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Lan Topologies Essay, Research Paper

When designing the layout of a local area network, an organization can choose from a variety of

different technologies on which to base the backbone of their LAN. The technology choices that the

organization adopts will play a critical role in the performance of its LAN. Two important LAN

backbone issues, which I will introduce, include:

? A network based on Ethernet or Token Ring technology

? Factors to consider when choosing a Network Operating System

LANs are also known as subnets, which are single networks. Overall, LANs use a hybrid

TCP/IP-IEEE (Institute of Electrical and Electronic Engineers) framework consisting of 6 layers:

application, transport, internet, logical link control layer, media access control layer, and physical

layer. The IEEE has many standards committees to develop standards for LANs. The most

widely known is the 802.3 Working Group, whose Ethernet standards dominate LAN technology.

Another is the 802.5 Working Group, which created the competing Token-Ring network

standards used in many organizations. At the media access control layer (MAC), CSMA/CD

(Carrier sense multiple access/collision detection) governs communication between stations.

Ethernet uses a bus transmission, which is characterized by broadcasting. One station transmits,

and this transmission is broadcasted to all other stations. In Ethernet 10Base-T and 100Base-TX,

the hub does the actual broadcasting. In contrast, Token Ring network stations connect to access

units, which are connected in a ring. A special frame called a token signals around the ring and

stations which grab hold of the token may transmit. Whereas Ethernet uses CSMA/CD for the

MAC layer, Token Ring networks use token passing to determine when each station may transmit.

The growth of today’s LAN traffic is pushing network administrators to look too higher-speed

network technologies to solve the bandwidth problem. Highly reliable networks are critical to the success

of the business organization, so ease of installation and support are primary considerations in the choice

of network technology. Network administrators today have several alternatives to choose from. Although

each network faces different issues, Gigabit Ethernet meets several key criteria for choosing a high-speed

network

? Easy, straightforward migration to higher performance levels without disruption

? Low cost of ownership—including both purchase cost and support cost

? Capability to support new applications and data types

One of the most important questions network administrators face is how to get higher bandwidth

without disrupting the existing network. Gigabit Ethernet follows the same form and function as its 10

Mbps and 100 Mbps Ethernet precursors, allowing a straightforward migration to higher-speed

networking. All three Ethernet speeds use the same IEEE 802.3 frame format, full-duplex operation and

flow control methods. In half-duplex mode, Gigabit Ethernet employs the same fundamental CSMA/CD

access method to resolve contention for the shared media. Simply stated, Gigabit Ethernet is Ethernet,

only faster. Because of the high scalability of Ethernet technology, it is simple to connect existing lower-

speed Ethernet devices to Gigabit Ethernet devices using LAN switches or routers to adapt one physical

line speed to the other. This evolutionary upgrade path allows Gigabit Ethernet to be seamlessly

integrated into existing Ethernet and Fast Ethernet networks.

Network administrators today face a myriad of internetworking choices and network design options.

They are combining routed and switched networks, and building intranets of increasing scale. Ethernet

networks are shared (using repeaters) and switched based on bandwidth and cost requirements. The

choice of a high-speed network, however, should not restrict the choice of internetworking or network

topology. Gigabit Ethernet can be switched, routed and shared. All of today’s internetworking

technologies, as well as such technologies such as IP-specific switching and Layer 3 switching, are fully

compatible with Gigabit Ethernet, just as they are with Ethernet and Fast Ethernet. Gigabit Ethernet is

available in a full duplex repeater (with the accompanying low cost per port) as well as on LAN switches

and routers.

Today, Ethernet networks are rapidly approaching the reliability level associated with their telephone

ancestors, and are relatively simple to understand and administer. The development of Gigabit Ethernet

extends the scalability of Ethernet even further. Now Ethernet scales from 10 to 100 to 1000 Mbps. There

has been a rapid overall decrease in the per-port price for Fast Ethernet and Ethernet products, and for

shared hub ports, the difference between their costs is narrowing. Over time, Gigabit

Ethernet per port costs can be expected to experience similar price decreases.

First introduced in 1985, the Token Ting evolution got underway in 1989 when the first

16mbps Token Ting adapter arrived. Token Ring switching later arrived in 1995 providing high

speed backbone connectivity between rings and enabling servers to benefit from dedicated full

duplex 16 Mbps connections. In his 1998 research paper “High Speed Token Ring”, Martin

Taylor explains that Token Ting switches provide high speed backbone connectivity between

multiple 4 or 16 Mbps rings. Servers can be connected directly to the switch, benefiting from a

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