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Managing Multivendor Networks
  
- 7 -
LANs and WANs
The Many Faces of Networking
efore the great LAN explosion, networking, for the most part, addressed the connection
of distributed devices to a central location. Although some pioneering companies,
such as Digital Equipment, offered LAN technology in these early days, the bulk of
the market was accustomed to a centralized computing environment.
In this centralized approach, the primary concern was to find the most practical
and economical way to connect terminals, printers, and other data collection/reception
devices to the primary location. When connectivity was required between systems,
the link was approached typically as a special-case, point-to-point operation, rather
than part of a peer-oriented, distributed processing network. However, as requests
mounted to link computer systems over wide areas, multiple, point-to-point operations
became very cost ineffective, and the door opened to such alternative wide-area connections
as X.25 and ISDN. Wide-area technologies have continued to evolve, and now include
Frame Relay, Asynchronous Transfer Mode (ATM), and Switched Multimegabit Data Service
(SMDS).
All things considered, this system-to-system connectivity hardly concerned the
end user--after all, this was the job of the communications analyst. But when the
LAN wave finally reached the PC on the end user's desk, that user suddenly encountered
and became concerned about connectivity issues. At first it was just local (LAN)
connectivity and terminal emulation. Then, as networks grew and costs increased,
products such as gateways, bridges and routers snaked their way into the LAN. Today,
the end user has an unprecedented amount of power at his or her disposal. Consolidated,
enterprise-wide data is no longer in the hands of a few technical elite; off-the-shelf
desktop software now gives the end user the ability to access data anywhere in the
enterprise--whether it is on the PC, server, minicomputer, or mainframe.
This progression of connectivity changed the role of the LAN. Whereas the LAN
began as a local computing environment (usually an island unto itself) it grew into
an area of computing, normally linked to other computing areas. The fact that one
computing area might be a LAN, another a mainframe, and yet another a combined midrange
computer and PC LAN has become almost irrelevant.
From this high-level perspective, the world of distributed networks can be broken
down into two large categories: local-area links and wide-area links. A local
area network (LAN) typically is limited to one geographic area and allows individual
workstations to access data or applications on a server. In smaller LANs, a peer-to-peer
arrangement can be deployed to allow each station to function as both server and
client. A wide area network (WAN), on the other hand, typically covers a large
geographic area, and often links together multiple LANs. Within each category, however,
are a wide variety of implementations and strategies.
LANs often play a pivotal role in modern networks. This chapter will address the
following LAN issues:
- Topology. LANs can be implemented in a variety of topologies (or structures),
such as star, bus, ring, hub, and so forth.
- Protocols. LANs can run token-passing or collision sensing protocols.
- LAN implementations. This section will examine IEEE standards and how
they compare to each other (and to Ethernet).
WANs can be used in both centralized and distributed processing environments to
tie all of the necessary devices together. This chapter will discuss the following
WAN issues:
- Point-to-point links. In the most basic of cases, creating a WAN might
simply involve tying together two LANs or two systems. These connections are most
often implemented using standard telephone links.
- Integrated Services Data Network (ISDN). This service is offered by the
telephone industry as a modern, high-speed, multi-point connectivity solution.
- X.25. This chapter will examine X.25 as a wide-area, packet-switching
network. The use of such networks has become a low-cost solution for low-volume networking
on a worldwide basis.
- Asynchronous Transfer Mode (ATM). ATM is a high-speed protocol that offers
every client on the network the capability to send data at speeds of up to 155 Mbps,
or nearly 15 times the speed of a standard Ethernet LAN. ATM is especially useful
for those applications with high bandwidth requirements, such as videoconferencing.
- Frame Relay. Frame Relay can carry multiple types of traffic, including
voice and Systems Network Architecture (SNA). It is extremely fast, and less costly
than a dedicated line solution.
- Switched Multimegabit Data Service (SMDS). SMDS is a connectionless service
and is simpler to implement than Frame Relay or ATM. SMDS is used to establish any-to-any
connectivity and is a highly scalable solution. Technologies such as frame relay
require permanent virtual circuits (PVCs) to be established between each and every
location; SMDS, on the other hand, takes a much simpler approach. Each workstation
on the network is given an address, and any site can communicate with any other site.
The administrator does not have to set up individual connections ahead of time.
- Fiber Distributed Data Interface (FDDI). This token-passing technology
uses optical fiber cabling, and can transmit data at 100 Mbps. Because of its superior
speed, FDDI is especially useful for sending large files such as graphics or digital
video. It is a useful method of adding bandwidth without having to make a costly,
long-term commitment.
- FDDI switching can also have a big impact on a backbone network. Digital Equipment
has led the way in FDDI switching with its GigaSwitch product; other vendors are
also now starting to offer FDDI switching products. FDDI switching, like other types
of LAN switching architectures, can significantly increase bandwidth and extend the
lifetime of the network.
- Fibre Channel. Fibre Channel is a high-speed architecture for connecting
network devices and high-speed hardware. This ANSI standard supports speeds of up
to 1.06 Gbps.
- Tools of the trade. Implementing wide-area solutions requires some special-purpose
devices or software that smooth out the differences between the local and wide-area
connections. These tools include bridges, routers, and gateways. They enable the
different LAN and WAN strategies to be mixed and matched in a single, unified network.
Local Area Networks
LANs became significant in the world of networking in the late 1980s, following
on the heels of the PC to become the preferred method for connecting multiple PCs
in a self-contained area.
Unfortunately, the networking software and operating systems used with the PC
LANs were quite different from the networking software used on midrange and mainframe
computers and office automation equipment (for example, dedicated word processing
machines and intelligent copiers). This, of course, set up the inevitable conflict
between PCs, office automation equipment, and the larger midrange and mainframe computers.
Although many computer-savvy corporations saw the conflict coming and took steps
to address it head on, other companies first became aware of the conflict when the
requests to run cable hit the maintenance department.
After all, from a simple and fundamental perspective, the laying of the cable
represents a major commitment. Installing the cable requires the unpleasant work
of snaking cable through ceilings and down walls. It requires that the cable be arranged
in such a way that it is manageable and easy to expand (from a networking perspective).
And even worse, the placing of cable is often regulated by local ordinances that
require special casings or materials (Teflon enclosures, for example) if the cable
runs near pipes, electrical work or people. In short, putting the cable in is almost
as much fun as simultaneously remodeling the kitchen and bathroom of your house.
LAN Topologies
Whether the purpose of the LAN is to interconnect PCs, minicomputers, or both
is almost irrelevant--the first issue is often choosing the topology of the LAN.
This choice dictates the cable, cabling methodology and the networking software that
can operate on the LAN. The three basic topologies are the ring, star, and bus (see
Figure 7.1).
FIG. 7.1 LAN Topologies
- Ring. As its name suggests, a ring LAN joins a set of attachment units
together via a series of point-to-point connections between each unit. Each attachment
unit, in turn, interfaces to one or more computers or computing devices. Information
flows from attachment unit to attachment unit in a single direction, thus forming
a ring network. Because each PC in a ring network acts as a repeater, performance
degrades with each additional PC. Consequently, this is typically appropriate only
in small networks.
- Star. In a star LAN, each computer or computer-related device is connected
on a point-to-point link to a central device called a hub. The hub acts as the LAN
traffic manager, setting up communication paths between two devices seeking to exchange
information. This configuration makes it very easy to isolate problem nodes, and
is one of the most common LAN models.
- Bus. The simplest form of bus LAN is a set of computers or devices connected
to a common, linear connection. Under the bus topology, information is transmitted
over the distance of the network, so each computer can pick up its intended information.
Links from the main bus line might break off into additional linear links with multiple
attachments; this type of bus structure is also referred to as a tree because multiple
branches reach out from the main trunk. This model is used in high-speed PBXs.
Star and ring network topologies are sometimes combined into one network to provide
a higher degree of fault tolerance. Because a star network is susceptible to a failure
in the hub, and a ring network is sensitive to a break in the ring, combining both
forms into one offers an alternate route in case one topology fails.
LAN Protocols
While the LAN topology defines the cabling methodology and the way that information
flows through the network, the LAN discipline determines how the computers interact
with each other on the LAN. The two most used protocols are token passing
and collision sensing.
In a token passing network, a special token is passed from computer to computer.
Possession of the token enables a computer to transmit on the network. When the original
transmission returns to the computer that sent it, that transmission is regarded
as complete (whether or not it was actually received) and a new token is generated
to flow to the next station (based on the LAN topology). When a message is successfully
received, the receiving station confirms receipt by changing a flag in the original
transmission. Therefore, by examining the original message when it returns, the sending
station can determine what happened at another end of the ring. Token passing dates
back to 1969 and is one of the earliest multiple-unit, peer-to-peer control procedures.
Token ring networks, although not as widely used as a CSMA/CD network, offer robust
performance because they furnish only a single channel; thereby avoiding any possibility
of collision.
The proper name for collision sensing is Carrier Sense Multiple Access with
Collision Detection (CSMA/CD). With this discipline, each computer listens to
the LAN to sense if another computer is transmitting. If someone else is active on
the LAN, the computer wishing to transmit waits for a preset amount of time before
trying again. When the computer perceives that the LAN is inactive, it transmits.
In the event that two computers transmit at the same time (and their data collides
and is hopelessly corrupted), both sides wait for different lengths of time before
attempting to retransmit. CSMA/CD dates back to the mid-1970s (when Ethernet was
in its infancy) and has grown to be the most common discipline for PC LANs.
The biggest difference between the two disciplines is that token passing is termed
a deterministic discipline while collision sensing is not. A token passing network
is deterministic because each computer is given the opportunity to transmit, but
only at preset time intervals, and only if it is in possession of the token. On a
collision sensing network, however, each computer must, in effect, compete for the
opportunity to transmit.
A third type of discipline, time division, is sometimes used in laboratory
environments for specialized controllers, technical equipment or wireless communications
networks. With this discipline, each unit is given specific amounts of time at specific
intervals to exchange data. Using time division in a conventional data processing
LAN, however, is extremely unusual, not to mention impractical.
LAN Implementations
Both the discipline and topology define a LAN implementation. Thus, a LAN might
be a token passing ring, a token passing bus, or a collision sensing bus.
The standards for LAN implementations can be roughly broken into two groups: those
that pre-date the work performed by the IEEE in this area, and those that were developed
by the IEEE. Of the LAN implementations that pre-date IEEE's involvement, Ethernet
and token ring implementations have stood the test of time and remain popular.
Ethernet was originally developed by Xerox Corporation in the 1970s as a 3 Mbps
bus LAN using the CSMA/CD discipline. Following the initial release of Ethernet,
both Digital Equipment and Intel joined the development effort and the three companies
released the specification for Ethernet version 1.0 in 1980. The most notable improvement
in Version 1.0 was the increase in the LAN speed from 3 Mbps to 10 Mbps. The Ethernet
specification was then revised again several years later as Ethernet II to provide
a higher degree of compatibility with the IEEE 802.3 standard. The 802.3 standard
has since grown to include a newer specification, known as Fast Ethernet, which boosts
the speed tenfold to 100 Mbps.
In addition, the IEEE 902.9a isochronous Ethernet standard provides a way for
two networks to run over 10Base-T wiring. IsoEthernet permits the integration of
LAN and WAN services, and can extend a company's existing investment in standard
Ethernet. IsoEthernet can deliver voice and video as well. In the past, multimedia
over Ethernet has been limited because of Ethernet's connectionless nature. Traditional
Ethernet generates bursty traffic, which is excellent for sending data, not suitable
for time-sensitive information such as video. This type of time-sensitive traffic
is highly dependent on all packets arriving in the correct order. IsoEthernet is
capable of multiplexing 56 Kbps/64 Kbps ISDN B channels and running both packet and
wideband circuit-switched multimedia services over Category 3 UTP cable. Its encoding
scheme also increases the available bandwidth from 10 Mbps to 16 Mbps. The extra
6 Mbps of bandwidth is used to create a multimedia pipe. IsoEthernet can be integrated
into an existing 10Base-T Ethernet with the addition of an isoEthernet hub, which
permits WAN and LAN services to be synchronized. Workstations must be equipped with
isoEthernet adapter cards, which are connected to the hub. An attachment unit
interface (AUI) then connects the isoEthernet and Ethernet hubs.
Token ring networks have been implemented on a variety of media at a variety of
speeds. Therefore, unlike Ethernet, token ring technology was not successfully introduced
into a generalized data processing network. IBM implemented token ring in its early
PC LANs, as did Apollo for its engineering workstations. But somehow, token ring
did not catch on as Ethernet did. There were several reasons for this, including
the fact that token ring is more expensive to deploy than Ethernet, requires more
planning, and is more difficult to install. More recently, however, token ring networks
have enjoyed a rebirth in popularity for several reasons.
The IEEE organization adopted token ring as a sanctioned network in its IEEE 802.5
spec-ifications.
The market for token ring switches is enjoying tremendous growth as corporate
networks continue to grow at an unprecedented pace. These switches provide users
on overcrowded LANs with their own personal 4 Mbps to 16 Mbps piece of bandwidth.
The switch can also be used to divide a large ring into smaller segments.
Until recently, equipment for switched token ring networks was largely unavailable.
However, token ring networks can suffer from the same geographic limitations as Ethernet,
and vendors are now stepping in to provide the switching equipment users require
to expand their token ring networks. Traditionally, two-port bridges are used in
token ring networks, which impose a significant limitation on its expandability.
Token ring switches can connect the separate rings to each other and to servers,
without the performance limitations of the past. Some products include both token
ring and Ethernet switching facilities in the same box. Most switches also accommodate
high-speed networking, such as ATM or FDDI; many also support RMON management.
Both token ring and Ethernet networks have bandwidth limitations. Many corporate
networks are beginning to reach those limitations, as they bring in more and larger
applications and experience a greater demand for data. Switching technology can help
overcome these limitations by extending an overcrowded network. Whether the switch
is used to divide the ring into smaller segments or to give each user a personal
slice of bandwidth, switches can greatly enhance network performance, thereby extending
the useful life of the existing network.
IBM has made a significant commitment to supporting token ring as the preferred
SNA LAN and has, in fact, provided connections for its broad range of computers and
communications controllers to token ring.
The architecture of the Fiber Distributed Data Interface (FDDI) is modeled
after token ring. FDDI is a high-speed WAN technology that runs at 100 Mbps. Therefore,
if you love FDDI, you must also at least have a passing respect for token ring.
Token ring technology was patented by a European engineer who forced those who
adopted it to pay a royalty. However, this patent has been successfully challenged,
so the economics of token ring networks has taken a turn for the better.
LAN Implementation Standards
Although both Ethernet and token ring networks function well, they were not recognized
as official standards because they were developed in the private, commercial sector.
To address this need for standardization, the IEEE studied these and other implementations
and developed a series of standards to properly define a series of LAN specifications.
In developing its standards, the IEEE had to walk the line between the OSI Reference
Model and the existing, well-known and widely accepted LAN implementations. In terms
of the OSI Reference Model, for example, IEEE carved the Data Link Layer (layer 2)
into two parts. The upper half of the layer that interfaces with the Network Layer
(layer 3) was termed the Logical Link Control (LLC). The LLC provides a common,
low level point of access, independent from the actual physical media.
The lower half of the layer that interfaces with the Physical Layer (layer 1)
was termed the Medium Access Control (MAC). The MAC addresses the specifics
of the physical network interface; therefore, separate MAC standards are defined
for CSMA/CD, token passing bus and token passing ring. However, note that a single
LLC specification addresses all three MACs.
A message passed from the Network Layer is processed by the LLC protocol, and
an LLC header is added to the data (see Figure 7.2). This new data structure is then
passed on to the MAC where another header and a trailer are added before the data
enters the physical network. The resulting structure that includes the MAC header,
the LLC header, the data and the MAC trailer is termed a frame.
IEEE 802.2
In IEEE terms, the 802.2 specification defines the LLC (see Figure 7.3). The 802.2
header consists of the following:
- Destination Service Access Point (DSAP). The DSAP is a seven-bit address
with an eighth bit to indicate if it is a specific address (0) or a group (broadcast)
address (1). The DSAP is not a station or device address; rather it designates the
service control point where the message should be routed.
- Source Service Access Point (SSAP). The SSAP is also a seven-bit address,
but in this case the eighth bit is used to indicate if the message is a command (0)
or a response (1). Like the DSAP, the SSAP designates a control point and not a station
address. In the case of the SSAP, this is the control point from which the message
originated.
- Control. The control field is either 8 or 16 bits long, with the length
indicated by the first two bits. The 16-bit fields are used to exchange sequence
numbers, while the 8-bit variation is used for unsequenced information.
Below the 802.2 LLC are the MACs for the various physical LAN implementations.
These standards are known as 802.3 for CSMA/CD, 802.4 for token passing bus, and
802.5 for token passing ring.
FIG. 7.2 IEEE LLC and MAC Layers
IEEE 802.3
The IEEE 802.3 standard specifies a CSMA /CD bus network that supports 10-Mbps
transmission over baseband, broadband, and twisted pair cable. This networking standard
closely resembles Ethernet. Both HP and IBM (and others) support the IEEE 802.3 networking
standards (HP for their native NS networking product, and IBM for their TCP/IP products).
The 802.3 header (see Figure 7.4) includes the following:
- Preamble. An 8-byte pattern of binary 1s and 0s used to establish synchronization.
The last bit of the preamble is always 0.
- Start Frame Delimiter. An 8-bit pattern indicating the formal start of
the frame.
- Destination Address. An address specifying a specific destination station,
a group of stations, or all stations in the LAN. This address can be 16 bits or 48
bits in length, but all stations in the LAN must adhere to one format or the other.
- Source Address. The address of the originating station. This address has
the same length requirements as the Destination Address.
- Length. The length, measured in bytes, of the actual data, including the
802.2 header. This is a 16-bit field.
FIG. 7.3 IEEE 802.2 LLC Header
Following the header is the 802.2 header and the actual data. At the end of the
data is the 802.3 trailer, which includes:
- Padding. Extra, nondata bytes can be inserted into the frame to make the
overall frame length more palatable to the physical network.
- Frame Check Sequence. At the end of the frame is a 32-bit Cyclic Redundancy
Check (CRC) on the data starting with the destination address and terminating at
the end of the data (not including any padding).
IEEE 802.4
The IEEE 802.4 specification defines a token passing bus that can operate at speeds
of 1, 5, or 10 Mbps. The 802.4 standard is, in many ways, a marriage of Ethernet
and token ring technologies. The physical topology for 802.4 is a bus, much like
in Ethernet, but the MAC-level discipline is a token-passing logical ring (as opposed
to a token-passing physical ring). Although the 802.4 specification does not have
as many active supporters as the 802.3 and 802.5 standards, its popularity is rapidly
growing. The format for 802.4 transmissions (see Figure 7.5) is as follows:
- Preamble. One or more bytes used for synchronization patterns.
- Start Frame Delimiter. An 8-bit pattern signaling the start of the frame.
- Frame Control. A 1-byte field used to indicate if the frame contains actual
data or if it is a control message.
- Destination Address. An address specifying a specific destination station,
a group of stations, or all stations in the LAN. This address can be 16 bits or 48
bits in length, but all stations in the LAN must adhere to one format or the other.
- Source Address. The address of the originating station. This address has
the same length requirements as the destination address.
FIG. 7.4 IEEE 802.3 CSMA/CD Frame
Following this header is the 802.2 header and the actual data. At the end of the
data is the 802.4 trailer, which includes the following:
- Frame Check Sequence. At the end of the frame is a 32-bit Cyclic Redundancy
Check (CRC) on the data starting with the Frame Control field and terminating at
the end of the data.
- End Delimiter. The 8-bit pattern signaling the end of the frame. The last
two bits of this field signal if the frame is the last frame to be transmitted and
whether any station has detected an error in the frame.
IEEE 802.5
The IEEE 802.5 standard specifies a token passing ring operating over shielded
twisted pair cables at speeds of 1, 4, or 16 Mbps. This standard is supported by
IBM in its Token Ring implementation. The 802.5 construction (see Figure 7.6) is
defined as follows:
- Start Frame Delimiter. An 8 bit-pattern signaling the start of the frame.
- Access Control. An 8-bit field used for priority and maintenance control.
Most important, one bit of this field is the token bit. If set to 1, the frame contains
data. If set to 0, the frame is actually a token that can be seized by a station
waiting to transmit. Also note that when the token bit is set to 0, the entire frame
consists only of the start frame delimiter, the access control byte and the end delimiter
byte.
- Frame Control. A 1-byte field used to indicate if the frame contains actual
data or a control message.
- Destination Address. An address specifying a specific destination station,
a group of stations, or all stations in the LAN. This address can be 16 bits or 48
bits in length, but all stations in the LAN must adhere to one format or the other.
- Source Address. The address of the originating station. This address has
the same length requirements as the destination address.
FIG. 7.5 IEEE 802.4 Token Bus Frame
FIG. 7.6 IEEE 802.5 Token-Ring Frame
Following this header is the 802.2 header and the actual data. At the end of the
data is the 802.5 trailer that includes the following:
- Frame Check Sequence. At the end of the frame is a 32-bit Cyclic Redundancy
Check (CRC) on the data starting with the Frame Control field and terminating at
the end of the data.
- End delimiter. The 8-bit pattern signaling the end of the frame. The last
two bits of this field signal if the frame is the last frame to be transmitted and
whether any station has detected an error in the frame.
- Frame Status. An 8-bit pattern indicating whether a station has recognized
the frame and also if the frame has been copied (received).
802.3 Versus Ethernet
The implementations of Ethernet and IEEE 802.3 are so compatible that computer
systems using each can coexist on the same network. The most significant difference
between the two is the way information is formatted into frames. Although both specifications
define the destination and origin of the information, the 802.3 frame includes significantly
more detail.
The Ethernet frame begins in the same fashion as the 802.3 frame with a preamble,
start delimiter, and then the destination and source addresses (see Figure 7.7 ).
The similarity stops here, because in Ethernet these addresses are followed by a
type field, which identifies which Ethernet service the frame applies to. However,
because the headers are so similar, these frames can coexist on the same LAN without
interfering with one another ( providing that the 802.3 frame uses 48-bit addresses
as does Ethernet).
FIG. 7.7 Comparison of Ethernet and 802.2/802.3
Frame Formats
NOTE: Ethernet Version 1.0 does not have the same level of compatibility
with IEEE 802.3 as Ethernet II. Specifically, the primary difference is that Ethernet
II and IEEE 802.3 both include a "heartbeat" function performed by the
transceivers (units that attach computer and computer equipment to the physical LAN)
to signal their ongoing operation (the absence of a heartbeat signals a failed or
failing transceiver).
Wide Area Networks
At a basic level, a WAN can be created by tying a series of simple, point-to-point
links together. On the other end of the spectrum, a WAN might comprise many different
systems and LANs, all interconnected using a variety of techniques, including standard
telephone lines, packet-switching networks and ISDN links. Between the two extremes
are networks that are superficially simple but technically complex, and those that
are superficially complex but technically simple.
Unlike LANs, which all accomplish the same purpose, WANs offer a unique variety
of technology and approaches. This discussion will focus on some of the better known
approaches--standard phone links, ISDN networking, and X.25 packet-switching--as
well as emerging methods such as ATM, Frame Relay, and SMDS.
Like LANs, WAN links are simply a way of transferring information from point A
to point B. Running on top of both types of links are networking protocols and services
that bring additional functions to the network. For example, IBM's SNA, Digital's
DECnet, TCP/IP, and many other networking protocols all include services that operate
over the physical links. Some protocols are specific to the LAN environment (such
as Digital's LAT or Novell's IPX), while other protocols are better suited for wide
area links (like IBM's SDLC or HP's implementation of HDLC).
The point is, in all cases, no network (wide or local) provides any value without
upper layers of protocols, services and applications.
Point-to-Point Links
In most cases, long-distance point-to-point links are routed through a telephone
carrier. From a practical point of view, the long-distance telephone carriers have
already done the work of establishing a wide area of physical links, so it makes
sense in some circumstances to use these existing connections.
Before the advent of high-speed digital lines, this world of long distance teleprocessing
was composed of dial lines and leased lines. Dial-up POTS (Plain Old Telephone
Service) lines are one of the few aspects of data communications that has not
changed much over the years, although higher-speed modems have enabled data to be
sent over them much faster. As the name implies, a dial line uses standard voice-grade
lines to create a temporary connection between two computing devices. POTS lines
can operate at speeds of up to 36,600 bps if a noise-free connection can be made.
A leased line is a permanent circuit installed between point A and point B. Because
they are permanent, leased lines can be conditioned to provide less noise and therefore
support high-speed operation (such as 28,800 bps) on a more reliable basis. Leased
lines have been greatly affected by the advent of digital phone circuits.
Before the advent of digital lines, point-to-point links used the same basic approach
to carry data as they did to carry voice. Although, as noted, leased lines could
be purchased with various levels of conditioning, they still used the same analog
approach for transmitting. Modems were developed to bridge the difference between
the analog nature of the phone system and the digital nature of computers. Modems
that translate between the digital and analog formats are described in greater detail
at the end of this chapter.
NOTE: The analog /digital translation process, MOdulating and
DEModulating, forms the etymology of the word "modem."
As technology and phone systems matured, the nature of the phone network became
much more sophisticated. Satellites were deployed to provide greater coverage without
costly physical connections, and digital circuits were added into many phone systems
to offer high transmission speeds with lower noise (and therefore fewer errors).
In particular, Digital Data Service (DDS) brought increased performance
to leased lines. When compared to digital networking, the analog phone system is
slow and error-prone. Furthermore, because the existing phone system was developed
to address voice transmissions, the way it handles data communications is less than
ideal. Digital service brought increased reliability and performance to leased line
networks.
Higher rates are provided through the use of T1 links. T1 links are multiple,
high-speed links packaged into a single unit. Specifically, a T1 line has an aggregate
throughput of 1.544 Mbps but is, in reality, composed of 24 64-Kbps digital lines.
A T1 user can dedicate these 24 lines to separate functions--for example, some
might carry voice, some video and some data. Or a T1 user can use multiplexing equipment
to run data across all (or a subset) of the separate lines concurrently, to effectively
achieve the full throughput. If a company does not need the full T1 bandwidth, it
might also choose fractional T1 service. In this case, only some of the T1 lines
are connected to the customer's premises. The availability of fractional T1 lines
is dependent on the local phone company's ability to find enough fractional users
to use up an entire T1 link.
Through bridges and routers, geographically distant LANs can be interconnected
over a T1 link. However, since T1 is strictly a North American standard, it cannot
be used to establish an intercontinental WAN.
NOTE: Because of the wide and diverse geography of the U.S., digital
and T1 services are not available in all areas of the country.
ISDN
ISDN is the planned replacement for the analog circuits used to provide voice
and data communications services worldwide. Development on ISDN was begun in the
1970s by AT&T and formalized in the early 1980s under the mantle of the Consultative
Committee for International Telegraph and Telephony (CCITT). Under the direction
of CCITT, ISDN became worldwide in scope, offering for the first time a fixed set
of interfaces and interface devices that were applicable globally. Thus, the computer
interface used in Germany for ISDN attachment would be the same interface used in
the U.S.
The customer's interface to ISDN is through a service node (see Figure
7.8). The purpose of the service node is to provide an interface from ISDN to the
customer phone system or PBX, a data communications device that interfaces to the
local computer equipment (analogous to a modem or CSU/DSU), or a hybrid device that
performs both functions. This service node interface enables the customer to access
( given proper security and compatible equipment) any other system also connected
to ISDN.
FIG. 7.8 ISDN Service Node Concept
Interface
For data communications, this approach is more flexible than the traditional point-to-point
leased line or dial-up connections previously discussed. Voice service, on the other
hand, will not be dramatically improved because it also operates at the equivalent
of 64 Kbps over the analog system. The digital nature of the network should, however,
remove some of the static often heard over phones. (Also remember that while static
is annoying to humans, it is devastating to data, and that's why data is not transmitted
across analog lines at these extremely fast speeds.)
From a point of entry perspective, ISDN offers two types of user interfaces:
- The Basic Rate Interface (BRI). This interface offers two 64-Kbps data
and/or voice circuits, known as the B channels, combined with a 16-Kbps management
and service circuit, or D channel.
- The Primary Rate Interface (PRI). This interface features 23 64-Kbps data
and/or voice circuits with an additional 64-Kbps circuit for management and ancillary
services.
Because the costs for a PRI far exceed the cost for a BRI, most business needs
are addressed by one or more BRIs. Still, given the higher speeds offered by the
PRI (a total of 1.544 Mbps for the PRI versus a total of 144 Kbps for the BRI), the
PRI is a viable contender to extending LANs using bridges and routers.
Also note that the PRI closely resembles a T1 link. The primary difference lies
in their use and network architecture. T1 is most often used to facilitate high-speed
point-to-point links, whereas ISDN is intended to interface a large number of systems
on a global basis. ISDN also differs from the T1-style link in that it features a
management circuit separate from the data circuits. This additional circuit is present
in both the Basic Rate and Primary Rate Interfaces and delivers some benefits that
are important to ISDN and its marketability. This circuit is separate from the data/voice
channels (see Figure 7.9). In fact, it is termed a D Channel, as opposed
to the B channels that carry the data and voice traffic.
FIG. 7.9 ISDN B and D Channels
This type D circuit can be used for a number of functions:
- Network management. If network monitoring and management functions are
separated from the network itself (which would be running over the D channels), then
two benefits accrue. First, the monitoring and management function does not adversely
affect network performance because it does not occur from within the network. Second,
if a failure occurs within the network, the monitoring and management structure is
still available to signal and alert operators to take corrective action.
- Faster call servicing. By using the D Channel to perform call set-up operations,
ISDN dramatically reduces the amount of time necessary to initially establish a call.
After the set-up occurs via the D Channel, the B Channel is instantly available for
the actual voice transmission.
- Automatic Number Identification (ANI). Although the introduction of this
feature has provoked political controversy in many states, ANI technology has many
reasonable applications, especially in the customer service area. Specifically, it
enables a phone call recipient to view (on a special display device) the phone number
of the person who is calling. In addition, call center applications are available
that will produce the caller's database record, order information, credit limits
or other pertinent data instantly on a computer screen, before the call is even answered.
- Advanced information forwarding. As customers learn to use ISDN, they
will invariably use the D Channel to send advanced information that relates to the
call going over the B Channel. For example, customers might direct their PBX to send
account information on the D Channel when they are making a voice call to a vendor.
Because the account information arrives before the voice call, it can often be processed
before the two parties converse. Thus, the vendor's representative might have the
customer's file on his or her workstation when the phone rings.
Beyond establishing a digital international data network, ISDN is a key piece
of the standards pie for several reasons:
- The U.S. government has developed federal standards based on ISDN and will soon
require federal agencies to comply with these standards.
- The phone companies are committed to ISDN. Like it or not, you will be using
ISDN at your home and office.
- ISDN has not overlooked fiber optic technology. Broadband ISDN (BISDN), which
uses higher speed and more reliable fiber optic communications, is being analyzed
as a transport within ISDN as well as a service in itself.
Laptop PC users now also have the ability to access a network with ISDN services
through new ISDN Basic Rate Interface PCMCIA cards.
The various Regional Bell Operating Companies (RBOCs) are now providing ISDN service
to most major cities, and the number of ISDN lines has increased dramatically over
the past few years. Additionally, all of the major commercial on-line services offer
ISDN access, and many Internet service providers are also offering ISDN Internet
access. The advantages are obvious; file downloads are lightning fast, and there
is less possibility of interruption due to line noise. PC vendors are accommodating
the increasing demand for ISDN by releasing ISDN modems, also known as ISDN terminal
adapters.
Although the cost of an ISDN connection is bound to decrease, costs vary tremendously
from region to region.
Asymmetric Digital Subscriber Line
(ADSL)
Although ISDN has been getting most of the press, a similar technology called
asymmetric digital subscriber line (ADSL) promises even more throughput over
an ordinary, narrow copper telephone line. ADSL accomplishes this remarkable feat
through a series of complex compression and digital signal processing algorithms,
and dynamic switching techniques. The ADSL transport technology boosts the capacity
of the existing phone line significantly more than ISDN. Duplex ADSL offers a downstream
data rate of up to 6 Mbps and an upstream channel running at 640 Kbps. ISDN, on the
other hand, ranges from 64 Kbps to 128 Kbps--faster than a standard modem, but still
too slow to handle that interactive TV and other services they keep telling us we'll
all have one of these days.
Most of the regional Bell operating companies are testing ADSL and making plans
to offer it to customers seeking high-bandwidth Internet access. Also, because it
offers two-way communications, some entertainment companies are considering it a
realistic possibility for interactive cable television.
ADSL gives the RBOCs an alternative to costly optical cables because it can transform
their existing copper-wire network into a high-performance system. Here's how it
works: The regular phone wire is configured for ADSL, and then connects to an ADSL
modem on one end and an ADSL circuit switch on the other. The connection then creates:
- A high-speed, unidirectional data channel capable of running between 1.5 Mbps
to 6.1 Mbps.
- A medium-speed duplex channel running between 16 Kbps and 640 Kbps.
- A standard analog connection.
Cable companies, set-top box makers, and TV couch potatoes drool over the possibilities.
Look for this technology to be making big news in the near future.
X.25
The CCITT developed the X.25 standard to define a reliable, relatively low cost
means of routing data through a shared network. An extremely important aspect of
X.25 is that the information being transmitted has been converted into packets.
Packets can be thought of as small fragments of information. Specifically, a block
of information is broken into smaller parts ( packets) before being transmitted
on the physical network. The packet methodology provides faster and more reliable
error detection and correction; it also prevents a system with a huge volume of information
to ship from tying up the network.
In addition to the raw information, each packet also contains information specifying
its origin, its destination, and a number indicating the "piece" of the
information to which it corresponds. This enables each packet to be treated as an
independent entity, so that packets from many different systems can be intermixed
on the network without concern about the order in which they are transmitted or even
the order in which they arrive. Each packet might take the best possible route available
at the time it is ready for transport.
The application end points of the information (that is, the terminal user and
the application program) rarely see the information in its packetized form. As part
of its interface with the network, the computer system converts the information into
packets, and then subsequently reassembles the packets into the original information
(see Figure 7.10).
FIG. 7.10 Conceptual Packetizing
This packet approach to transmitting data is extremely pervasive in the networking
world. In addition to being used by X.25, this approach is also used by most LANs
and many other data communications protocols (although they are usually referred
to as frames, as discussed in the LAN section of this chapter). Specific to X.25
networks, however, is the concept of a packet switching network (PSN).
A PSN is a WAN through which packets are sent. The precise route that packets
take from point A to point B is not fixed and is immaterial to the equipment at point
A or point B, which checks only to see whether the packets arrive intact (again,
order is not a major concern).
Because they don't have prescribed data routes, PSNs are often shown as clouds
in many networking diagrams (see Figure 7.11). When depicted in this manner, information
goes into the cloud at some point and comes out at another. What goes on within the
cloud is not the concern of mere humans.
FIG. 7.11 Typical X.25 Representation
The inside of the cloud, however, is composed of packet-switching nodes
(also called PSNs, just to make life confusing). The switching nodes can take routes
to other switching nodes, and thus can route or reroute data as necessary. For example,
if a switching node has a packet to forward and the best possible switching node
to receive it is busy, the node holding the packet will reroute it to another node
for subsequent rerouting (see Figure 7.12).
FIG. 7.12 Inside of the X.25 "Cloud"
Packet-switching networks often are associated with public data networks (PDNs),
but this relationship is certainly casual. A PDN is normally a telephone system (or
telephone company in the U.S.) that offers data services to the public. It does not
have to use packet-switching to move information from point to point. If a PDN does
offer the services of a packet-switching network, it might be referred to as a packet-switching
data network (PSDN) or even a packet-switching public data network (PSPDN). Clearly,
the abbreviations are almost endless.
Furthermore, implementation of packet-switching networks is not limited to telephone
companies. In fact, PSNs can be constructed of telephone links, fiber optic links,
microwave links, satellite links, and other forms of communications. Many large corporations
have used these diverse communication techniques to construct their own private PSN.
Because, in the final analysis, a packet switching network is a cost-effective WAN,
organizations with widely dispersed equipment find this approach most effective in
terms of both cost and function.
The traditional packet-switching cloud is shown in Figure 7.13.
FIG. 7.13 X.25 Interfaces
Moving outside of this cloud, the interfaces between the computer equipment and
the cloud generally fall into one of two types of devices:
- PAD. The packet assembly/disassembly device is a piece of hardware that
interfaces between the network and computer equipment incapable of sending or receiving
packets. This function is defined in CCITT standard X.3. The purpose of the PAD,
then, is to handle the conversion of the raw data into packets for transmission into
the packet-switching cloud and, conversely, handle the reassembly of information
from packets received from the cloud. PADs most often are used to interface terminals
into the packet- switching network, but they are also used to interface computer
systems that cannot handle packet transformations on their own.
- A communications controller running (normally) the LAP-B protocol. Rather
than use an external device, such as a PAD, most computers use an internal interface
to directly connect to the packet-switching network. These interfaces and their corresponding
software drivers provide much of the same function provided by a PAD. The advantage
to putting these interfaces into a computer is that computer software can directly
access the link (whereas in the PAD the link was external and, for the most part,
invisible to the software). For example, an office automation package can communicate
with a counterpart package operating on the other "side" of the cloud.
For terminal traffic over packet-switching networks, two additional standards
come into play. First, the CCITT X.28 standard defines the interface between
an asynchronous terminal and a PAD. Second, the CCITT X.29 standard defines
the control procedures for information exchanges between a PAD and another PAD (or
an integrated controller). Just as X.25 has become synonymous with packet-switching
networks, X.29 has become synonymous with interfacing terminals over packet-switching
networks.
LAN Switches
Switches are used to extend overcrowded networks by providing each end user with
his own piece of 4 Mbps or 16 Mbps bandwidth. In many cases, this might be more than
each end user needs. In this event, the token ring switch can be used to break one
big token ring into multiple, smaller rings. This approach will also significantly
increase performance.
Switch Technology--Token Ring and FDDI
Many vendors are bringing token ring LAN switches to the market. A number of alliances
illustrate the strength of this market, such as a recent noteworthy alliance between
Bay Networks (Santa Clara, California) and IBM. Other network vendors, such as Cisco
Systems and Cabletron Systems, have made similar deals with third parties. As more
vendors go into this market and volume increases, token ring switch products are
expected to come down in price and enjoy higher demand.
FDDI switching is another promising technology for extending network life and
bandwidth. Digital's GigaSwitch is the leading FDDI switching product, although several
other vendors are preparing to release FDDI switches as well.
Token ring networks, like Ethernet networks, have bandwidth limitations, and many
are starting to reach those limitations because of the bigger applications and greater
demands for data that companies are experiencing. The lower pricing structures of
Ethernet and token ring LAN switching devices might encourage individual business
units to make their own purchases. In terms of the overall enterprise, however, this
can be disastrous. It is essential for individual departments to consider the overall
corporate direction when making such purchases, and to make sure that the technology
they are purchasing is compatible with the existing infrastructure and corporate
data needs assessments. If not, they might wind up spending much more money because
they now have to buy additional equipment to connect with the corporate switches
and to address data type and volume transmission requirements decided on by corporate
information communication needs.
Tools of the Trade
Needless to say, computers and networks do not connect to each other as easy as
phones plug into wall jacks. In networks, the tools of connectivity handle conversion
between analog and digital formats, between one type of physical interface and another,
or between one transmission media and another. In short, these tools are the nuts
and bolts of the erector set called networking.
For LANs, one set of tools is required to make the physical attachment between
the interface in the computer (for example, an Ethernet adapter in a VAX or a token
ring adapter in an AS/400) and the physical network. The tools include:
- The Attachment Unit Interface (AUI). This is the cable that attaches the
interface in the computer to the MAU described below.
- The Medium Attachment Unit (MAU). Also known as a Multistation Access
Unit when used with a token ring network. This device attaches one or more AUIs to
the physical LAN. A MAU can provide one-to-one connection with a computer or it can
be a hub to several systems.
When two LANs are joined together, a bridge or router is normally used. When a
bridge links two or more LANs, those LANs form a single, logical LAN. In this case,
all information routed through one LAN goes over the bridge and through the attached
network. Because of this traffic, high speed links are normally required to keep
the bridge from slowing the performance of the network. And finally, because bridges
are implemented at such a low level, all protocols can operate over a bridge.
A router also connects two or more LANs, but routers are much more selective about
the information that they allow to cross over. Specifically, routers are aware (through
self-learning or manual configuration) of which computer addresses apply to which
LANs. Therefore, rather than pass all information across, routers transmit only information
pertinent to the other LAN. Because only selected traffic travels across the link,
lower speed links can be used without affecting overall LAN performance. The router
can also act as a firewall to prevent unwanted access to the network from
outside.
Routers cannot be used in all types of networks, though. Because routers depend
on the network to supply an internetwork address (an address that is globally unique),
those network protocols that do not support this type of addressing cannot be used
with routers. Digital's LAT protocol, for example, has no facilities for internetwork
addressing, and therefore will not travel over a router (but it will travel over
a bridge). And because routers and bridges have their advantages and disadvantages,
the two are often combined into one piece of equipment (in this case some protocols
are bridged and others are routed). These devices are often called brouters.
When a computer, bridge, or router must interface to the telephone system (analog,
digital or T1), more special devices are needed. They are as follows:
- Modem. For traditional (analog) phone lines, modems (MOdulator/DEModulators)
provide the conversion between the digital computer output and the analog phone transmissions.
The interface between the computer (or router or bridge) and the modem is normally
a well-defined standard such as EIA RS-232 or CCITT V.32.
- CSU and DSU. For digital links (DDS or fractional T1 lines), two devices
are required. A channel service unit (CSU) interfaces with the telephone-side of
the link and with a data service unit (DSU) that, in turn, interfaces with the computer
system (or router or bridge). The attachment to the DSU is a well-defined standard
like EIA RS-232 or CCITT V.35. In most cases, the CSU and DSU are combined into a
single, physical unit.
- Gateway. A gateway attaches seemingly incompatible networks, such as IBM's
SNA and Digital's DECnet. In a nutshell, a gateway is a complicated form of protocol
converter--it converts multiple protocols and emulates multiple devices to provide
a wide variety of services. Gateways can be used to link electronic mail systems,
to enable one type or terminal to access another type of host, to provide file transfer
between networks, or to perform all of these functions.
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