LAN Network Specifications
Introduction. LANs - Local Area Networks are really the basic building blocks of all internetworks. These technologies are implemented at the Data Link Layer of the OSI model or layer 2. This is because these network technologies are largely determined by the physical media they share and how they control access to this shared medium. This Data Link Layer is also called the MAC - Media Access Layer. The basic traffic format at this level is called a frame.
So, in LANs, communication can only deal with MAC addresses which are serial number like device identifiers. Things like IP addresses are only necessary when routing data across LAN segments through an internetwork. These 2nd layer technologies can only support switched internetwork operations. They are only good for local areas or simple paths over longer distances, where not much guidance to deliver the data is needed.
As such, LANs can be classified into two major categories
Access LANs - these accept cabling or wireless connection from devices, tie workgroups together and share local resources such as departmental printers and servers. These segments are formed/focused by hubs or access switches. These are usually within a department or floor of a building.
Backbone LANs - these link access LANs, such as database servers, mail servers and other larger coverage area shared devices. These are formed/focused by routers or LAN switches. These are usually across an entire building or office campus.
Some Other Important Issues. Now I want to spend a little time discussing some issues about quality of service QoS. I want to develop these issues at present so we can use them to compare different technologies.
Certain types of traffic need predictability more than others. For example, phone conversations can't tolerate delays in transmission because it would disturb the flow of conversation. Video streaming has somewhat similar requirements, though the situation is more flexible. Latency Sensitivity is the term used to characterize these sorts of sensitivity to delays in the transmission. Priority Sensitivity is somewhat similar, but has more to do with the order in which data is received. For example, in video streaming, some small delays are tolerable, particularly with buffering, but if they aren't received in the proper order then there are going to be major problems.
A broadcast message goes to every receiving device within a particular domain. Obviously, we all get radio and television broadcasts through different media. Broadcasting on the internet can result in huge overheads on infrastructure demand and is not something generally acceptable. A multicast message goes only to predetermined receiving devices within the broadcast domain. While getting multicasting to work properly is a real problem, it still is going to be preferable to broadcasting on the internet.
Ethernet. This is the most common form of LAN. This seems to be due to economies of scale and some other issues that will be presented in this page. It was first developed by Xerox in 1970. Then Digital Equipment Company, Intel and Xerox released a second version often called DIX Ethernet in 1982 an effort to overcome many of the difficulties in the original.
Ethernet communications operate by contention. Devices sharing an Ethernet listen for other traffic and transmit only if the LAN is clear. If two stations send at about the same time and their messages collide, both transmissions are aborted and they wait for a randomly generated period of time before retransmitting. Ethernet uses what is called the CSMA/CD - Carrier Sense Multiple Access with Collision Detection protocol to regulate traffic.
In addition, because the connection medium is shared, every device on an Ethernet LAN segment receives every message and checks to see whether the destination address matches its own. If there is a match then the message is accepted and processed. If no match is found, the message is dropped.
All of this makes it sound as if Ethernet is very inefficient and it is. So much of the available bandwidth is lost to aborted transmissions that the theoretical effective bandwidth is only about 37% of what is actually available! On the other hand, the equipment required to deal with such simplistic protocols is so inexpensive in comparison to other approaches that it ends up usually being the most cost effective.
Ethernet has a pretty large variety of implementation options. Three of the most well known are described in the following tables.
Ethernet Option Speed Connections
Original 1Mbps coax cable or 10BaseT twisted pair
Fast 100Mbps 100BaseTX twisted pair or 100BaseFX fiber optic
Gigabit 1000Mbps 1000BaseTX twisted pair or 1000BaseFX fiber optic
A currently popular configuration is to have Fast Ethernet Access LANs interconnected through a Gigabit Ethernet backbone LAN.
Token Ring. Token Ring used to be Ethernet's main competition. By its own standards it is incompatible with Ethernet in terms of NICs, cable connectors and the software that must be used. Token Ring tends to be most widely used in enterprises dominated by IBM based computing.
Token Ring gets its name from the fact that it defines attached devices into a logical, though not likely physical, ring. We call it is logical ring because it passes signals as if the devices were attached by a single looped cable. Physically, they may well be in a hub and spoke layout called a star topology.
Token Ring avoids contention over a LAN segment by using a token passing protocol. Only the device in possession of the token is allowed to transmit, thus eliminating collisions. This approach definitely increases Token Ring's effective use of available bandwidth. Testing shows that Token Rings can attain up to 75% of the available bandwidth.
Unfortunately, the costs of implementing Token Rings are relatively higher than the costs of implementing Ethernet. It is reasonably estimated that the LAN segments must have at least about 40 users to become cost effective. Once the traffic on a LAN segment increases to certain levels the collisions involved in Ethernet become relatively more costly to the users. The following diagram represents the break even.
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