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Transparent LAN Service:

The Simplest Form of VirtualPrivate Network

Larscom2 Text 6/9/99 10:25 AM Page 1

Table of ContentsTransparent LAN Service:The Simplest

Form of Virtual Private Network. . . . . . . . . . . . . 2What is Transparent LAN Service? . . . . . . . . . . . . . 3Why Do Corporations Need Transparent

LAN Service?. . . . . . . . . . . . . . . . . . . . . . . . . . . 4How Does Transparent LAN Service

Address These Problems? . . . . . . . . . . . . . . . . . . 4Comparison of Transparent LAN

Service to Alternatives . . . . . . . . . . . . . . . . . . . . . 9How Transparent LAN Service is Delivered . . . . . . 11Why is Transparent LAN Service Attractive

to Network Service Providers?. . . . . . . . . . . . . . 18Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Case Study 1 . . . . . . . . . . . . . . . . . . . . . . . . . . 22Case Study 2 . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25About the Editors…Jerry Ryan is a principal at ATG and the Editor-in-Chief oftechguide.com. He is the author of numerous technology papers on various aspects of networking. Mr. Ryan has developed and taughtmany courses in network analysis and design for carriers, governmentagencies and private industry. He has provided consulting support in the area of WAN and LAN network design, negotiation with carriers forcontract pricing and services, technology acquisition, customized soft-ware development for network administration, billing and auditing oftelecommunication expenses, project management, and RFP generation.Mr. Ryan has been a member of the Networld+Interop ProgramCommittee and the ComNet steering Committee. He holds a B.S.degree in electrical engineering.

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security issue, but also allow service providers to guar-antee bandwidth to their customers. Becausetransparent LAN service is fully managed, the provideralso installs and maintains the equipment, relieving theend user of all such tasks.

What is Transparent LANService?

Transparent LAN is a flexible, high-speed VPNservice that hides the complexity associated with thewide area network (WAN) from the end user. Withtransparent LAN service, a provider interconnects anenterprise’s local area networks (LANs), regardless oftheir physical location, in such a way that the WANservices supporting the network are not apparent to thecustomer.

With this service offering, users avoid the difficultchallenges of owning and operating their own widearea network infrastructure. Gone are the headaches ofcomplex wide-area network design, routing protocoltuning and optimization, network management issues,plus the risk and expense of keeping up with each newadvance of technology. The organization’s MIS teamneed only retain the skills and disciplines that pertainto managing a LAN and internal applications.

Transparent LAN service delivers benefits to endusers and service providers alike. End users receive anaffordable solution that is easy to manage. Serviceproviders have an opportunity to not only solve theircustomers’ internetworking problems, but to provideadditional value by taking over the managementburden and risk of wide area connections.

Technology Guide • 3

Transparent LAN Service:The Simplest Form of VirtualPrivate Network

A virtual private network (VPN) is a service inwhich a carrier provides an end user with what appearsto be a network of dedicated resources, when in factthe information is running over a shared infrastructure.The network infrastructure that supports VPNs can beeither the Internet or another type of backbone, suchas a service provider’s ATM network. VPNs deliver thebenefits of a private network (security and availability)and the benefits of a public network (economies ofscale and freedom from management burden).

Since information on a VPN is travelling on ashared facility, security is often considered the mostimportant feature of a VPN, and it represents one ofthe greatest challenges to service providers. AnInternet-based VPN might use encryption, firewalls,and other security techniques. Realistically, however,data is only secure on the service provider’s portion ofthe Internet when it travels through encrypted tunnels.On the other hand, VPNs that are built on ATM-based transparent LAN services derive their securityfrom ATM Permanent Virtual Circuits (PVCs), whichare typically managed by a single service provider. Thisminimizes security risks and enhances VPNperformance.

Transparent LAN service is a specialized VPNoffering that is straightforward for service providers toprovision and simple for end users to implement. WithATM, customers work with familiar technology, andATM’s inherent characteristics not only resolve the

2 • Transparent LAN Service: The Simplest Form of Virtual Private Network


WAN design and management from the enterprise tothe carrier, however, is probably the single most impor-tant value of transparent LAN service.

Support For Any ProtocolTransparent LAN service is a protocol indepen-

dent service. It is not restricted to routed IP and mayeliminate the need for encapsulating other protocolsinto IP for transmission across the WAN. All LANprotocols can continue to run in their native formacross the WAN. IP, IPX, SNA, Appletalk, and evenproprietary protocols can be transmitted seamlesslyacross a transparent LAN service.

Scalable ServiceThe most natural model for Transparent LAN

Service is for full LAN bandwidth operation (such as10 Mbps for Ethernet). However, using traffic manage-ment capabilities inherent in their network infrastruc-ture, service providers can offer transparent LANconnections at subrate speeds. For example, a nativeEthernet connection could be offered at 2 Mbps or aFast Ethernet connection could be offered at 30 Mbps.This flexibility allows the service provider to offer andprice the service in increments that best meet thecustomers’ needs. This also allows the customer to buy,for example, a Fast Ethernet connection at 20 Mbpsand have built-in scalability up to 100 Mbps withouthaving to upgrade any equipment or change thenetwork design.

Comprehensive ServiceTransparent LAN service encompasses all LAN

types: Ethernet, Fast Ethernet, Token Ring, and FDDI.Although Ethernet is the predominant LAN type,


Why Do Corporations NeedTransparent LAN Service?

Typically, an MIS manager’s challenges includethe following:

• Great user expectations

• Perennial budget shortfall

• Resource shortage combined with desire to focuson core competencies

• Constantly evolving complexity of WANtechnology combined with WAN skills shortage

How Does Transparent LANService Address TheseProblems?

To understand the benefits of transparent LANservice, we must first understand what transparentLAN service is.

Native Interconnection Between LANsTransparent LAN service gives the appearance

that several sites are all connected to the same LANsegment. Transparent LAN service can be thought ofas the service provider building a “campus backbone”over the wide area, where each site represents a loca-tion on the campus and the transparent LAN serviceacts as the campus backbone LAN. The network thatsupports this service offering is sophisticated, robust,and typically based on ATM. Shifting the burden of



Platform For Value Added ServicesA feature of transparent LAN service that is both

subtle and profound is that it can become a platformfor the service provider to offer additional value-addedservices over the same infrastructure. Because theservice provider connects directly to the customer’sLAN, as opposed to an extraneous wide-area interface,the provider can offer a routed connection to theInternet or secure community-of-interest network,(internet), all from the one LAN connection point. Thisallows the customer to subscribe to additional value-added services in the most convenient and low-risk waypossible.

Benefits of Transparent LAN ServiceTo an end user, the major benefits of transparent

LAN service are:

• Simplicity. The customer does not have to learncomplex technologies such as frame relay orATM. The customer works only with familiarLAN technology and leverages the serviceprovider’s expertise in the wide-area.

• High speed, and just the right speed. TransparentLAN service can be offered at full native LANspeeds (10 Mbps Ethernet, 4 or 16 Mbps TokenRing, 100 Mbps Fast Ethernet or FDDI). Notethat when the service is offered at native LANspeed, there is no speed mismatch between theLAN and WAN and, therefore, no bandwidthbottleneck. When a customer doesn’t require fullLAN-speed bandwidth, the service can be scaledappropriately (for example, to 2 Mbps forEthernet LANs or 20 Mbps for Fast EthernetLANs). The customer only pays for the bandwidthhe needs. When a customer’s needs change, theservice can easily be upgraded without new equip-ment or network reconfiguration.


other LANs have significant positions in some marketsthat, because of their need for high-speed, reliableLAN interconnection, have found transparent LANsolutions very attractive. Examples include the financeindustry, health care, and Internet service providers.Therefore, a transparent LAN offering will typicallyinclude Token Ring and FDDI options.

Flexible ServiceAlthough the basic model of transparent LAN is a

simple homogeneous LAN segment, the service typi-cally has a great number of options that allow theservice offering to be tailored to meet the particularneeds of the customer. For instance, a transparentLAN service may allow bridging betweenheterogeneous LANs, typically the ability to connect ata higher speed to a LAN like FDDI or Fast Ethernet ata primary location and regular Ethernet at regionallocations.

Managed ServiceIn addition to network design and management

functions, the service provider can offer other value-added management functions such as:

• Customer management views. The capability foreach customer to have a management view intotheir own backbone LAN. To be meaningful, thisview must present the management information ina LAN format.

• Usage-based billing. The option for the user topay by usage rather than by connection.

• Customer network monitoring. The ability of theservice provider to monitor and troubleshootcustomer-premise equipment (CPE).



Comparison of TransparentLAN Service to Alternatives

The following sections compare transparent LANservice to the alternatives of using leased lines orpublic data services.

Comparison to Leased LinesFor networks that consist of only two locations,

private lines may offer the most cost-effective solution.However, because of the high equipment requirementsand the dedicated nature of private line networks, theyare neither scalable nor flexible. With private lines,every time another network location is added, there areadditional on-site and off-site costs. Adding, moving, ordeleting a network location in a leased-line networknecessitates a substantial amount of time to reconfigureexisting locations.

In addition, leased lines may not provide the exactbandwidth that will suit a company’s needs. T1/E1lines offer only 1.544 or 2.048 Mbps of service, whichdoes not provide much room for growing demands.T3/E3 lines offer 45 or 34 Mbps of bandwidth —more bandwidth than most companies currently need.Fractional T3/E3 services provide an alternative whichmore closely matches companies’ current bandwidthrequirements and which provides room for futuregrowth, especially in locations where higher-speedaccess services are unavailable or too expensive.

Private-line networks require a degree of manage-ment that is not required with transparent LANservice. The MIS manager is responsible for creatingthe architecture of the network, managing the termi-nating equipment (CSU/DSUs and/or inverse multi-plexers), troubleshooting, and network modification.


• Improved access to information. TransparentLAN service provides customers with ubiquitoushigh-speed access to information at all intercon-nected sites. It also delivers the bandwidth neces-sary to support the latest applications.

• Cost savings. Subscribing to a transparent LANservice is generally less expensive than the cost ofbuilding, managing, and maintaining a wide areanetwork, especially when the costs of recruiting,hiring, and training administrative and technicalpersonnel are factored in. Transparent LANservice can also be a great value compared toother services that interconnect LANs (frame relay,native ATM service). A cost/performancecomparison of transparent LAN service to alter-nate solutions will be covered in greater detail.

• Less risk. Customers protect themselves from tech-nology obsolescence.

• Future-proof solution. Providers can add new sitesand upgrade to higher-speed service as needschange.

• Efficient utilization of resources. With transparentLAN service, companies can centralize their serverresources, since employees at each site enjoy high-speed access to servers housed in a single location.Centralizing resources allows companies to enjoycost savings because of the reduction in capitalequipment and maintenance costs.

• Single point of contact. A service provider takesfull responsibility for isolating, identifying, andrectifying network problems. The customer knowswho to call.

• Overcomes distance limitations of LANs.Transparent LAN service can surpass the 2.8 kilo-meter limit of Ethernet and span an extendedmetropolitan area or wide area.



When comparing transparent LAN service toalternate solutions, performance must be considered.While native ATM services are offered at very highspeeds, many frame relay services are offered at speedslower than LAN speeds, which can result in a networkbottleneck. Even in locations where higher speed framerelay services are available, they can still be slower thanLAN speed and could impact network performance.

Some service providers offer managed frame relayservices that do reduce the customer’s managementburden. Managed services include WAN CPE and itsmanagement and maintenance with the frame relayservice. For customers who are currently frame relayusers, this is one way to relieve network administration.However, the burden of the WAN design, planning,and optimization is still on the customer. Those whoare not familiar with frame relay must learn the tech-nology and understand how to apply it to theirnetwork requirements. And, of course, the network willstill be limited to the speeds supported by the framerelay service, often sub-LAN speeds.

How Transparent LAN Serviceis Delivered

The BackboneTransparent LAN services can be supported by

many different network technologies, including leasedlines, xDSL, Frame Relay, or ATM. The choice ofservice type will depend greatly on the speeds neededto meet the performance goals of the business applica-tion and traffic volume. In some cases leased lines orFrame Relay will suffice, but other cases may need thehigher capacity of ATM or SONET.


Comparison to Public Data ServicesPublic data services include frame relay and native

ATM. Transparent LAN service offers several advan-tages over both frame relay and ATM public dataservices. With frame relay and native ATM services,the end user is generally responsible for testing,installing, and implementing WAN equipment. Withtransparent LAN service, most service providersinclude the WAN equipment and assume responsibilityfor managing and maintaining the edge equipment(i.e., the equipment that interfaces the serviceprovider’s network to the customer’s network).

As we’ve already discussed in general terms, trans-parent LAN service is a less complex service thanframe relay and ATM. From the MIS manager’sperspective, fewer network components are involvedwith transparent LAN service. With public frame relayservices, an access piece (i.e., local access circuit), framerelay port, and permanent virtual circuits (PVCs) aredefined for each location. Native ATM service is morecomplicated because each PVC is assigned class-of-service and perhaps quality-of-service parameters. Withtransparent LAN service, there are two main compo-nents: the LAN interface at each location and thePVCs between locations. Local access circuits are oftenincluded in the service.

With frame relay and ATM services, the customerhas to make a significant investment in WAN accessequipment because the equipment is not included aspart of the service offering. In addition, piecing aframe relay or ATM network together can be timeconsuming. The process includes bidding, evaluation,installation of equipment and access circuits, end-to-end testing, and troubleshooting. This entire processcan take six months or longer. Providers of public dataservices assume that end users will employ a dedicatedtelecom person for maintenance.



Technology Guide • 1312 • Transparent LAN Service: The Simplest Form of Virtual Private Network

• Versatile. Whether the market demandsEthernet, Fast Ethernet or, eventually, GigabitEthernet, ATM will be able to support it. And, byusing ATM as a single infrastructure supportingother value-added ATM-based services such ashigh-speed Internet access and intranet/extranetofferings, the service provider can defray the infra-structure cost across several different services.

Compared to traditional LAN technologies, ATMcan be complicated. Transparent LAN service is oneway for end users to take advantage of the benefits ofATM without being exposed to its complexities.

At the EdgeTo subscribe to a transparent LAN service, the end

user need only provide the service provider with aLAN interface from each site to be interconnected.Since the end user’s network is running LAN protocolsand the service provider’s backbone is most likelyATM, a device at the “edge” of the provider’s network(where the end user’s network begins) is needed to packLAN frames into ATM cells. Larscom’s EDGE equip-ment is designed for this role.

EDGE devices, which are owned by the serviceprovider, are the service-enabling intelligence in thenetwork. Larscom’s EDGE concentrator can be housedat the customer premises or in the service provider’spoint of presence. The service provider connects fromthe concentrator to the customer’s LAN segment ateach site. The concentrators are then interconnectedvia permanent virtual circuits (PVCs), either in a fullmesh or in a more streamlined topology (as in Figure1). With the appropriate separation of customers’traffic both in the concentrators and in the allocationof PVCs, the service provider creates a virtual privatenetwork for that individual customer.

ATM is the technology of choice for many dataservices because of the following distinguishing charac-teristics:

• Efficient multiplexing of data streams.Despite a “cell tax” overhead of 10 to 12%, ATMis the most efficient, high-speed statistical multi-plexing technology. A single ATM network oper-ated by a service provider is more efficient than acollection of equivalent-capacity private networks.

• Quality of service. ATM has Quality ofService (QoS) and traffic management controlsthat allow service providers to offer and ensureservice level guarantees to their customers. Andbecause the data is handled in small, uniformcells, the network has very fine grained servicecontrols. Therefore, the benefit of the “cell tax” isprecise control of traffic. ATM also supportsdifferent classes of service (such as lower-prioritydata and higher priority voice traffic).

• Scalability. Providers can offer their customersthe bandwidth they need today and can easilyincrease the bandwidth as customers’ needschange. And, because ATM allocates bandwidthon demand, service providers can oversubscribethe network. The bandwidth customers need isavailable when they need it and what they don’tuse is available for other customers. ATM is theonly available technology that can scale up to 2.4gigabits per second.

• Traffic integration. An ATM infrastructurecan support all traffic types, including voice andvideo.

• Future-proof. ATM can support a variety ofsimultaneous, value-added services such as high-speed Internet access and intranet/extranet offerings.


Figure 2: A multi-customer implementation of transparent LANservice. Customer XYZ and customer ABC share the capacity of theATM pipe at site 1.

The concentrator must be able to allocate the appro-priate bandwidth to each customer according to a servicelevel agreement. It is also quite common that a singlecustomer will require the use of more than one port onthe concentrator (when several LANs are located in onebuilding). In these cases, it is possible to allocate differentamounts of bandwidth for different LANs. For instance,suppose a company’s engineering department andmarketing department have separate LANs. The multi-service access concentrator can be configured so that theengineering department can communicate with anotherlocation at a speed of 5 Mbps and the marketing depart-ment can communicate at a speed of 8 Mbps.

These edge devices also enable advanced featuresof transparent LAN service. Multiservice accessconcentrators which have been designed for serviceproviders, like Larscom’s EDGE series, support sophisti-cated features such as customer management viewsand usage-based billing.

Customer XYZ

Site 1

Customer XYZ

Site 2

Customer XYZ

Site 3

Customer ABC

Site 2

Customer ABC

Site 3

EDGE 85 Edge device or multiservice access concentrator

Permanent virtual circuits

EDGE 85

EDGE 85

EDGE 85

EDGE 85

EDGE 85

ATMNetwork


Figure 1: Transparent LAN service establishes a virtual privatenetwork interconnecting four customer sites.

The traffic management capabilities of the accessconcentrators and of the ATM network should allowservice providers to create secure, multi-customer imple-mentations, in which many customers share the capacityof an ATM pipe (as in Figure 2). This is especiallyimportant in metropolitan areas and large business parkswhere a service provider generally has many customersin close proximity. By consolidating customers’ traffic atthe edge of the network, the service provider canleverage its infrastructure investment. In order for acarrier to create a multi-customer environment, theconcentrator must be designed to support manycustomers; that is, it must have a watertight scheme forensuring the security and integrity of customers’ data.

Customer

Site 1

Customer

Site 3

Customer

Site 2

Customer

Site 4

EDGE 85

EDGE 85 Edge device or multiservice access concentrator


EDGE 85

EDGE 85

EDGE 85

ATM

Network




provider would configure a virtual access router in oneof the edge devices and a virtual circuit from one ofthe customer’s sites to an ISP’s POP. All of the sitesthat are interconnected by the transparent LAN servicecan now share the 2 Mbps connection to the Internet.

Figure 3: Transparent LAN service establishes a virtual privatenetwork for athe customer, interconecting all sites at 6 Mbps. High-speed Internet access (2 Mbps) is shared among all sites.

The customer does not need to purchase an accessrouter or any other equipment to receive service. Theservice is delivered to the end user through the singlenetwork connection that was established with theservice provider’s network when transparent LANservice was first offered. In a similar way, the providercan deliver high-speed Intranet or extranet services oroffer access to other information services over the singleLAN connection. The LAN connection becomes the“information plug” through which the customer canpurchase network solutions to his business problems.

Customer

Site 1

Customer

Site 2

Customer Site 4

EDGE 85

EDGE 85

EDGE 85

EDGE 85

ATM

Network

Internet

Edge device or multiservice access concentrator



Why is Transparent LANService Attractive to NetworkService Providers?

The case has been made that transparent LANservice is valuable to customers. Transparent LANservice is also appealing to service providers because it isa “value-added” offering. It is a fully managed service,not just the provisioning of network “pipes” (as is thecase with leased lines). Transparent LAN service buildsthe provider’s reputation as a strategic solution provider.

Transparent LAN service involves the sharing offiber, T1, and/or E1 lines by many customers, so itoffers an economy of scale. Additional data, voice, orvideo services can be sold to existing transparent LANcustomers without additional edge equipment, andnew customers can often share existing edgeequipment. So, the sale of each new service can bemore profitable than the one before.

Examining the inherent economies of scale leadsus to the following. Because the service providerconnects directly to the customer’s LAN, as opposed toan extraneous wide area interface, the service providercan offer additional routing based connections to theInternet, secure community of interest networks(intranets), all from the one LAN connection point.This is best illustrated with an example.

In Figure 3, the service provider has created avirtual private network on a transparent LAN model at6 Mbps. To provide this customer with a high-speedconnection to the Internet (such as 2 Mbps), the



Case Studies

Transparent LAN service answers real-world busi-ness questions such as:

• How can the user provide many regional officeswith high-speed access to centrally located servers?

• How can the user continue to run the businesswhile in the midst of massive reorganization ordepartmental relocations?

• How can the user ensure that the database infor-mation needed by employees is always available even in case of disaster?

The answers are in the following examples oftransparent LAN service:

• Establishing a virtual campus network. This is themost obvious application of transparent LANservice, wherein a company wants to interconnectLANs in buildings not located on single campus.The interconnected LANs can be located across ametropolitan area, across the country or evenbetween countries.

• Relocation service. A transparent LAN service canbe established between locations to facilitate acompany that is either moving to an entirely newlocation or that is expanding into a new location.

• Server replication. A transparent LAN service canbe implemented to ensure that database serverslocated in two separate buildings both receiveupdates in real-time and are always in sync.

• Disaster recovery. When an end user has aprimary and back-up database server and theprimary database crashes, the disaster recoveryservice ensures that traffic is automatically re-routed to the back-up server.

Case Studies • 21

Conclusion

Transparent LAN service is a high-speed, virtualprivate network solution that removes the burden ofwide area networking from the shoulders of MISmanagers. The greatest benefit that transparent LANservice delivers is simplicity — from the end userperspective, transparent LAN service is a much simplersolution for interconnecting LANs than any of thealternatives. It is also the most cost-effective and safesolution because the end user is not required to makerisky or expensive bets on technology.

Transparent LAN service is an appealing proposi-tion to service providers as well. Because it is a value-added service, it can be key to a service provider’sdifferentiation strategy. Because it is a flexible and scal-able service, a provider can satisfy its customers’current and future needs. And, because it can beoffered over an infrastructure that allows the serviceprovider to maximize economies of scale, serviceproviders can profitably deliver service at attractiveprices.



It is important to note that with transparent LANservice, each site has a 45 Mbps link to the other sites,independent of how many sites are interconnected. Inother words, the switched architecture of thetransparent LAN service prevents the sites fromcompeting for the same bandwidth, as they would bein a shared LAN implementation.

An appealing feature of transparent LAN serviceis its network management data. Network problems aremanaged by the service provider, but the bank’s MISmanager can build confidence in the system by moni-toring connectivity and performance. Customer viewsof management data help the customer determinewhether a network problem is internal or external, andtherefore help minimize the number of service calls tothe provider.

Figure 4: Virtual Campus Network for a Bank with RemoteBranches.

Ethernetor Fast

Ethernet

#1

Branch Bank

Ethernetor Fast

Ethernet

#5

Branch Bank

Ethernetor Fast

Ethernet

#2

Branch Bank

Ethernetor Fast

Ethernet

#3

Branch Bank

Ethernetor Fast

Ethernet

#4

Branch Bank

Ethernetor Fast

Ethernet

Bank Headquarters

EDGE 85

EDGE 85

EDGE 85

EDGE 85

EDGE 85

EDGE 85

Case Studies • 23

Following are two actual transparent LAN serviceapplications. The first, a virtual campus network appli-cation, was installed by one of the largest inter-exchange carriers (IXCs) in the United States. Thesecond, a relocation application, is offered by Telia, thelargest network service provider in Sweden.

Case Study 1: Virtual CampusNetwork Application

A financial institution with its headquarters andfive regional facilities located in a metropolitan areahad a requirement to interconnect all of its locations atvery high speeds. Each site had an Ethernet network inplace. One of the reasons that this financial institutionwas seeking a high-speed LAN interconnection solutionwas to implement “imaging”, one of the newest tech-nology trends in banking, in which the bank keeps anelectronic record of every check and transactionconducted by its customers.

The transparent LAN service depicted in Figure 4 isa seamless and easy-to-manage solution. The serviceprovider connects the Ethernet at each branch to anATM switch using a T3 circuit at 45 Mbps, or a set ofup to eight T1 circuits offering up to 12 Mbps. In thefuture, the provider can upgrade the interconnections toOC3c at 155 Mbps or increase the number of T1circuits to each branch. Because the service is fullymanaged by the provider, and because the end userneed not work with WAN technology, it appears to thebank as though there is a single Ethernet segment inter-connecting the sites.



Case Study 2: RelocationApplication

Skanska, a large construction firm in Sweden,wanted to move a workgroup from headquarters to ajob site and maintain access to the central resources atheadquarters. A high-capacity network was needed forreal-time access to technical drawings and bandwidth-intensive applications on the servers at headquarters.

Skanska subscribed to Telia’s transparent LANservice at native Ethernet speed, which provided atransmission capacity of 10 Mbps (as in Figure 5).Telia fully integrated the local office network withSkanska’s headquarters. The link provided by Telia wascompletely transparent to Skanska.

Because it subscribes to the transparent LANservice, Skanska can maintain servers at a single,central location. All network-layer addressing ispreserved, meaning that network administrators arenot forced to create new subnets and reconfigure work-station addresses. And, the remote workgroup canaccess all applications and data from the centrallylocated servers as quickly and easily as if the employeeswere located down the hall from those servers.

Figure 5: Relocation application of transparent LAN serviceproviding a remote warn group with 10 Mbs access to centrally locatedservers.

Telia's

ATM

Network

Central

Server

Skanska's

main office

Skanska's

remote work group

EDGE 85EDGE 85

access to centrally located servers.


Glossary

Asynchronous Transfer Mode (ATM)—(1) TheCCITT standard for cell relay wherein information formultiple types of services (voice, video, data) is conveyedin small, fixed-size cells. ATM is a connection-orientedtechnology used in both LAN and WAN environments.(2) A fast-packet switching technology allowing free allo-cation of capacity to each channel. The SONETsynchronous payload envelope is a variation of ATM.(3) ATM is an international ISDN high-speed, high-volume, packet switching transmission protocolstandard. ATM currently accommodates transmissionspeeds from 64 Kbps to 622 Mbps.

Backbone—(1) The part of a network used as theprimary path for transporting traffic between networksegments. (2) A high-speed line or series of connectionsthat forms a major pathway within a network.

Bridge—A device that connects and passes packetsbetween two network segments. Bridges operate atLayer 2 of the OSI reference model (the data-link layer)and are insensitive to upper-layer protocols. A bridgeexamines all frames arriving on its ports and will filter,forward, or flood a frame depending on the frame’sLayer 2 destination address.

Channel Service Unit/Data Service Unit(CSU/DSU)—A digital interface unit that connectsend user equipment to the local digital telephone loop.

Customer Premises Equipment (CPE)—Terminating equipment, such as terminals, phones,routers and modems, supplied by the phone company,installed at customer sites, and connected to the phonecompany network.

Glossary • 25


Interexchange Carrier (IXC) or InterexchangeCommon Carrier—(1) Any individual, partnership,association, joint-stock company, trust, governmentalentity or corporation engaged for hire in interstate orforeign communication by wire or radio, between twoor more exchanges. (2) A long-distance telephonecompany offering circuit-switched, leased-line and/orpacket-switched services. Inverse Multiplexer—A devicethat divides a single high-speed data stream into severalfor transmission over a set of lower-speed physical lines.Also see “IMA”.

Leased Line—A dedicated, physical telecommunica-tions circuit connecting two or more customer premises.

Local Area Network (LAN)—A network covering arelatively small geographic area (usually not larger thana floor or small building). Compared to WANs, LANsare characterized by relatively high data rates.

Managed Frame Relay Service—A service offeringin which Frame Relay is provided to the customer in afully configured and managed manner.

Multiplexer (MUX)—A device that combines severaldata streams into one for transmission over a single,higher-speed physical line.

Native ATM service—ATM service provided by thecarrier to the customer in its standardized form.

Network Service Provider—A vendor of networkservices - either a local exchange carrier (LEC), compet-itive LEC, Interexchnage Carrier (IXC) or InternetService Provider (ISP).

Glossary • 27

Edge (of the network)—The remote customerpremises, usually configured with a LAN, that accessesthe wider network through an edge access device, suchas an ATM multiservice access concentrator.

Ethernet—The electrical standard that describes acommon method of building a LAN with twisted pairor coaxial cable. Ethernet can be used with almost anykind of computer.

Fast Ethernet—Term given to IEEE 802.3u (calledFast Ethernet) for Ethernet operating at 100 Mbps overCategory-3 or Category-5 UTP cable.

Fiber Distributed Data Interface (FDDI)—AnANSI-defined standard specifying a 100 Mbps token-passing network using fiber-optic cable. Uses a dual-ringarchitecture to provide redundancy.

Frame Relay—Interface for packet-switchingnetworks. Considered more efficient than X.25 which itis expected to replace. Frame relay technology canhandle “bursty” communications that have rapidlychanging bandwidth requirements.

IMA (Inverse Multiplexing over ATM)—A tech-nique of transmitting an ATM data stream over a set ofT1 or E1 circuits, for use when a higher-speed fiberoptic link such as OC3c, DS3, or E3 is not available.

Internet—A collection of networks interconnected bya set of routers which allow them to function as a single,large, virtual network such as the World Wide Web.

Intranet—A private, company-internal network thatsupports web servers and web browsers.



Spanning Tree—An algorithm, the original version ofwhich was invented by Digital Equipment Corporation,used to prevent bridging loops by creating a spanningtree. The algorithm is now documented in the IEEE802.1d specification, although the Digital algorithm andthe IEEE 802.1d algorithm are not the same, and theyare not compatible.

Synchronous Optical Network (SONET)—(1) Aset of standards for transmitting digital information overoptical networks. “Synchronous” indicates that all piecesof the SONET signal can be tied to a single clock. (2) ACCITT standard for synchronous transmission up tomultigigabit speeds. (3) A standard for fiber optics.

Transparent LAN Service—A VPN offered bycarriers based on LAN bridging at the customerpremises. Key characteristics of transparent LANservice are that it supports native LAN speeds in orderto eliminate the bottleneck between the LAN and WANand that it is delivered in such a way that the wide areais transparent to the end user.

Token Ring—As defined in IEEE 802.5, a communi-cations method that uses a token to control access to theLAN. The difference between a token bus and a tokenring is that with a token ring LAN does not use amaster controller to control the token. Instead, eachcomputer knows the address of the computer thatshould receive the token next. When a computer withthe token has nothing to transmit, it passes the token tothe next computer in line.

UNI ATM Service—The User to Network Interfaceof ATM—The point of connection between the userand the carrier’s network.

Glossary • 29

OSI Network Address—The address, consisting ofup to 20 octets, used to locate an OSI Transport entity.The address is formatted into an Initial Domain Partwhich is standardized for each of several addressingdomains, and a Domain Specific Part which is theresponsibility of the addressing authority for thatdomain.

Permanent Virtual Circuit (PVC)—A definedvirtual link with fixed end-points that are set-up by thenetwork manager. A single virtual path may supportmultiple PVCs.

Point-to-Point Network—a WAN in which all of thecustomer sites are connected via direct leased lines.

Points of Presence (POP)—A term used byInternet service providers to indicate the number ofgeographical locations from which they provide accessto the Internet.

Private Line—A leased line.

Quality of Service (QoS)—Term for the set ofparameters and their values which determine theperformance of a given virtual circuit.

Route—A path through an internetwork.

Scalability—The ability to add capacity to thenetwork element to accommodate growth.

Server—(1) A software application that responds withrequested information or executes tasks on the behalf ofa client application. Also, a network host, such as a webserver, running a set of protocol server applications.(2) Any computer that allows other computers toconnect to it. Most commonly, servers are dedicatedmachines. Most machines using UNIX are servers.



Usage Based Billing—Service billing based on theamount of usage incurred by the customer.

Value Added Services—Carrier provided enhance-ments to basic services.

Virtual Private Network (VPN)—A networkservice offered by public carriers in which the customeris provided a network that in many ways appears as if itis a private network (customer-unique addressing,network management capabilities, dynamic reconfigu-ration, etc.).

Wide Area Network (WAN)—(1) A network whichencompasses interconnectivity between devices over awide geographic area. Such networks require publicrights-of-way and operate over long distances. (2) Anetwork that covers an area larger than a singlebuilding or campus.

30 • Transparent LAN Service: The Simplest Form of Virtual Private Network Notes • 31

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32 • Inverse Multiplexing—Scalable Bandwidth Solutions for the WAN

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