1 Introduction to Network Layer Lesson 09 NETS2150/ School of Information Technologies

2 Lesson Outline Switching is an effective way of sharing network resources Circuit switching Packet Switching

3 Position of network layer McGraw-Hill © The McGraw-Hill Companies, Inc., 2004

4 Network Layer in an Internetwork McGraw-Hill © The McGraw-Hill Companies, Inc., 2004 Link 1 Link 2Link 3

5 Network layer functions transport packet from sending to receiving hosts network layer protocols in every host, router three important functions: path determination: route taken by packets from source to dest. Routing algorithms forwarding: move packets from router ’ s input to appropriate router output call setup: some network architectures require router call setup along path before data flows network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical

6 Switching in Wide Area Switched Networks McGraw-Hill © The McGraw-Hill Companies, Inc., 2004

7 Switching Networks Long distance transmission is typically done over a network of switched nodes not through dedicated mesh lines Nodes not concerned with content of data Data routed by being switched from node to node

8 Switching Nodes Switching nodes may connect to other nodes only, or to end systems and other nodes Some redundant connections are desirable for reliability Two different switching technologies: Circuit switching Packet switching

9 An Example Network

10 Circuit Switching Developed for voice traffic Provides dedicated communication path between two stations Connected sequence of links Resources reserved for exclusive use Done at the physical layer Transparent connection Three phases in communication Establish Transfer Disconnect

11 Circuit Switching (2) Connection setup takes time Once connected, transfer is transparent Developed for voice traffic (phone)

12 Public Circuit Switched Network Twisted-pair (subscriber line) Subscribers, subs. Line, exchanges, and trunks Multiple voice frequency circuits

13 Circuit Establishment Local call Long-distance call Line c + one channel on the trunk to the ex

14 Circuit switching Disadvantages Circuit switching designed for voice Resources dedicated to a particular call Much of the time a data connection is idle, unused capacity is wasted Data rate is fixed Both ends must operate at the same rate Solution: Packet Switching

15 Packet Switching Principles Developed for bursty data traffic Data transmitted in small packets Typically 1000 octets Longer messages split into series of packets Each packet contains a portion of user data plus some control info (header) Control info Routing (addressing) info Packets are received, stored briefly (buffered) and passed on to the next node Store and forward Not concerned with the content of the data

16 The use of packets

17 A packet ’ s trip from Src to Dest Source Destination

18 Packet Switching … Source and Dest. layer 2 addresses Source and Dest. IP addresses Routing table

19 Advantages Line efficiency Single node to node link can be shared by many packets over time Packets queued and transmitted Data rate conversion Each end system connects to the local node at its own speed Nodes buffer data if required to equalize rates Packets are accepted even when network is busy (as opposed to call dropping) Delivery may slow down Priorities can be used Based on the priority, some packets can experience less delay

20 Packet Switching Technique End system breaks a long message into packets Packets sent one at a time to the network Packets handled in two modes: Datagram  used in today ’ s Internet Virtual circuit  used in ATM, frame-relay, X.25

21 Datagram Mode: the Internet model No call setup at network layer Each packet treated independently No reference to packets handled before from the same message no network-level concept of “ connection ” Packets can take any practical route Packets may arrive out of order Packets may go missing: Best-effort service! Up to receiver to re-order packets and recover from missing packets

22 Datagram Mode Illustration Packets for same destination may not follow the same route May arrive out of sequence Exit node or the destination does the re-ordering Exit node Pkt re-ordered

23 Virtual Circuit Mode Preplanned route established before any packets sent Call request and clear packets to establish and drop circuit (handshake) Each packet contains a virtual circuit identifier instead of destination address Every router on source-dest path maintains “ state ” for each passing virtual circuit (VC) No routing decisions required for each packet transport-layer connection only involved two end systems link, router resources (bandwidth, buffers) may be allocated to VC to get circuit-like performance. Non-dedicated path

24 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today ’ s Internet application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 3. Accept call4. Call connected 5. Data flow begins 6. Receive data

25 Virtual Circuit Diagram

26 Packet Size Packet size and transmission time Breaking a message into smaller packets Transmission time drops Too many smaller packets is not good either! Processing and queuing delays increase when there are more packets to handle, for a single message

27 Packet Size & Transmission time … Total tx time: 43*3=129 octet-times 92 octet- times 77 octet-times 84 octet-times! More and smaller packets mean more of the headers, increasing the octet-time

28 Virtual Circuits vs Datagram Virtual circuits Network can provide sequencing and error control Packets are forwarded more quickly No routing decisions to make Less reliable Loss of a node looses all circuits through that node Datagram No call setup phase Better if few packets More flexible Routing can be used to avoid congested parts of the network

29 Datagram or VC network: why? Internet data exchange among computers “ elastic ” service, no strict timing req. “ smart ” end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at “ edge ” many link types different characteristics uniform service difficult ATM evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service “ dumb ” end systems telephones complexity inside network

30 Circuit v Packet Switching Performance comparison involves: Propagation delay Transmission time Node processing delay Constant factors Variable factor

31 Event Timing Sequence Node delay Single block Packetised data

32 Network layer functions transport packet from sending to receiving hosts network layer protocols in every host, router three important functions: path determination: route taken by packets from source to dest. Routing algorithms forwarding: move packets from router ’ s input to appropriate router output call setup: some network architectures require router call setup along path before data flows network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical

33 Required Reading Circuit Switching Packet Switching Virtual circuit Datagram Read Stallings 10.1,10.2, and 10.6 Next: Routing