9-Jun-14Data Link Layer Guided Media Media that provide a conduit from one device to another. Signals travelling along any of these media is directed and contained by the physical limits of the medium. Examples are: Twisted-pair cable Coaxial cable Fiber-optic cable

9-Jun-14Data Link Layer Twisted-Pair Cable Uses two metallic (copper) conductors with individual plastic insulator each. Accepts and transports signals in the form of electric current. One wire is used to carry signals to the receiver and the other is used as a ground reference. The receiver uses the difference between the two signal levels in the wires. Noise or crosstalk effect is balanced by twisting the wires.

Figure 7.3 Twisted-pair cable

9-Jun-14Data Link Layer Twisted-Pair (Continued…) Twisted-pair cable can be divided into: Unshielded (UTP) – The most common twisted-pair cable used in data communication. Shielded (STP) – IBMs version for its use. It has a metal foil of braided-mesh covering encasing each pair of the insulated conductors. It improves the quality of the cable, but it is bulkier.

Figure 7.4 UTP and STP

9-Jun-14Data Link Layer Twisted-Pair (Continued…) Standards: The Electronic Industries Association (EIA) has developed standards to classify UTP cable into seven categories based on their quality. Category 1 is the lowest in quality and category 7 is the highest. Connectors: The most common is RJ45, which is a keyed connector.

Table 7.1 Categories of unshielded twisted-pair cables CategoryBandwidthData RateDigital/AnalogUse 1very low< 100 kbpsAnalogTelephone 2 < 2 MHz2 MbpsAnalog/digitalT-1 lines 3 16 MHz 10 MbpsDigitalLANs 4 20 MHz 20 MbpsDigitalLANs MHz 100 MbpsDigitalLANs 6 (draft) 200 MHz 200 MbpsDigitalLANs 7 (draft) 600 MHz 600 MbpsDigitalLANs

Figure 7.5 UTP connector

9-Jun-14Data Link Layer Twisted-Pair (Continued…) Performance: A twisted-pair cable can pass a wide range of frequencies. However, the attenuation (dB/mi) sharply increases with frequencies above 100KHz. Applications: Voice and data channels in telephone lines. High data rate connections in DSL lines. 10Base-T and 100Base-T LANs.

Figure 7.6 UTP performance

9-Jun-14Data Link Layer Coaxial Cable Uses central core conductor of solid or stranded copper wire enclosed in an insulating sheath. The insulating sheath is then encased in an outer conductor of metal foil, braid or a combination of the two. The metallic wrapping serves both as a shield against noise and as a second conductor that completes the circuit. This outer conductor is also enclosed in another insulating sheath and the whole thing is protected by a plastic cover.

Figure 7.7 Coaxial cable

9-Jun-14Data Link Layer Coaxial Cable (Continued…) Standards: Categorised by their radio government (RG) ratings with each number denotes a unique set of physical specifications. CategoryImpedanceUse RG Cable TV RG Thin Ethernet RG Thick Ethernet

9-Jun-14Data Link Layer Coaxial Cable (Continued…) Connectors: The most commonly used connectors are the Bayone-Neill-Concelman (BNC) connectors. The three popular types of BNC are: BNC – used to connect the end of the cable to a device, such as TV set. BNC T – used in Ethernet networks to branch out a cable for connection to a computer or other devices. BNC terminator – used at the end of the cable to prevent the signal reflection.

Figure 7.8 BNC connectors

9-Jun-14Data Link Layer Coaxial Cable (Continued…) Performance: Coaxial cable has a much higher bandwidth compared to the twisted-pair cable. However, the signal weakens rapidly and it requires frequent use of repeaters. Applications: Analogue and digital telephone networks. Cable TV. Traditional Ethernet LANs.

Figure 7.9 Coaxial cable performance

9-Jun-14Data Link Layer Fiber-Optic Cable Made of glass or plastic and transmits signals in the form of light. Making use of the property of light that changes direction when travelling through substances of differing density. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.

Figure 7.14 Fiber construction

Figure 7.10 Bending of light ray

Figure 7.11 Optical fiber

9-Jun-14Data Link Layer Fiber-Optic Cable (Continued…) Sizes: Defined by the ratio of the diameter of their core to the diameter of their cladding, both expressed in micrometers. 7/ / /125 50/125 Type Core 125 Cladding

9-Jun-14Data Link Layer Fiber-Optic (Continued…) Connectors: Uses three types of connectors: Subscriber channel (SC) – uses push/pull locking system. Straight-tip (ST) – uses bayonet locking system and more reliable than SC. MT-RJ – a new connector with the same size as RJ45. Performance: Attenuation is flatter than in the case of TP and coaxial cables. Therefore, less repeater is needed for fiber-optic cable.

Figure 7.15 Fiber-optic cable connectors

Figure 7.16 Optical fiber performance

9-Jun-14Data Link Layer Fiber-Optic (Continued…) Applications: Used often in the backbone networks. Used in hybrid cable TV network as the backbone. Used in 100Base-FX and 1000Base-X LANs