A primary consideration in selecting communications technology for a LAN is the speed of data transmission required and the need to support other types of transmission (such as video) on the same medium.
A second important consideration is the selection of an access protocol to support the traffic distribution and management control requirements of the LAN-attached devices. An access protocol defines how the devices share the media and support basic configuration and recovery functions.
LAN interconnection is an increasingly important aspect of LAN design. As LANs grow with respect to numbers of attached devices and requirements for increased application connectivity, methods for providing backbone
configurations and access to external systems may impact selection of different LAN solutions and technologies. While interconnection can be achieved in different ways, bridge stations are usually the most attractive and best performing approach. However, systems called routers and gateways might also be considered as alternative solutions. These different network
interconnection techniques will be introduced in section" Physical Layer and MAC Sublayer" on page 57.
End-to-end connectivity within interconnected LANs implies the availability of routes between any two end stations. In communications environments evolving as quickly and continuously as LANs, the ability to cst3.blish these routes dynamically is preferable to approaches requiring pre-definition and maintenance of large tables and directories. Two important approaches to bridging will be discussed: source routing and transparent bridging.
2.1 Physical LAN Attarhment
The purpose of physical LAN attachment is to provide an interface between the transmission medium and the LAN station's access protocol to the transmission-medium. It provides mechanical, electrical, functional and procedural
specifications for implementing a LAN.
The mechanical definition specifies the type of connectors to be used. The electrical definition, if applicable, states what voltages are to be used, while the functional definition defines the meaning of the voltages on the different pins.
Finally the procedural definition states the sequence of events to be followed to transmit and receive data, including the encoding scheme specifying how digital data is to be represented by electrical or optical pulses.
Many different types of media can be used for the physical layer. For example, telephone twisted pair, coax cable, shielded copper cable and fiber optics are the main types used for LANs. Different transmission techniques may be applied to each of these media types. They are generally categorized as baseband or broadband transmission techniques.
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2.1.1 Media Types
2.1.1.1 Coaxial Cable
Coaxial (coax) cable has low attenuation characteristics and can drive signals at high data rates for relatively long distances. Coax is an unbalanced cable (the two conductors have a different impedance to ground) with the shield being used as one of the conductors. This structure (a central core with a shield around it) does not generate as much RF noise as unshielded twisted pair cable does when used with high data rates.
Coax cable can be used for both baseband and broadband transmission.
2.1.1.2 Telephone Twisted pair Cable
Telephone twisted pair (unshielded, voice-grade twisted pair) cable can be used for data transmission when data signal strength is filtered, and distances are restricted. It is used by many PABX manufacturers to carry voice and data.
Unshielded telephone twisted pair cable tends to suffer from high attenuation (loss of signal strength due to inherent media characteristics) and is very susceptible to noise if located near strong electromagnetic fields (power cables, etc.). A result of attenuation is a reduction in the driving distance4 , number of attachments and bandwidth potential of the LAN using this medium. When used for high rates of data transmission (for example, 1Mbps or higher), it wi"
radiate radio frequency (RF) emissions. Filters can be used to reduce this.
However, filters increase loss in signal strength and add to the cost of the cabling and attachments. TTP wire also suffers from crosstalk5 between qdjacent twisted pairs. Voice Grade Twisted Pair wir!ng is generally undesirable for data transmission in excess of 4Mbps.
The IBM Token-Ring Network allows the use of TTP wire in 4 Mbps segments of the LAN.
2.1.1.3 Data-Grade M·edia (DGM), Shielded Twisted Pair Cable
This type of cable has one or more twisted pairs within a shield. The shielding reduces its susceptibility to low levels of noise and its own generation of radio frequency interference, thus making it more suitable for data transmission.
Shielded data grade cable can be used with data rates in excess of 20 Mbps over most distances encountered within buildings.
The cable can be constructed with two twisted pairs within a shield, while maintaining a low level of crosstalk5 due to the shielding and the way in which the pairs are twisted around each other. In addition to providing two data paths, the twisting and shielding of DGM cable provides greater immunity to external interference than coaxial cables which use the shield as one of the conductors. Data grade twisted pair cable is a balanced medium better suited to the differential encoding schemes used by some LANs.
4 Driving distance refers to the distance a signal can propagate in a specific medium without requiring regeneration.
5 Crosstalk refers to a signal in one cable generating a parasite signal in an adjacent cable.
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LAN ConceptsWhile this type of cable can be used for baseband and broadband transmission, it is primarily used for baseband.
The IBM Token-Ring Network uses DGM Twisted Pair media for both 4 and 16 Mbps LAN segments 6.
2.1.1.4 Fiber Optic Cable
Fiber optic cable presents an attractive solution for high speed transmission rates used in backbone local area networks. The cable is relatively immune to the types of electrical noise and grounding trouble that can plague metallic conductors in some environments. Thus, it is also an ideal medium for outdoor connections or for factories or locations in which cabling has to run near higher voltage wiring. Fiber optic cable also has extremely high data transfer
capability (hundreds of megabits per second, terabits per second) with very little Signal attenuation (signal loss due to the medium). Because of the high data rates and the distances that fiber optic cables can carry a signal without regeneration, its use in telephone networks, channel extenders on mainframe computers, and backbone LANsis rapidly increasing.
In comparison with transmission of electrical signals on copper media, it is difficult to tap an optical signal from a fiber optic cable without the inherent optical signal loss being detected. Therefore fiber optic cable has potential for greater security than metallic conductors.
Fiber optic cable is normally used for baseband transmission.
IBM includes fiber optic cable as part of the IBM Cabling System, referred to as
"Type 5 cable". Other fiber optic cables may also be used.
2.1.2 Network Topologies
Numerous topologies are used for local area networks. This section briefly discusses four different topologies. The term topology refers to the way in which network devices are interconnected. For example, if every station is directly connected to every other station, this is called mesh topology. Figure 3 on page 10 shows the four network topologies to be described plus two
variations of popular LAN topologies.
6 LAN segment refers to a single physical local area network running its own independent MAC protocol.
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