Internet Architecture and Protocol

Transcription

1 Internet Architecture and Protocol Set# 04 Wide Area Networks Delivered By: Engr Tahir Niazi

2 Wide Area Network Basics Cover large geographical area Network of Networks WANs used to be characterized with slow, noisy lines. Today WANs are very high speed with very low error rates. WANs usually follow a mesh topology.

3 Wide Area Network Basics

4 Wide Area Network Basics A station is a device that interfaces a user to a network. A node is a device that allows one or more stations to access the physical network and is a transfer point for passing information through a network. A node is often a computer, a router, or a telephone switch. The sub-network or physical network is the underlying connection of nodes and telecommunication links.

5 Wide Area Network Basics

6 Network Layer Concerned with getting packets from source to destination. The network layer must know the topology of the subnet and choose appropriate paths through it. When source and destination are in different networks, the network layer (IP) must deal with these differences. * Key issue: what service does the network layer provide to the transport layer (connection-oriented or connectionless).

7 Messages Messages Network service Transport layer Segments Transport layer Network service End system A Network layer Data link layer Network layer Data link layer Netwok layer Data link layer Network layer Data link layer End system B Physical layer Physical layer Physical layer Physical layer

8 Types of Network Structures Circuit switched network - a sub-network in which a dedicated circuit is established between sender and receiver and all data passes over this circuit. The telephone system is a common example. The connection is dedicated until one party or another terminates the connection.

9 Types of Network Structures

10 Types of Network Structures Packet switched network - a network in which all data messages are transmitted using fixed-sized packages, called packets. More efficient use of a telecommunications line since packets from multiple sources can share the medium. One form of packet switched network is the datagram. With a datagram, each packet is on its own and may follow its own path. Virtual circuit packet switched network create a logical path through the subnet and all packets from one connection follow this path.

11 Connection-oriented versus Connectionless The network structure is the underlying physical component of a network. What about the software or application that uses the network? A network application can be either connection-oriented or connectionless.

12 Connection-oriented versus Connectionless A connection-oriented application requires both sender and receiver to create a connection before any data is transferred. Applications such as large file transfers and sensitive transactions such as banking and business are typically connection-oriented. A connectionless application does not create a connection first but simply sends the data. Electronic mail is a common example.

13 Connection-oriented versus Connectionless

14 Connection-oriented versus Connectionless

15 Connection-oriented versus Connectionless A connection-oriented application can operate over both a circuit switched network or a packet switched network. A connectionless application can also operate over both a circuit switched network or a packet switched network but a packet switched network may be more efficient.

16 Switched Network

17 Nodes a collection of nodes and connections is a communications network nodes may connect to other nodes only, or to stations and other nodes network is usually partially connected some redundant connections are desirable have two different switching technologies circuit switching packet switching

18 Circuit Switching uses a dedicated path between two stations has three phases establish transfer disconnect inefficient channel capacity dedicated for duration of connection if no data, capacity wasted set up (connection) takes time once connected, transfer is transparent

19 Public Circuit Switched Network

20 Circuit Establishment

21 Circuit Switch Elements

22 Circuit Switch Element Digital switch Transparent signal path between any pair of attached devices. The connection must allow full-duplex transmission. Network interface represents the functions and hardware needed to connect digital devices, such as data processing devices and digital telephones Control unit Establishes connections. This is generally done on demand, that is, at the request of an attached device. Control unit must maintain the connection. Control unit must tear down the connection

23 Blocking or Non-blocking blocking network may be unable to connect stations because all paths are in use used on voice systems non-blocking network permits all stations to connect at once used for some data connections

24 Traditional Circuit Switching

25 Softswitch

26 Softswitch Softswitch can convert a stream of digitized voice bits into packets. This opens up a number of options for transmission, including the increasingly popular voice over IP (Internet Protocol) approach. In softswitch terminology, the physical switching function is performed by a media gateway (MG) call processing logic resides in a media gateway controller (MGC).

27 Packet Switching circuit switching was designed for voice packet switching was designed for data transmitted in small packets packets contains user data and control info user data may be part of a larger message control info includes routing (addressing) info packets are received, stored briefly (buffered) and pass on to the next node

28 Packet Switching

29 Advantages line efficiency single link shared by many packets over time packets queued and transmitted as fast as possible data rate conversion stations connects to local node at own speed nodes buffer data if required to equalize rates packets accepted even when network is busy priorities can be used

30 Switching Techniques station breaks long message into packets packets sent one at a time to the network packets can be handled in two ways datagram virtual circuit

31 Datagram Diagram

32 Virtual Circuit Diagram

33 Virtual Circuits vs. Datagram virtual circuits network can provide sequencing and error control packets are forwarded more quickly less reliable datagram no call setup phase more flexible more reliable

34 Circuit vs. Packet Switching range of other characteristics, including: Transparency (constant data rate) amount of overhead Circuit switching is essentially a transparent service. Once a connection is established, a constant data rate is provided to the connected stations. This is not the case with packet switching, which typically introduces variable delay, so that data arrive in a choppy manner, or even in a different order than they were transmitted. In addition there is no overhead required to accommodate circuit switching. Once a connection is established, the analog or digital data are passed through, as is. For packet switching, analog data must be converted to digital before transmission; in addition, each packet includes overhead bits, such as the destination address

35 OPERATION

36 OPERATION Circuit switching, there is a delay before the message is sent. First, a Call Request signal is sent. If the destination station is not busy, a Call Accepted signal returns. Note a processing delay is incurred at each node during the call request. On return, this processing is not needed because the connection is already set up. After the connection is set up, the message is sent as a single block, with no noticeable delay at the switching nodes.

37 Virtual circuit packet switching appears similar to circuit switching. A virtual circuit is requested using a Call Request packet, which incurs a delay at each node, and is accepted with a Call Accept packet. In contrast to the circuit-switching case, the call acceptance also experiences node delays, since each packet is queued at each node and must wait its turn for transmission. Once the virtual circuit is established, the message is transmitted in packets. Packet switching involves some delay at each node in the path. Worse, this delay is variable and will increase with increased load.

38 Datagram packet switching does not require a call setup. Thus, for short messages, it will be faster than virtual circuit and perhaps circuit switching. However, because each individual datagram is routed independently, the processing at each node may be longer than for virtual circuit packets.

39 Summary of Network Structures

40 X.25 key features include: Interface between devices and network control packets over the link multiplexing of virtual circuits at layer 3 layers 2 and 3 include flow and error control hence have considerable overhead not appropriate for modern digital systems with high reliability Have low data rate 64 kbps

41 Frame Relay packet switching systems have large overheads to compensate for errors modern systems are more reliable errors can be caught in end system Frame Relay provides higher speeds 44 Mbps with most error control overhead removed

42 Asynchronous Transfer Mode ATM evolution of frame relay fixed packet (called cell) length with little overhead for error control anything from 10Mbps to Gbps constant data rate using packet switching technique with multiple virtual circuits

43 Routing Each node in a WAN is a router that accepts an input packet, examines the destination address, and forwards the packet on to a particular telecommunications line. How does a router decide which line to transmit on? A router must select the one transmission line that will best provide a path to the destination and in an optimal manner. Often many possible routes exist between sender and receiver.

44 Routing

45 Summary Wide area network WAN Network layer Types of Networks Connection oriented and connectionless Circuit switching Packet switching WAN technologies

46 Q & A