Voice over IP

Lecture# 5, 6

NETWORK EDGE

Asst. Prof. Muhammad Adil

�� The computers connected to the internet are referred to as

“End Systems”.

� The internet’s End Systems include desktop computers (PC), Macs , portable computers, PDA , Phones with wireless internet , web TV , digital cameras , home application etc.

� They are also referred to as “hosts” because they run (host) application programs like Web server programs, an e-mail reader program or an e-mail sever program.

� Hosts are also categorized in to two types. i.e. clients and server.

END SYSTEM

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�Clients:

� A client tends to be desktop and mobile PCs, PDAs etc.

Servers:A server is a more powerful machine that stores and distributes web pages, streams video and audio, relays e-mail etc.

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END SYSTEM

�Clients & Servers Applications:

� In context of networking software, A “Client program” is a program running on one end System that requests and receives a service from a “Server program” running on another system.

� Undoubtedly, the client-server model is the most powerful structure for internet applications.

� File transfer, e-mail, remote login, web, news groups and many more applications have adopted this client-server model.

� Client-server internet applications are called “Distributed Applications” because the client program runs on one computer and the server program runs on another computer.

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END SYSTEM

��TCP/IP Networks and in particular the internet

provide two types of services to End-Systems Applications: Connectionless and Connection-oriented services

�Developers creating an internet applications e.g. an e-mail application , a file transfer application or a web application , must design the application to use one of those services

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CONNECTIONLESS AND CONNECTED SERVICES

�� These services are based on connection-oriented transport i.e.

TCP. � Here, the client and server programs send control packets to

each other before sending packets with actual data to be transferred, which is known as handshaking.

� Handshaking procedure alerts the clients and server to be prepared for transmission of data packets.

� As soon as the handshaking procedure is finished, a connection is said to be established between the two systems.

� TCP is said to be connection-oriented because of “handshake” process.

� TCP is defined in RFC 793, RFC 1122, RFC 1323, RFC 2018 and RFC 2581.

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Connection-Oriented Services

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Connection-Oriented Services

�Connection-Oriented transfer is “Reliable Data Transfer“ because an application can rely on the connection to deliver all of its data without errors and in proper order.

� Reliability in internet is achieved by acknowledgements and retransmissions.

� For example, let an application has developed a connection between end system X and Y. When Y receives a packet from X, it sends an acknowledgement; this makes X know that Y has received the sent packets, definitely when X does not receive an Ack from Y, it assumes the packet loss and thus retransmits the packet.

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Connection-Oriented Services

�� Here, there is no handshaking, so when one side

wants to transmit packets to other side it simply sends the packets.

� Due to non-handshaking prior to packet transmission, data can be deliver sooner.

� But there is no reliable data transfer, so a source never knows which packets are lost.

� There is no flow control or congestion control.

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Connectionless Services

�� The internet connectionless services are called user

datagram protocol (UDP).

� UPD is defined in RFC 768.

� TCP is mostly used by application like Telnet (remote login), SMTP (e-mail), FTP (file transfer) and HTTP (for the web).

� UDP is used by many applications also, like many of the emerging multimedia applications such as internet phone and video conferencing.

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Connectionless Services

�� It consists of a set of switches connected by physical

links.

� A connection between two systems is a dedicated path made of one or more links. A link is divided into n channels.

� Each connection uses only one dedicated channel on a link.

� Each link is divided into channels using FDM or TDM.

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CIRCUIT-SWITCHED NETWORK

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CIRCUIT-SWITCHED NETWORK

�� The end systems like phones or computers are directly connected to

switch.

� Circuit switching takes place at the physical layer.

Communication in a circuit switched network requires 3-Phases.

� Setup Phase: A dedicated circuit needs to be established before the two or multiple parties can communicate. End systems are connected by means of dedicated lines (channels) to switches, so connection setup means creating dedicated channels between the switches. So when a connection is established an Ack: from Y is sent back to X.

� Data Transfer Phase: After the establishment of the dedicated circuit (channels), the two parties can transfer data.

� Teardown Phase: When one of the parties needs to disconnect, a signal is sent to each switch to release the resources.

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CIRCUIT-SWITCHED NETWORK

�� A network in which data are transmitted in form of

independent units called packets is called a”Packet-Switching Network”.

� Data transmission using a packet-switch network is called “Packet-Switching”.

� Applications exchange messages over network.

� Messages may perform control tasks like “Handshaking” or may contain data e.g. an e-mail message, a GIF image, an MP4 video file etc.

� The source breaks long messages into smaller chunks of data called “Packets”.

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PACKET-SWITCHING

�� Each of these packets travel through communication links

called “Packet Switches”.

� Two important type of devices are Routers and Link-Layer switches.

� The rate at which packets are transmitted over a communication link is equal to the full-transmission rate of the link.

� Most packet switches use Store-and-Forwarding transmission at the inputs to the link. i.e. the switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outgoing link.

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PACKET-SWITCHING

�� So a store-and-forwarding delay at the input to each

link is introduce.

� If a packet consists of L bits and the packet is to be forwarded onto a link of R bps then the store-and-forwarding delay at the switch is L/R seconds.

� This delay is proportional to the packet length in bits

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PACKET-SWITCHING

�OUTPUT BUFFER/QUEUE:� Each packet switch has multiple links attached to it, and for each

link there is an output buffer or output Queue, that stores the packet that the router is about to send into that link.

� Output buffer is very important. If a packet is to be transmitted over an already busy link, then the newly arrived packet must wait in the output buffer.

� This also adds the output buffer Queuing delay. This delay is variable and depends upon the level of congestion in the network.

� As amount of buffer space is finite, so there is a possibility an for arriving packet that the buffer is completely filled with other packets waiting for transmission, that will result in packet loss; Either the arriving packet or one of the already queued packet will be dropped.

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PACKET-SWITCHING

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PACKET-SWITCHING

�� In this figure packets of equal size are to be transmitted over

the network.� Hosts A and B are sending packets to Host E. � At first the packets are sent along 10 Mbps Ethernet links to the

first packet switch. � The packet switch directs these packets to the 1.5 Mbps link.� Congestion will occur if the arrival rate of packets exceeds the

rate at which the switch can forward packets across the 1.5 output link.

� If there are Q links between the two hosts, each of rate R bps. Let queuing delay and end-to-end propagation delays are negligible, and there is no connection establishment, then the total delay for such a case is QL/R

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PACKET-SWITCHING

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