4Transmission medium and physical layer A transmission media defined as anything that carry information between a source to a destination- Located below the physical layer and are directly controlled by the physical layer
67-1 GUIDED MEDIAGuided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable.Twisted –pair cables and coaxial cable:use metallic (copper) conductors that transport signals in the form of electric currentOptical fiber : transport signals in the form of the light
7Twisted-pair cableOne of the wire used to carry signal and the other as a ground. The receiver uses the difference between the two.If the two wires are parallel, the effect of interference noise and crosstalk is bigTwisting the pair of wire balance the effect of unwanted signal and reduce it.The number of twists per unit of length effects on the quality of the cable
8Applications of Twisted pair Coaxial (Coax) cableCoax cable carries signals of higher frequency ranges than those in Twisted pair cablebecause the two media are constructed quite differentlyThe outer conductor serves both as a shield against noise and as second conductor, which complete the circuitApplications of Twisted pairUsed in1.telephone lines to provide voice and data channels (local loop)2.The DSL lines that are used by the telephone companies to provide high-data-rate connections3.Local area networks, such as 10-base-Tand 100base-T
10Coaxial cableCoax cable carries signals of higher frequency ranges than those in Twisted pair cable because the two media are constructed quite differently.The outer conductor serves both as a shield against noise and as second conductor, which complete the circuit
11Applications of coaxial cable 1.Analog telephone network where a single cable could carry 10,000 voice signals. Later it was used in Digital telephone networks where cable can carry 600Mbps2.Cable TV network: hybrid network use coaxial cable only at the network boundaries , near the consumer. Cable TV use RG-593.Traditional Ethernet LANs.10-base-2 or “Thin Ethernet”, uses RG-58 coax cable to transmit data at 10 Mbps with a range of 185m.10-base-5,or “Thick Ethernet”, uses RG-11 to transmit 10 Mbps with rang of 500 m
13Fiber Optic CableIs made of glass or plastic and transmit signals in the form of light.Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one substance enters another substance of different density , the ray change direction as shown:I: angle of incidence: the angle the ray makes with line perpendicular to the interface between the two substancesCritical angle: property of substance, its value differs from one substance to another
14Optical fiberFiber Optical : uses reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic
15Applications for Fiber Optic cable Used in :1.Cable TV network: hybrid network use a combination of optical fiber and coax cable. Optical provides the backbone while coaxial cable provide the connation to the user.2.Local area networks such as 100base-FX(fast Ethernet) and 1000base-XLANs.3.Backbone networks because its wide bandwidth
16Advantages of fiber-optical 1.Higher Bandwidth2.Less signal attenuation it needs repeater every 50km, where twisted and coaxial need it every 5km.3.Immunity to electromagnetic interference (noise)4.Resistance to corrosive materials. Glass is more resistance to corrosive material than copper5.Light weight. Fiber cables are much lighter than copper cables6.Greater immunity to tapping: copper cables create antenna effects that can easily be tapped
17Disadvantages of fiber-optical 1.Installation and maintenance. It’s a new technology. Its installation and maintenance require expertise that is not yet available every where.2.Unidirectional light propagation. If we need bidirectional , two fibers are needed.3.Cost. The cable and the interfaces are more expensive than those of other guided media. If the demand of BW is not high , often use of optical fiber can not be justified
20Building large networks A network is a set of connected devices.When ever we have multiple devices, we have the problem of how to connect them!Point-to-point (mesh or star topology): impossible for large networks.Multipoint (bus topology): does not work for large network since the distances between devices and the total number of devices increase beyond the capacity of the media and equipments.
21Switching is the solution A switched network consists of a series of interlinked nodes, called switches.Switches are devices capable of making temporary connections between any two or more devices connected to the switch.
24Topics discussed in this section: 8-1 CIRCUIT-SWITCHED NETWORKSA circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.Topics discussed in this section:Three Phases Efficiency DelayCircuit-Switched Technology in Telephone Networks
25NoteA circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels.
27NoteIn circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase.
28the circuit-switching approach. NoteSwitching at the physical layer in the traditional telephone network usesthe circuit-switching approach.
29Topics discussed in this section: 8-2 DATAGRAM NETWORKSIn data communications, we need to send messages from one end system to another. If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. The size of the packet is determined by the network and the governing protocol.Topics discussed in this section:Routing Table Efficiency DelayDatagram Networks in the Internet
30In a packet-switched network, there is no resource reservation; NoteIn a packet-switched network, there is no resource reservation;resources are allocated on demand.
31Figure 8.7 A datagram network with four switches (routers)
36NoteSwitching in the Internet is done by using the datagram approach to packet switching at the network layer.
37Topics discussed in this section: 8-3 VIRTUAL-CIRCUIT NETWORKSA virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both.Topics discussed in this section:Addressing Three PhasesEfficiency DelayCircuit-Switched Technology in WANs
40Figure 8.12 Switch and tables in a virtual-circuit network
41Figure 8.13 Source-to-destination data transfer in a virtual-circuit network
42Figure 8.14 Setup request in a virtual-circuit network
43Figure 8.15 Setup acknowledgment in a virtual-circuit network
44NoteIn virtual-circuit switching, all packets belonging to the same source and destination travel the same path;but the packets may arrive at the destination with different delays if resource allocation is on demand.