that prefixes the data. Capacity is allocated to a conversation when that conversation needs it.

0 Networking multipiexers--high-end statistical m ultiplexers performing any combination of concentration,

adaptation, and intelligent routing. Usually supporting several TI /E I aggregates, they enable multiple term ina

devices and host computers to exchange information over star or mesh networks.

l Access tnultiplexers--provide access to an enterprise backbone for a remote office environment. Access

r”

multiplexers usually concentrate data from multiple devices onto single or dual trunk connections leading to the

0 1995 McGraw-H i l l , Incorporated Reproduct lo n Proh i b i ted.

Datapro in format ion Serv ices Group, De l ran NJ 0 8 0 7 5 US A

May 1995



“Datapro Commun ications Analyst” Data Corn

Concepts

municatio ns: Basic 7001DNW

networking multiplexer. As concentrators, they perform little or no intelligent routing.

l Voice/data multiplexers--can combine both voice and data switching.

l Digital access devices such as inverse multiplexers--provide dial-up access to multiple, independent 56K or

64K bps channels, usually over a single ne twork access trunk. This capability provides variable bandw idth for a

single application or dynamically shared ban dwidth for two or more concurrknt applications.

Statistical multiplexing differs from the other strategies in that it dynamically allocates capacity. The benefit this

provides is better utilization of the shared comm unications circuit; fixed allocation of resources wastes capacity

because it is assigned to a conversation, even when it is idle.Multiplexers that operate between two points and provide the appearance of a series of dedicated circuits are

called “point-to-point” multiplexers. Multiplexers that can be used to create more complex interconnections of

circuits and switch data between them are called “ networking multiplexers” or “hubs,”

Switching Devices

The comm unications processor, usually designed for one or more vendor-specific families of compu ting and

comm unications equipme nt, performs iletwork control. intelligent routing, and concentration functions. As a

front end to a host computer, it serves as a master processor. relieving the host of the overhead involved in

messag e ha ndling and network control. As an intelligent switch, it routes messa ges across the network. either

under the control of a higher-level comm unications processor or as a peer of other intelligent switches. Remote

concentrators control a commu nity of terminals or distributed ap plication processors. gathering, queuing, and

multiplexing their transmissio ns onto one or more high-speed network trunks. These concentrators also often

provide protocol conversion and gateway functions to attached devices.

Point-to-point multiplexing of a carrier circuit is useful in sharing costs where there is a co-located commu nity

of users who need access to a common resource elsewhere. Unfortunately, this simple situation is not pervasive

in business; complex patterns of access are now the rule. Furthermore, higher-capacity circuits often need a

number of users to be justified.

Figure “Transit Routing and Switding Techniqzres” shows that it is possible to concentrate traffic onto a trunk line

by bringing some traffic to one trunk termination from locations even more remote (locations “C” and “D” on

the figure). This would also provide users in that remote location with access to ant’ information resource

located at the near trunk term ination (“A” on the figure). B ut to support this config&ation, there mu st be a

network device at location “A” that can distinguish between traffic from “C” or “D” which is destined for “A”

and then destined for location “B.” This requires a routing or switching function.

Switching devices are the core of modern

traffic for ecoi jomy aiId ful I connect ivity to

without them. Private d ata networks can be

data networks, because the collateral need

support all types of information connectio

defined as networks in which user devices

s of concentration of

ns cannot be readily met

provide this switching,

and public n etworks are those where the switching is provided by the carrier as a part of the service.

There are four major classes of switching devices:

I. Connection switches such as PBX es provide a large population of users (telephone users, terminals, etc.) with

access to a more limited numb er of shared resources (tie lines, computer ports, etc.), by allowing users to select

a destination through a dial-like mech anism.

2. Concentrator hubs, such as comm unications controllers or packet nodes, route information amon g multiple

trunk lines and user connections based on information provided within the protocol u sed. These devices sup port

multiple protocols makin g them “protocol independent” and are normally either provided by the computervendors or conform to a vendor-proprietary or formal standa rd specification. They are therefore limited to use

within networks whose protocols conform to those specifications.

3. Multiplexer hubs, such as TI/EI TDM nodes, route information in a way that is information-format

independent. These devices can be used in any tvpe of network, as IOII~ as the capacity requirements for the

application can be met with the carrier services and equipme nt.

4. Internetwork switches connect multiple data networks. These are most often used to link LAN s and are further

classified according to the OSI protocol level at which they operate (see Figure “ The OS/ MO &/ Network”).

Bridges are internetworking devices operating at Level 2, the Data Link Layer. Routers operate at 0% Level 3,

the Network Layer, and gateways operate at any level above Level 3 .

0 1995 McGraw -Hil l, Incorpo rated R eproduct io In Proh i b i ted.

Datapro In format ion Serv ices Grou PI Delran NJ 08075 USA

May 1995



“Datapro Communications Analyst" Data Comm

Concepts

unications I: Basic 1001 DNW 10

Switching devices are often called “nodes” because they form the junctions between routes or trunks in a data

network. Because the control of node operation and routing of information will control overall information flow,

the “ managem ent” of a network is tvpically based on the mana geme nt of the nodes within it,r’

Network Monitoring and Management

Wh en an application uses a carrier circuit to connect users to information sources, the failure of that circuit will

cause the user to lose service. Because this is a localized problem. the business may elect to tolerate it for theperiod needed for the carrier to restore operation. But when a shared circuit or a node in a data netw ork fails,

many users may lose their ability to interact with information resources, and company operations may suffer.

The more complex the network. and the more shared resources are used, the greater the need for explicit

attention to service assurance mana geme nt.

There are two goals to service assura nce: restoration of acceptable service in the shortest possible time and

recomm issioning of the failed facility itself. The former can be accom plished through the use of alternate routes

for information. The latter will require identification of the specific c omponent of the network that is faulty and

the support of the provider of that compone nt.

A broader set of goals, comprising network mana geme nt as a whole, build s from the service assurance goals

listed above to include requirem ents for capacity planning for future application needs, accoun ting for network

resource usage for billing, and control of access to network and network-projected resources to prevent

unauthorized intrusions or information da mag e.

There are three basic types of tools that are applied to the meeting of network mana geme nt goals: network

monitoring systems, test systems, and network mana geme nt systems. The operation of these devices in a

network is shown in Figure “ Net-war-k Monitor-ing and Management.”

Network Monitoring Equipmen t

The goal of network monitoring is the exam ination of the protocol exchanges at points within the network, so

that conditions there can be compared to normal operating conditions and reasons for differences determined.

Depending OJ I where this monitoring process occurs, the device u sed is called either an interface monitor or a

data line monitor. Both types of devices are inserted into a connection and report on what passes through them.

Interface monitors are used to test the boundary between services and devices, verifying the state of the

interface. Interface monitors display the status of the “ control signals” mentioned earlier in this report, which

advise each element of the interface on the status of the other. Interface monitors normally have L ED or LCD

indicators to display important status conditions and may also provide a mea ns of manip ulating the control

signals to force specific conditions. Because these devices “ break out” the control lines at the interface, they are

often called “breakout boxes.” Most are handhe ld an d inexpensive devices.

Data line monitors also show the condition of the interface where they are inserted, and the data flow through

the interface. Because this data flow is normally defined by a protocol, most data line monitors include the

capability to interpret the protocol and to indicate unusua l situations. At the very least, they will normally

provide a formatted display of the messa ges being exchanged on a small CRT display or LCD display. Data line

monitors are normally microprocessor based and are considerably more expensive than interface monitors.

The use of either a data line monitor or an interface monitor is affected by two facts. First, the monitor mus t be

placed into the circuit a t the point being tested. T his requires either moving the monitor (and an operator) to that

point or providing a “test point” at the location with remote access provisions. Second. the monitor must be usedby someone who is skilled at its operation and can interpret the results.

Network monitors are gaining greater intelligence by using expert technology to monitor, analyze, and

automatically diagnose problems in a comm unications network. These intelligent protocol analyzers can decode

several hund red protocols an d recomm end problem-solving action. These instrumen ts can also perform trend

analysis and display and indicate instantaneous traffic in graphical or tabular formats.

Network Testing Devices

Monitoring is a passive function; it relies on the interception of messa ges across an interface where test access is

available. Sometim es a comm unications problem manifests itself by the failure of either partner to attempt

0 1995 McGraw-H/I I , Incorporated. Reproduct io n Proh ib t ted

Datapro In format Ion Serv ices Group, De l ran NJ 08075 USA

May 1995




Concepts

IOOIDNW

comm unication at all, resulting in nothing to monitor. Whe n this happen s, or where a component of the network

is to be examined outside a connection dialog, an active testing device is needed. -

The simplest testing devices generate patterns of data. either to display o n a terminal or to “loop back” to the

source for comparison. These are often called “b it error rate” or “bit tine error rate” testers. Most are similar to

interface monitors in size and are restricted to use with very simple protocols, since they tack the intelligence to

obey complex rules for information exchange.

Data line monitors, with microprocessor intelligence, provide active testing capabilities of even complex

protocols. Called “protocol emulation,” this testing capa bility allows a user to certify the basic operation of a

device and confirm the essential characteristics of a carrier service.

Testing devices can sometim es be used by inexperienced personnel, providing that the device has a simple

“go/no go” indicator. In most cases. however, testing devices have the same constraints on usage a s apply to

network monitors.

Network Management Systems

Whe n a network device has microprocessor intelligence, it is said to be “sm art.” Such devices can often monitor

their own operation and display it on a panel or through a terminal interface. This allows the device to be

mana ged without the aid of minitoring devices. W here terminal access is provided, mana geme nt can be

exercised over a data connection from a remote location.

As the numbe r of devices in a network increases, it becomes inconvenient to maintain terminals that control

each device individually. Switching a terminal from one device to another. while theoretically practical. would

risk losing information that might be presented when the terminal is connected elsewhere. To help operators

control complex networks of many devices, num erous vendors offer network manag emen t systems. These not

only provide for the monitoring and control of many “sm art” devices, but also for the collection of data that is

useful in network planning or billing. In addition to monitoring and alarm tasks, a true network mana gemen t

system supports higher-level services. A network mana geme nt system records and processes information from

its monitors, as well as information on the network’s configuration supplied by administrators and operators.

Most element mana geme nt products are designed to control a specific device from a single vendor. Others

may control several types of devices from a given vendor. Mana geme nt systems that cross device or vendor

boundaries are called integrated m anagem ent systems.

Unti I recently. the network manag emen t system market was dominated by proprietary-based manag emen t

systems sold by LAN and internetworking device vendors. Generally, each system could only manage certainelement types. It was not uncomm on to have separate mana geme nt systems for each device class, even if all the

equipment was from the same vendor. For example. a system m ight manage modems, but not Tl CS U/DSUs.

Standa rds-based, open systems m itigate that trend. Although ha rdware vendors are still the primary suppliers

of mana geme nt systems, the systems are much more flexible since they can often control devices based on the

same standards. The Simple N etwork Management Protocol (SNM P) has been a primary focus in the rise of

standards-based management systems, but more platforms are featuring Comm on Management Information

Protocol (C MIP) a s well. Integrated mana geme nt systems help users mana ge networks by establishing a single

location to control network operations, regardless of the type and source of devices on the network.

As local area networks grow larger and are connected to one another to other resources, the need to control the

interconnected systems increases. One successful approac h is the three-tiered mana geme nt svstem. Network

nodes and devices constitute the first tier. Individual standalon e m anagem ent systems--form&g the second

tier--can still be used to control v arious resources. More frequently, however, these element manag emen tsystems--a “ manager of managers”-- occupy the third tier and provide a comm on mana geme nt interface. Many

element mana gers can link directly! with HP OpenV iew, AT&T Accum aster Integrator, IBM NetView , or Digital

DECmcc Management Stations.

Future issues in Data Communications

Network services evolve in response to the changes in dema nd and the change s in technology. Both are changing

rapidly. not only because of internal technical adv ances, such as high-speed computer chips, but because of

globalization of the marketplace and its pressure on business to expand the geographic scale of operations.

0 1995 McGraw-HI11 , Incorporated. Reproduction Prohibited.

Data pro I nformatlon Services Group, Delran NJ 08075 USA

May 1995



“Datapro Communications Analyst”Data Communications: Basic 1001 DNW

Concepts

12

Data comm unications and data networks are more than products and services. P ersonnel must plan. select,

install, and tnaintain equipm ent and coordinate services. Because data networks project computer information,

they are closely tied to computer planning and operations. Because they may utilize carrier facilities like Tl in

conjunction with voice services, data networks are also often linked to telecomm unications planning and

operations.

Many businesses have computer and comm unications organizations that have been inherited from a period

when the role of data communications was very different than it is today. In some cases, this results in a

subordination of data comm unications issues to data processing interests. In others, a “runaway” network planhas little relationship to the computers and terminals that must be served. But in most cases. defects in

organizational coordination hampe r the diagnosis of problems and the support of users.

As data networks become more critical to business, they must be placed in a reasonable planning and support

context. C omputer technology and wide area transport services are becoming less expensive daily. If the

enterprise of the future is to be dependent on networks, cost-benefit constraints of networks must be addressed in

planning. and the needs of the networked enterprise must be met in technical support.

Planning Considerations

Com panies traditionally plan network services based OII a set of dema nds presented by the information

technology and compu ting planning tasks and the constraints set by the carrier service structure in place in the

areas to be served. T his presume s that the network is relatively flexible to meet any information technology (IT)

goals and that computer technologv is relatively inflexible. Cost trends, cited above, clearly show this to be

false, even in the present.

In modern IT planning, network constraints on the compu ting systems relationships are considered as early in

the process as computer technology and sofiware constraints. The goal of most businesse s is the enterprise

network, a collection of comm unications services that will meet the information transfer dem ands for all types

of information, now and in the future.

Because comp uting relationships are changing rapidly in the face of plumm eting desktop technologv costs,

networks are increasingly supporting client/server and peer comm unications models. As shown earlie;, this

creates a need for increased connectivity along the “ fringes” of the network. The enterprise network of the future

is therefore likely to have fewer preferred information paths and be much m ore dependent on nodes and shared

trunks than was the case only a decade ago.

Increasingly, users require higher-capacity network services that are priced according to actual usage

(connection time and/or bandwidth allocated) rather than on the peak capacity available. New technologies, such

as switched digital (n x 64K bps) transmission , frame-relay data transport, Switched Multi-m egabit Data Service

(SMD S), and ATM , are enabling such services to evolve.

Broadban d networks of the future will probably have a stronger carrier compon ent, much like virtual networks

of today, b ecause of the need to concentrate the traffic of many users to create econom ical service connections.

The carrier component of broadband services is evolving from Sonet and BISDN principles. Future broadband

networks will also require local distribution of high bandw idth traffic. The local distribution strategies are

evolving from hub and FDD I principles. Somew here, the two trends must mesh.

As users plan for the future, the role of carrier “ virtual” networks for data comm unications will also become

more imp ortant. A virtual netwo rk offers users the services of a private network without the equipmen t

investment. This insulates the user against the problems of change, but it places the burden on the carrier. Users

who are totally dependent on virtual services may find that those services cannot be adapted rapidly to takeadvantage of new opportunities. A balance of risk and opportunity will be required to support the best

competitive b usiness comm unications and information processing structure.

Supporting the Network

As networks evolve and equipme nt change s, the user is often faced with rapidly changing network problems and

conditions. even when applications being supported have not changed . User frustration is increased when a

“service” facility suddenly introduces a new set of problems and support dem ands, even though no perceptible

application benefit has been gained. “ We have been running this program for four years” is a refrain often heard.

While it is often true, the fact is that the network underpinning s may hav e changed many times in that period.

0 1995 McGraw-Hr l l Incorporated Reproduct ron Proh ib t ted

Datapro In format ion Servtces Group Del ran NJ 08075 USA

May 1995





Figure 1.

Analog a nd Digital Coding Tecttniques

User Sy8t2m“A

/I

Presentation

1ession

Network

Analog Carrier Modulated

by a Digital Signal

A Digital Signal

Figure 2.

Synchnmous andAsynchronous Tmnsmission Blocking

Techniques

Time

Start and Stop Bits

Synchronous Transmission

Data Characters with No Separation of 8its

Block Start (Sync or Flag) Block End (CRC)

User System “B”

Application

RWentatiOIlI

session

Network

Network

hpofi

44Network

DataLink

Ph@d

Data Link

Physical

Data Link

Physical

Figute 3.

The O S1 Model New ark



Figure 4.

Communications N etwork

Reiuti onships and Topologies

Figure 5.

Transit Routing and Swit ching

Techniques

Host / Term inal

Relationship

Cl ient /Server Relat ionship

Intern&work Relat ionship

Peer Relutlonship

Location ‘C’l\

8 Y

D \ \\ \

\ L/

\ \

Traffic Exiting a t “ A’

A8

Location ‘B ’

A “ Switching or Routing’ Node



lb

n t

Figrvc 6,Network Monitoring amiManagement

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