Computer Network Notes CS 602 LN 5

COMPUTER NETWORK LECTURE NOTES – Volume-5

SUJOY BHOWMICK (Asst. Prof, IT) PAGE NO : 1

Transmission Media

Figure :Transmission medium and physical layer

Classes of transmission media

GUIDED MEDIA

Guided 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 Cable Coaxial Cable Fiber-Optic Cable

Twisted-pair cable



Figure : UTP and STP cables

Table : Categories of unshielded twisted-pair cables

Figure: UTP CONNECTOR



Figure : Coaxial cable

Table : Categories of coaxial cables

Figure : BNC connectors

Figure : Bending of light ray

Figure : Optical fiber



Types of Propagation modes (Optical Fiber Communication)

Figure : Transmission Modes (OFC)

Figure : Fiber construction



Figure : Fiber-optic cable connectors

UNGUIDED MEDIA: WIRELESS

Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Type of Wireless Media: Radio Waves Microwaves Infrared

Figure : Electromagnetic spectrum for wireless communication



Figure : Propagation methods

Table : Transmission Bands

Figure: Wireless transmission waves



Figure : Omnidirectional antenna

Radio waves are used for multicast communications, such as radio and television, and paging systems.

Figure: Unidirectional antennas Microwaves are used for unicast communication such as cellular telephones, satellite

networks, and wireless LANs.

Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation.



Switching

Figure : Switched network Taxonomy of switched networks:

CIRCUIT-SWITCHED NETWORKS

A 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.

A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels.



Figure : A trivial circuit-switched network

In 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.

As a trivial example, let us use a circuit-switched network to connect eight telephones in a small area. Communication is through 4-kHz voice channels. We assume that each link uses FDM to connect a maximum of two voice channels. The bandwidth of each link is then 8 kHz. Figure above shows the situation. Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of course the situation may change when new connections are made. The switch controls the connections.



As another example, consider a circuit-switched network that connects computers in two remote offices of a private company. The offices are connected using a T-1 line leased from a communication service provider. There are two 4 × 8 (4 inputs and 8 outputs) switches in this network. For each switch, four output ports are folded into the input ports to allow communication between computers in the same office. Four other output ports allow communication between the two offices. Figure above shows the situation.

Figure: Delay in Circuit Switch Network

Switching at the physical layer in the traditional telephone network uses the circuit-switching approach. DATAGRAM NETWORKS

In 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.

In a packet-switched network, there is no resource reservation; resources are allocated on demand.



Figure : A datagram network with four switches (routers)

<<< Figure : Routing table in a datagram network A switch in a datagram network uses a routing table that is based on the destination

address. The destination address in the header of a packet in a datagram network remains the

same during the entire journey of the packet.

Figure : Delay in a datagram network

Switching in the Internet is done by using the datagram approach to packet switching at the network layer.



VIRTUAL-CIRCUIT NETWORKS A virtual-circuit network is a cross between a circuit-switched network and a

datagram network. It has some characteristics of both.

Figure : Virtual-circuit network

<<< Figure : Virtual-circuit identifier

Figure : Switch and tables in a virtual-circuit network



Figure : Source-to-destination data transfer in a virtual-circuit network

Figure : Setup request in a virtual-circuit network



Figure : Setup acknowledgment in a virtual-circuit network

In 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.

Figure : Delay in a virtual-circuit network

Switching at the data link layer in a switched WAN is normally implemented by using virtual-circuit techniques. STRUCTURE OF A SWITCH

We use switches in circuit-switched and packet-switched networks. In this section, we discuss the structures of the switches used in each type of network.



Figure : Crossbar switch with three inputs and four outputs

Figure : Multistage switch

In a three-stage switch, the total number of crosspoints is 2kN + k(N/n)2; which is much smaller than the number of crosspoints in a single-stage switch (N2).



Figure : Time-slot interchange

Figure : Time-space-time switch

<<< Figure : Input port

<<< Figure : Output port



TELEPHONE NETWORK Telephone networks use circuit switching. The telephone network had its beginnings

in the late 1800s. The entire network, which is referred to as the plain old telephone system (POTS), was originally an analog system using analog signals to transmit voice.

The tasks of data transfer and signaling are separated in modern telephone networks: data transfer is done by one network, signaling by another.

Figure : Modulation/demodulation (MODEM)

Endoffices

Local loop

Trunk

Tandemoffices Regional offices

Trunk

• • •



Figure : Uploading and downloading in 56K modems

DIGITAL SUBSCRIBER LINE

After traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing local loops. ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. The existing local loops can handle bandwidths up to 1.1 MHz. ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line.



FIGURE: ASDL TECHNOLOGY

CABLE TV NETWORKS The cable TV network started as a video service provider, but it has moved to the

business of Internet access. Traditional Cable Connection is Unidirectional.

Figure : Traditional cable TV network

Figure : Hybrid fiber-coaxial (HFC) network

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Computer Network Notes CS 602 LN 5