Local Area Networks (LAN) Parts Of Network Hardware

Local Area Networks:- 
  

Local Area Networks, generally called Lans,are privately-owned networks within a single building 

or campus of up to a few kilometers in size. they are widely used to connect personal computers

and workstation in company offices and factories to share resource (e.g; printers) and exchange 

information. Lans are distinguished from other kinds of networks by three characteristice: (1) their size, (2) their transmission technology, and (3) there topology.

    

    LANs are restricted in size, which means that the worst-case transmission time is bounded and 

known in advance. knowing this bound makes it possible to use certain kinds of design that would 

not otherwise be possible. it also simplifies network management.

    

    LANs often use a transmission technology consisting of a single cable to which all the machine 

 

are attached, like the telephone company part lines once used in rural areas. Traditional LANs run at speed of 10 to 100 Mbps, have low delay (tens of microseconds), and make very few 

 

errors. Newer LANs may operate at higher speeds, up to hundreds of megabits/sec. in this book, 

we will adhere to tradition and measure line speeds in megabits/sec (Mbps), not megabytes/sec 

(MB/Sec). A Megabit is 1,000,000 bits, not 1,048,576 (2`20) bits.

Various Topologies are possible for broadcast LANs. First Figure shows to of them. In a bus (i.e; 

liner cable) network, at any instant one machine is the master and is allowed to transmit. All other 

machines are required to refrain from sending. An arbitration mechanism is needed  to resolve 

conflicts when two or more machines want to transmit simultaneously. the arbitration mechanism 

may be centralized or distributed . IEEE 802.3, popularly called Ethernet Tm , For example, is a 

bus based broadcast network with decentralized control operating at 10 or 100 Mbps. Computers 

on an Ethernet can transmit whenever they want to; if two or more packets collide, each computer 

just waits a random time and tries again later. 

   A second type of  broadcast system is the ring. In a ring, each bit propagates around on its own, 

not waiting for the rest of the packets to which it belongs, Typically, each bit circumnavigates the 

entire ring in the time it takes to transmit a few bits, often before the complete packet has even 

transmitted, Like all other broadcast system, some rule is needed for arbitrating simultaneous 

accesses to the ring. Various methods are in use and will be discussed later in this book. IEEE 

802.5 (the IBM token ring), is a popular ring-based LAN operating at 4 and 16 Mbps.

  Broadcast networks can be further divided into static and dynamic, depending on how to channel 

is allocated. A typical static allocation would be to divide up time in to discrete intervals and run a 

round robin algorithm, allowing each machine to broadcast only when its time slot comes up. Static 

allocation wastes channel capacity when a machine has nothing to say during its allocated slot, so 

most systems attempt to allocate the channel dynamically (i.e. on demand).

 Dynamics allocation methods for a common channel are either centralized or decentralized. In the 

centralized channel allocation method, there is a single entity, for example a bus arbitration unit, 

which determines who goes next. It might to this by accepting requests and making a decision 

according to some internal algorithm. In the decentralized channel allocation method, there is no 

central entity; each machine must decide for itself whether or not a transmit. You might think that 

this always leads to chaos, but it does not. Later we will study many algorithms design to bring 

order out of the potential chaos. 

    The other kind of of LAN is built using point-to point lines. individual lines connect a specific 

machine. Such a LAN is really a miniature wide area network. We will look at these later.