INTRODUCTORY COMPUTER

Transcription

1 INTRODUCTORY COMPUTER NETWORKS TYPES OF NETWORKS Faramarz Hendessi

2 Introductory Computer Networks Lecture 4 Fall 2010 Isfahan University of technology Dr. Faramarz Hendessi 2

3 Types of Networks Circuit switching versus message switching.

4 A Generic Switch incoming links Switch outgoing links How to Demultiplex? How to Switch? How to Multiplex?

5 Circuit Switching Old telephone technology For each connection, physical switches are set tin the telephone network kto create a physical circuit That s the job of the switching office

6 Circuit Switching - Example Physical copper connection set up when call is made Switching offices

7 Circuit Switching g( (cont d) Switches are set up at the beginning of the connection and maintained throughout the connection Network resources reserved and dedicateddi d from sender to receiver Not a very efficient strategy A connection holds a physical line even during silence periods (when there is nothing to transmit)

8 Circuit Switching: TDMA and TDMA FDMA Example: 4 users frequency time TDMA frequency time

9 Circuit Switching: Mltil Multiplexing/Demultiplexingi ltil i Frames Slots = Time divided in frames and frames divided in slots Relative slot position inside a frame determines which conversation the data belongs to E.g., slot 0 belongs to red conversation Needs synchronization between sender and receiver In case of non-permanent conversations Needs to dynamic bind a slot to a conservation How to do this? If a conversation does not use its circuit the capacity is lost!

10 Circuit Switching Three phases 1. circuit establishment 2. data transfer 3. circuit termination If circuit not available: busy Examples Telephone networks ISDN (Integrated Services Digital Networks)

11 Timing in Circuit Switching Host 1 Host 2 Switch 1 Switch 2 Circuit Establishment Transmission delay propagation delay between Host 1 and Switch1 propagation delay between Host 1 and Host 2 Transmission Circuit Termination Information time

12 Message Switching No physical path established! Whenever sender has data to send, sends it. Data stored at first router then forwarded. Store-and-forward networks. Sharing by taking turns. Analogy: conveyor belt in a warehouse. Items are picked from the storage room and placed on the conveyor belt every time a customer makes an order. Different customers may request a different number of items. Different users items may be interspersed on the conveyor belt (they are multiplexed ).

13 Packet Switching Upper bound on size of unit to be handled at the network layer. Why? Fairness. What kind of implementation used by Internet?

14 Packet Switching Example A Header Payload C C D C D C D D B D

15 Packet Switching Each packet is composed by the payload (the data we want to transmit) and a header. The header contains information useful for network layer functions. Contains: Source (sender s) s) address Destination (recipient s) address Packet size Sequence number Error checking information

16 The Internet Example of packet switching network!

17 Packet Switching g( (cont d) The header introduces overhead, that is, additional bits to be sent. Therefore, it is not wise to have packets that are too small. What happens if the payload is just 1 bit?

18 Packet Switching g( (cont d) In general, packets need not be of the same size Maximum transmission unit (MTU). No minimum size. But, header size is fixed (e.g., 20 bytes for TCP/IP). Original data chopped up into packets. The application (e.g., ) does not know that the data to be transmitted is packetized. When packets are received, they are put together before the application accesses the data

19 Packet Switching g( (cont d) What kind of delay should we expect? Time-division multiplexing: constant delay. Packet switching multiplexing: variable delay (it depends d on the traffic on the line). Conveyor belt example: if there are many customers before you, you may have to wait more.

20 Packet Switching: Mltil Multiplexing/Demultiplexingi ltil i Data from any conversation can be transmitted at any given time A single conversation can use the entire link capacity if it is alone How to demultiplex? Use meta-data (header) to describe data

21 Packet Switching Data are sent as formatted bit-sequences, so-called packets. Packets have the following structure: Header Data Trailer Header and Trailer carry control information (e.g., destination address, check sum) At each node the entire packet is received, stored briefly, and then forwarded to the next node based on the header information (Store-and-Forward Networks) Allows statistical multiplexing

22 Packet Switch incoming links Node Memory outgoing links

23 Datagram Packet Switching Host C Host A Host D Node 1 Node 2 Node 3 Node 5 Host B Node 4 Node 6 Node 7 Host E Each packet is independently switched Each packet header contains destination address

24 Timing of Datagram Packet Switching Host 1 Host 2 Switch 1 Switch 2 transmission time of Packet 1 at Host 1 Packet 1 Packet 2 Packet 3 propagation delay between Host 1 and Switch 2 Packet 1 Packet 2 Packet 3 processing delay of Packet 1 at Switch 2 Packet 1 Packet 2 Packet 3

25 Switching Circuit- Message- Packet Switching

26 Circuit Switching vs Packet Switching Circuit switching Must set up a connection (initial delay). Resources are dedicated Therefore they may be used inefficiently! But, performance is predictable as resources are reserved. Packet switching Very small set-up delay. Efficient shared use of resources. Possible congestion and consequent packet dropping Performance is unpredictable and is a function of current traffic conditions.

27 Virtual-Circuit Packet Switching Communication with virtual circuits takes place in three phases 1. VC establishment 2. data transfer 3. VC disconnect Note: packet headers don t need to contain the full destination address of the packet

28 Virtual-Circuit Packet Switching Hybrid of circuit switching and packet switching Data is transmitted as packets All packets from one packet stream are sent along a preestablished path (=virtual circuit) Packet header only contains local virtual circuit identifier (VCI) Demultiplexing and switching based on VCI Guarantees in-sequence delivery of packets Example: ATM networks

29 Timing of Virtual-Circuit Packet Switching Host 1 Host 2 Node 1 Node 2 VC establishment propagation delay between Host 1 and Node 1 Packet 1 Data transfer Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 VC termination

30 Virtual Circuit Switching Host C Host A Host D Node 1 Node 2 Node 3 Node 5 Host B Node 4 Node 6 Node 7 Host E

31 A Taxonomy of Communication Networks Communication networks can be classified based on the way in which the nodes exchange information: Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Virtual Circuit it Network Network

32 Packet-Switching vs. Circuit- Switching Most important advantage of packet-switching over circuit switching: ability to exploit statistical multiplexing More efficient bandwidth usage However,,packet-switching needs to buffer and deal with congestion More complex switches Harder to provide good network services (e.g., delay and bandwidth guarantees)

33 Types of Network Services Connectionless versus connection-oriented. oriented.

34 Datagram and Virtual Circuit Packet switching networks can provide 2 different types of services to transport layer. Virtual circuit or connection-oriented service. Datagram or connectionless service.

35 Virtual Circuit Analogy to physical circuits used by telephone networks. At connection establishment time, path from source to destination is selected and used throughout connection lifetime. When connection is over, virtual circuit terminated.

36 Datagram No logical connection. Each packet (datagram) routed independently; successive packets may follow different routes. More work at intermediate routers, but more robust and adaptive to failures and congestion.

37 Datagram network! The Internet Datagrams are formed by header and payload. IP Datagrams can have different sizes Header is fixed (20 bytes) Data area can contain between 1 byte and 65 KB

38 Forwarding Datagrams Header contains all information needed to deliver datagrams to destination. Destination address. Source address. Router examines header of each datagram and forwards it along gpath to destination.

39 Routers For VCs, routers keep a table with (VC number, outgoing interface) entries. Packets only need to carry VC number. For datagrams, routing table. (destination, outgoing interface) entries. Each packet must carry destination address.

40 Examples Internet Layer Connectionless Internet Protocol (IP) Task is to deliver packets to destination Transport Layer Transmission Control Protocol (TCP) Connection-oriented Reliable User Datagram Protocol (UDP) Connectionless Unreliable

41 Network Taxonomy Telecommunication networks Circuit-switched networks Packet-switched networks FDM TDM Networks with VCs Datagram Networks Datagram network is not either connection-oriented or connectionless. Internet t provides both connection-oriented i t (TCP) and connectionless services (UDP) to apps.