In the last fifteen years, local area networks (LANs) have gonefrom bein g an experimental technology to becoming a key business toolused by companies w orldwide. A LAN is a high-speed communicationssystem designed to link computer s and other data processing devicestogether within a small geographic area such as a workgroup,department, or a single floor of a multi-story building. Sever al LANscan be interconnected within a building or campus of buildings, aswell, to extend connectivity.
LANs have become popular because they allow users to share vitalcomputing r
esources electronically, including expensive hardware suchas printers and CD RO
M drives, application programs and, mostimportantly, the information the users
need to do their jobs. Priorto the development of LAN technology, individual c
omputers wereisolated froms each other and limited in their range of applicatio
ns.By linking these individual computers over local area networks theirusefuln
ess and productivity have been increased enormously. But a LANby its very natu
re is a local network, confined to a fairly small areasuch as a building or eve
n a single floor of a building. To realizethe full benefit of computer network
ing, it is critical to link theindividual LANs into an enterprise-wide backbone
network that connectsall of the company
Today
Today local area networking is a shared accesstechnology. Thi s means that all of the devices attached to the LANshare a single communication s medium, usually a coaxial, twisted pair,or fiber optic cable. Figure 1 illustrates thisconcept: several computers are connected to a si ngle cable whichserves as the communications medium for all of them. The physi calconnection to the network is made by putting a networkinterface car d (NIC) inside the computer and connecting it tothe network cable. On ce the physical connection is in place it is upto the network software to manag e communications between stations onthe network.
When Station B sends a packet to another station on the LAN itpasses by all of the stations connected to that LAN. On the busnetwork i llustrated here the electrical signal representing the packettravels away from the sending station in both directions on the sharedcable. All stations will se e the packet but only the station it isaddressed to will pay attention to it. strong> |
In a shared media network, when one station wishes to send amessage to another station it uses the software in the worksta tion toput the message in an envelope. This envelope, called apacket , consists of message data surrounded by aheader and trailer that carryspecial information used by the network softw are to the destinationstation. One of the pieces of information placed in the packet headeris the address of the destination station.
The NIC then transmits the packet onto the LAN. The packet istransmitted as a stream of da ta bits represented by changes inelectrical signals. As it travels along the s hared cable all of thestations attached to it see the packet. As it goes by th e NIC in eachstation, the NIC checks the destination address in the packet head erto determine if the packet is addressed to it. When the packet passesthe st ation to which it is addressed, the NIC at that station copiesthe packet and th en takes the data out of the envelope and gives it tothe computer.
Figure 1 shows one source station sending asingle message packe t to one destination station. If the message thesource station wants to send i s too big to fit into one packet it willsend the message in a series of packets . On a shared access LAN,however, many stations all share the same cable. Sin ce eachindividual packet is small it takes very little time to travel to theen ds of the cable where the electrical signal dissipates. So after apacket carry ing a message between one pair of stations passes alongthe cable another statio n can transmit a packet to whatever station itneeds to send a message. In this way many devices can share the sameLAN medium.
The most widely used LAN technology in use today is Ethernet. It strikes a good balance between speed,price, ease of instal lation, and supportability. Approximately 80percent of all LAN connections ins talled use Ethernet.
The Ethernet standard is defined by the Institute o
f Electrical andElectronic Engineers (IEEE) in a specification commonly known a
sIEEE 802.3. The 802.3 specification covers rules forconfigu
ring Ethernet LANs, the types of media that can be used, andhow the elements of
the network should interact. The Ethernetprotocol provides the services calle
d for in the Physical and DataLink Layers of the OSI reference model
(please
One element of the 802.3 spe cification states that Ethernetnetworks run at a data rate of 10 million bits p er second (10 Mbps) or100 million bits per second (100Mbps). This means that w hen a stationtransmits a packet onto the Ethernet medium it travels along that medium at 10 Mbps.
Another important element defined by the 802.3 specif ication is theaccess method to be used by stations connected to an Ethernet LAN . Itis called Carrier Sense Multiple Access with CollisionDetection ( CSMA/CD). In this method each station contendsfor access to the share d medium. It is possible for two stations totry sending packets at the same ti me which results in a collision onthe LAN. In Ethernet networks collisions are considered normal eventsand the CSMA/CD access method is designed to quickly r estore thenetwork to normal activity after a collision occurs.
An important part of designing and installing a LAN is selectingthe appropriate medium and topology for the environment. Ethernet networks can be configured in either a star or bustopology and installed using any of three differentmedia.
Coaxial cable was th e original LAN medium and it is used in what iscalled a bus topology (Figure 1 depicts a typicalbus topology). In this configuration the coaxial cable forms a singlebus to which all stations are attached. This topol ogy is rarely usedin new LAN installations today because it is relatively diffi cult toaccommodate adding new users or moving existing users from onelocation to another. It is also difficult to troubleshoot problems ona bus LAN unless i t is very small.
Figure 2 illustrates a star topology
In a star topology all stations are wired to a centralwiring concentrator called a hub. Similar to a bus topology, packetssent from one station to another are repeated to all ports on the hub. This allows all stations to see each packet sent on the network, butonly the st ation a packet is addressed to pays attention toit. |
As previously noted, Ethernet networks can be built using threed ifferent types of media: shielded and unshielded twisted pair,coaxial and fiber optic cables. By far the most common is twistedpair because it is associated with the more popular star topology. Itis inexpensive, and very easy to instal l, troubleshoot, and repair.Twisted pair cable comes both unshielded and shield ed.Unshielded twisted pair (UTP) cable used for LANs issimila r to telephone cable, but has somewhat more stringentspecifications regarding i ts susceptibility to outsideelectromagnetic interference (EMI) than commontelephone wire. Shielded twisted pair (STP), as i tsname implies, comes with a shielding around the cable to provide moreprotect ion against EMI.
Of the two types of twisted pair cable, UTP is by far t he mostcommonly used. The specification for running Ethernet on UTP iscalled 10BASE-T. This stands for 10 Mbps, basebandsignaling (the sig naling method used by Ethernet networks), overtwisted pair cable. Other Ethern et specifications include10BASE-5, which uses a thick coaxial cable, and10BASE-2 which uses a thin coaxial cable media.Toda y, 10BASE-5 is seldom installed in new Ethernet networks, and10BASE-2 is used o nly in very small office networks. An additionalstandard allows 10BASE-F Ether net to run on fiber optic cable
Another major LAN technology in use today is TokenRing. Toke n Ring rules are defined in the IEEE 802.5specification. Like Ethernet, the To ken Ring protocol providesservices at the Physical and Data Link Layers of the OSI model. TokenRing networks can be run at two different data rates, 4 Mbps o r 16Mbps.
The access method used on Token Ring networks is called token passing. Token passing is a deterministicaccess method in wh ich collisions are prevented by assuring that onlyone station can transmit at a ny given time. This is accomplished bypassing a special packet called a token from onestation to another around a ring. A station can only send a packetwhen it gets the free token. When a station gets a free token and transmits a packet it travels in one direction around the ring,passing all of the other stations along the way. As with Ethernet,the packet is usually addre ssed to a single station, and when itpasses by that station the packet is copie d. The packet continues totravel around the ring until it returns to the sendi ng station whichremoves it and sends a free token to the next station around th ering.
Token Ring networks use what is called a ringtopology. However, it is actually imple mented in what canbest be described as a collapsed ring that l ooks likea physical star topology (see Figure 3). In Token Ring LANs eachstat ion is connected to a Token Ring wiring concentrator, called amulti-sta tion access unit (MAU), using an individualrun of twisted pair cable. Like Ethernet hubs, MAUs are located inwiring closets.
The ring topolog y as used in Token Ring networks is a collapsed ring that looks like a physical star. Each station is connected to a Token Ring wiring connector by a single twi sted pair cable with two wire pairs. One pair serves as the |
Fiber Distributed Data Interfac e, commonlyknown as FDDI, provides data transport at 100 Mbps, a much higher datarate than Ethernet or Token Ring. Originally FDDI networks required fiber optic cable, but today they can be run on UTP, as well. Fiberis still p referred in many FDDI networks because it can be used overmuch greater distance s than UTP cable. Like Token Ring, FDDI uses atoken passing media access metho d. It is also usually configured in acollapsed ring, or physical star, topolog y. FDDI is used primarily asa backbone, a segment of network that links severalindividual workgroup or department LANs together in a single building.It is also used to link several building LANs together in a campusenv ironment.
Both the Ethernet star topology a
nd the
The hub is one of the most important elements of a LAN. It is acentral connection poi nt for wiring the network (refer to Figure 4),and all stations on the LAN are l inked to each other through the hub.The term hub is generally associated with 10BASE-TEthernet networks while the term multistation a ccess unit(MAU), is used to refer to the Token Ring wiringconcentrato r. These two LAN technologies use different media accessmethods hubs and MAUs perform different media access functionsinternally, but at one level they perfo rm the same function: they areboth network wiring concentrators.
The cor nerstone of the network is the intelligent hub, orconcentrator, which serves as the control point for systems activity,management and growth. By integrating a ny combination of connectivity,internetworking and management capabilities into intelligent hubs,network managers can create the perfect physical networkinfr astructure for their environment. |
A typical
hub has multiple user ports to which computers andperipheral devices such as s
ervers are attached. Each port supports asingle 10BASE-T twisted pair connectio
n from a network station. Whenan Ethernet packet is transmitted to the hub by
one station it isrepeated, or copied, over onto all of the oth
er portsof the hub. In this way all of the stations
Intelligent hubs have been defined as manageablehubs , meaning that each of the ports on the hub can beconfigured, monitored, a nd enabled or disabled by a network operatorfrom a hub management console. Hub management can also includegathering information on a variety of network param eters, such as thenumbers of packets that pass through the hub and each of its ports,what types of packets they are, whether the packets contain errors,and h ow many collisions have occurred. Each hub vendor has some typeof management p ackage they sell with their product. These applicationsvary in how much informa tion they can gather, what commands can beissued, and how the information is pr esented to the networkoperator.
Both hubs a
nd MAUs come in three configurations:stand-alone hubs,
stackable hubs,and modular hubs. Some products are
combinations ofthe best configurations. Stand-alone hubs are
Netw ork A illustrates four 10BASE-T hubs connected together bya single cable. This cable could be a coaxial or an optical fibercable. All of the hubs are part of a single LAN. Network B illustratestwo 10BASE-T hubs cascaded. Here the cable c onnecting the two ports isunshielded twisted pair wire. All of the hubs that ar e cascaded inthis fashion are part of a single LAN. |
A third type of hub is the stackabl
e hub.Stackable hubs look and act like stand-alone hubs except that se
veralof them can be
Each hub usually represents a singl e LAN. In most organizations itis desirable to interconnect all of the LANs, w hich means linking hubsin some way. One way to link hubs is to use an i nter-repeaterlink or cascaded segment. This type of connection simply repeats all of the packets from one hub to the other hub it is linkedto, so th at in effect the two hubs are part of the same LAN.
Modular hubs are popular in networks because theyare easil y expanded and always have a management option. A modularhub starts with a cha ssis, or card cage, with multiple card slots,each of which accepts a communicat ions card, or module. Each moduleacts like a stand-alone hub; when the communi cations modules areplaced in the card slots in the chassis, they connect to ac ommunications backplane that links them together so that a stationconnected to a port on one module can easily communicate with astation on another module. F igure 6 illustrates a modular hub.
Modular hubs typically range i n size from four to fourteen slots,so the network can be easily expanded. Typi cally several slots in amodular hub will be filled with 10BASE-T Ethernet modul es. Forinstance, with ten modules, each supporting twelve users, a single hub could support 120 users over 10BASE-T. The modules are linked by thehigh-speed backplane, which can also be used to connect thecommunications modules to a ma nagement module that manages all of thecards in the chassis. In addition to us ing one management module fora large number of ports, all of the modules share a common powersupply. Another advantage of some modular hubs is that Ethernet, Token Ring, and even FDDI communications modules can be placed in thesame chas sis, using the same common power supplies.
The term internetworking refers to linkingind ividual local area networks together to form a single internetwork.This interne twork is sometimes called an enterprisenetwork because it inte rconnects all of the computer networksthroughout the entire enterprise. Workgr oup LANs on different floorsof a building or in separate buildings on a busines s campus can belinked together so that all of the computing systems at that sit e areinterconnected. Geographically distant company sites can also be tiedtog ether in the enterprise-wide internetwork.
An individual LAN is subject
to limits on such things as how far itcan extend, how many stations can be conn
ected to it, how fast datacan be transmitted between stations, and how much tra
ffic it cansupport. If a company wants to go beyond those limits
There are tw
o main reasons for implementing multiple LANs andinternetworking them. One is
to extend the geographic coverage of thenetwork beyond what a single LAN can su
pport
There are three major types of dev ice used for internetworking:bridges, routers, and switches. Today the most co mmonly usedinternetworking devices are high-speed routers, especially inwide-a rea internetworks linking geographically remote sites. Butrouters are also hea vily used in building and campus internetworks, aswell. Bridges have also been popular even though the offer lessfunctionality than routers because they are less expensive topurchase, implement and maintain.
LAN switches are a n
ew class of internetworking device, and manypeople believe that switched intern
etworks will become the most commondesign for linking building and campus LANs
in the future.Today
Bridges and routers are bothspe
cial kinds of devices used for internetworking LANs
Bridges and routers link adjacent LANs. Local bri dges and routerswere first used to extend the area a network could cover by all owingusers to connect two adjacent LANs to maintain performance by reducingthe number of users per segment. Both Ethernet and Token Ring specifylimits on ma ximum distances between workstations and hubs, hubs andhubs, and a maximum numb er of stations that can be connected to asingle LAN. To provide network connec tivity for more people, orextend it to cover a larger area, it is sometimes nec essary to linktwo different LANs or LAN segments. Bridges and routers can both provide this function.
Today, however, these internetworking devices ar e also increasinglyused to segment LANs to maintain performanc e byreducing the number of users per segment. When users on a single LANbegin to experience slower response times, the culprit is oftencongestion: too much traffic on the LAN. One method users areemploying to deal with this is to brea k large LANs with many usersinto smaller LANs, each with fewer users. Adding n ew network usersmay require the organization to create new LANs to accommodate them.Implementing new applications on an existing LAN can create so muchincrem ental traffic that the organization may need to break the LANinto smaller LANs segments to maintain acceptable performancelevels.
In all of these cas es, it is still critical that users on one LANbe able to reach resources on oth er LANs within the organization. Butthe LANs must be connected in such a way t hat packets arefiltered, so that only those packets that need topass from one LAN to another are forwarded across the link. Thiskeeps the p ackets sent between two stations on any one LAN fromcrossing over onto the othe r LANs and thereby congesting them. Ageneral rule of thumb says that 80 percen t of the packets transmittedon a typical workgroup or department LAN are destin ed for stations onthat LAN. Both bridges and routers can be used to segment LA Ns.
Bridges are the simpler, and often less expensive
, type of device.Bridges filter packets between LANs by making a simpleforward
/don
Routers are
more complex internetworking devices and are alsotypically more expensive than
bridges. They use Network LayerProtocol Information within ea
ch packet to route it from oneLAN to another. This means that a router must be
able to recognizeall of the different Network Layer Protocols that may be used
on thenetworks it is linking together. This is where the termmultipr
otocol router comes from
Switch
es are another type of device used to linkseveral separate LANs and pr
ovide packet filtering between them. ALAN switch is a device with multiple por
ts, each of which can supporta single end station or an entire Ethernet or Toke
n Ring LAN. With adifferent LAN connected to each of the switch
Switches are used to increase performance on anorgani
zation
Deploying technology c alled dedicated LANs isanother advantage of using switches. E ach port on an Ethernet switchsupports a dedicated 10 Mbps Ethernet LAN. Usual ly these LANscomprise multiple stations linked to a 10BASE-T hub (see Figure 7) ,but it is also possible to connect a single high-performance station,such as a server, to a switch port. In this case, that one stationhas an uncontested 1 0 Mbps Ethernet LAN all to itself. Packetsforwarded over it from other ports o n the switch will never produceany collisions because there are no other statio ns on the LAN at thatport.
Using LAN switches a llows a network designer to create severalsmall network segments. These smaller segments mean that fewerstations are competing for bandwidth, thereby diminish ing networkcongestion. |
As was noted earli
er, LAN switching is a relatively new technology.Today
LAN technology is evo
lving. In the early 1980s LANs were strictlylocal area networks, linking small
groups of computers in companydepartments. As workgroup LANs proliferated ove
r the past ten years,users began connecting them to form internetworks, first w
ith bridgesand later with routers. Today
The next few years will see ne tworks evolve to include moresophisticated LAN switches and switching hubs. The y will be designedusing several different types of components, both old and new .Ethernet and Token Ring LANs will be built with stackable workgrouphubs which , in turn, will be interconnected by larger modular hubsthat may incorporate LA N switching functionality. Large networks willinclude another layer of consoli dation with networkcenter hubs linking workgroup hubs and swit ches. Routerswill continue to be used as gateways to the wide-area network lin kingother buildings and remote sites.
For networks to deliver the perfo
rmance today
Born from the merger of SynOptics Communications and WellfleetCommuni
cations on October 20, 1994, Bay Networks, Inc., is one of theworld
Headquartered in Santa Clara, California, Bay Networks m anufacturesand markets a comprehensive line of networking equipment used to bui ldboth small and large-scale corporate networks for companies around theworld.
Through both direct and indirect channels, the company sells acomplete line of intelligent hubs, high-speed switches, multiprotocolrouters and sophis ticated network management systems to virtuallyevery Fortune 100 company.
The foundation of Bay Networks networking solutions is its systemof intellig ent hardware and software products. Designed to meetcurrent and future networki ng needs, these solutions provide theflexibility to create a network today that can easily grow into avast, multi-enterprise network in the future.
Ba
y Networks
Bay Networks markets these products to large and small en d-userorganizations through a combination of original equipmentmanufacturers ( OEMs), distributors, value-added resellers and a directsales force. Typical ta rget users include worldwide retailers, foodservice companies, financial instit utions, technology manufacturers,telecommunications companies, hospitals and un iversities, andgovernment organizations.
A representative list of Bay N
etworks customers include: AT&T,Australia Department of Social Services, Bank o
f InternationalSettlements, Bear Stearns, Boeing Aircraft, British Petroleum, C
haseManhattan Bank, Ford Motor Company, General Motors, McDonald
The company has a co mbined market value in excess of $2.5 billionand employs a staff of over 3,000 around the world. As a market shareleader with over 23 percent of the estimate d $4.6 billioninternetworking market, Bay Networks has an installed base of ove r 10million end-user connection for its intelligent hubs products and morethan 200,000 port connections installed for its router products.
The company pioneered the networking industry in the mid-eightiesby innovating the ability to run Ethernet networks over common phonewire as well as being one of the fir st companies to bring to markethigh-speed, multiprotocol routing.
Addit ionally, Bay Networks has a number of strategic developmentand technology partn erships with a variety of industry leadingcompanies including: IBM, Novell, Int el, Hewlett-Packard and SunMicrosystems.
Publicly held and traded on NA SDAQ, Bay Networks is led by PaulSeverino (Wellfleet), chairman of the board an d by Andy Ludwick(SynOptics) president and CEO.
Asynchronous Transfer Mode (ATM)
Backbone
Bridges
Carrier Sense Multiple Access with Collision Detection(CSMA/CD)
Collision
EMI
Ethernet
Fiber Distributed Data Interface (FDDI)
IEEE 802.3
IEEE 802.5
Intelligent Hubs
Inter-repeater link
LAN Internetwork
Local Area Network (LAN)
Manageable Hubs
Modular hubs
Network Inte
rface Card (NIC)
Multi-station Access Un
it (MAU)
Open Systems Interconnect Reference
Model (OSI)
Packet
Routers
Shared Access
Shielded twisted pair (STP)
Stackable hubs
Stand-alone hubs
Structured wiring architecture
Switches
10BA
SE-T
10BASE-5
10BASE-2
Token Passing
Token Ring
LANs ar
e complex systems that implement many different services inorder to provide com
munication between all of the types of devicesthat can be connected to them. A
communications model called the OpenSystems Interconnect (OSI) Reference Model
was developed by theInternational Standards Organization (ISO) to define all o
f theservices a LAN should provide (see Figure 8). This model definesseven la
yers, each of which provides a subset of all of the LANservices. This layered
approach allows small groups of relatedservices to be implemented in a modular
fashion that makes designingnetwork software much more flexible. A network sof
tware module thatimplements services at the Network and Transport Layers of the
modelcan be paired up with different Physical and Data Link Layer modulesdepe
nding on the requirements of the user
The Internation al Standards Organization (ISO), the primarystandard-setting body in the data c ommunications industry, developed aframework for LAN standards called the Open Systems Interconnectionreference model. This reference model represents a stand ard approachto communicate information throughout a network so that a variety o findependently developed computer and communications devices caninteroperate.< /strong> |
But the OSI model doesn
Because they are approved and published by the IEEE, both theEthernet and Token Ring protocols are said to be industry standards.Any company can acquire the specificat ions and design Ethernet orToken ring NICs. Users can purchase an Ethernet NIC, for example,from any vendor and be assured that it will operate in a network withEthernet NICs from other vendors. This degree ofinteroperability is highly desirable.
However, there are many more protocols for providing services atthe higher layers of the OSI model and very few of them have beenapproved by an international standards bodies. In fact, most upperlayer LAN protocols are incorporated into
proprietary networkoperating systems, such as Novell
Ethernet and Token Ring technologies are just one part of acomplete LAN. They provide the services specified in the Physical andData Link Layers of the OSI model, but several other services must beadded on top o f the connectivity of Ethernet or Token Ring. Networkoperating systems (NOSs) are most often used to provide the additionalcommunications services.
A NOS defines client and server systems. Clients are individualuser workstations attached to the ne twork where application programsare run and data is generated. Servers are shared network resourcesthat provide hard disk space for users to store files, printerservices, and a number of other network services. The networkoperating system provides a set of protocols in software that run onboth servers and client systems and allow them to communicate witheach other, share files, printers and other network resources.