ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS
The transmission medium employed
The transmission technique (i.e., baseband or broadband)
Network access protocol
The incorporation of such devices as repeaters, bridges, routers, and switching hubs
The access standard used, where there may be some upper limit on the number of
accesses per segment
The preponderance of LANs operate without error correction with BERs specified in the
range of 1
The most common application of a LAN is to interconnect data terminals and other
processing resources, where all the devices reside in a single building or complex of
buildings, and usually these resources have a common owner. A LAN permits effective
cost sharing of high-value data processing equipment, such as mass storage media, main-
frame computers or minicomputers, and high-speed printers. Resource sharing is probably
equally as important where a LAN serves as the access vehicle for an intranet. Resource
sharing in this context means that each LAN user has access to all other users' file and
other data resources.
The interconnection of LANs in the local area (e.g., the floor of a building) with a
high-speed backbone (e.g., between building floors) is very prevalent. LANs may connect
through wide area networks (WANs) to other distant LANs. This is frame relay's principal
application. The interface to the WAN may be via a smart bridge or a router.
There are two generic transmission techniques utilized by LANs: baseband and broad-
band. Baseband transmission can be defined as the direct application of the baseband
signal to the transmission medium. Broadband transmission, in this context, is where the
baseband signal from the data device is translated in frequency to a particular frequency
slot in the RF spectrum. Broadband transmission requires a modem to carry out the trans-
lation. Baseband transmission may require some sort of signal-conditioning device. With
broadband LAN transmission we usually think of simultaneous multiple RF carriers that
are separated in the frequency domain. Present broadband technology comes from the
cable television (CATV) industry.
The discussion in this section will essentially cover baseband LANs. An important
aspect is that only one user at a time may access a LAN segment at a time. We expect
segments to be isolated one from another by smart bridges or switched hubs.
There are three types of basic LAN topology
: bus, ring, and star. These are illustrated in
Figure 11.1 along with the tree network, which is a simple derivative of the conventional
A bus is a length of transmission medium from which users tap into, as shown in
Figure 11.1a. Originally the medium was coaxial cable. Today coaxial cable is being
phased out in favor of unshielded twisted pair (UTP) or fiber-optic cable.
A ring is simply a bus that is folded back onto itself. A ring topology is illustrated in
Figure 11.1b. User traffic flows in one direction around the ring. In some other approaches
a second transmission medium is added for flow in the opposite direction. Such a dual
counterrotating ring concept improves reliability in case of a failed station or a cut in
Topology means the logical and/or physical arrangement of stations on a network. In other words, topology
tells how these assets are connected together.