1 Physical and Data Link Layer Computer Network System Sirak Kaewjamnong

2 Network Devices

3 Circuit Switching AB Sou rce Destin ation  It ’ s the method used by the telephone network.  A call has three phases: 1.Establish circuit from end-to-end ( “ dialing ” ), 2.Communicate, 3.Close circuit  Originally, a circuit was an end-to-end physical wire.  Nowadays, a circuit is like a virtual private wire: each call has its own private, guaranteed data rate from end- to-end.

4 Circuit Switching : T elephone Network Sourc e “ Calle r ” Central Office “ C.O. ” Destinati on “ Callee ” Central Office “ C.O. ” Trunk Exchange Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000 calls.

5 Packet Switching A R1 R2 R4 R3 B Sourc e Destinati on  It ’ s the method used by the Internet.  Each packet is individually routed packet-by-packet, using the router ’ s local routing table.  The routers maintain no per-flow state.  Different packets may take different paths.  Several packets may arrive for the same output link at the same time, therefore a packet switch has buffers.

6 Packet Switching : Simple router model R1 Link 1 Link 2 Link 3 Link 4 Link 1, ingressLink 1, egress Link 2, ingressLink 2, egress Link 3, ingressLink 3, egress Link 4, ingressLink 4, egress

7 Why does the Internet use packet switching? Efficient use of expensive links: –The links are assumed to be expensive and scarce. –Packet switching allows many, bursty flows to share the same link efficiently. –“ Circuit switching is rarely used for data networks,... because of very inefficient use of the links ” - Gallager Resilience to failure of links & routers: –“ For high reliability,... [the Internet] was to be a datagram subnet, so if some lines and [routers] were destroyed, messages could be... rerouted ” - Tanenbaum Source: Networking 101

8 Packet Switching Host A Host B R1 R2 R3 A R1 R2 R4 R3 B TRANSP 1 TRANSP 2 TRANSP 3 TRANSP 4 PROP 1 PROP 2 PROP 3 PROP 4 Sourc e Destinati on “ Store-and-Forward ” at each Router

9 Packet Switching Why not send the entire message in one packet? Breaking message into packets allows parallel transmission across all links, reducing end to end latency. It also prevents a link from being “ hogged ” for a long time by one message. Host A Host B R1 R2 R3 M/R Host A Host B R1 R2 R3 M/R

10 Common Data Link

11 LAN and WAN Layers Physical and Data Link layer for LAN and WAN

12 Medium Access Control Protocol for controlling access to transmission medium Defined as part of Data Link layer The protocol performs: –Perform functions related to medium access (MAC sublayer) –Concerned with the transmission of a link-Level between two node (LLC sublayer)

13 IEEE Operation Every node can receive a transmission by all other nodes Need addressing scheme to identify destination Only destination copies frame to it, all other nodes discarded the frame

14 Token Ring / IEEE 802.5

15 IEEE Operation Token ring LANs continuously pass a token or a token ring frame

16 Fiber Distributed Data Interface (FDDI) Devices on FDDI maintain connectivity on dual ring

17 WAN Protocols SDLC : Synchronous Data Link Control for IBM SNA network HDLC : High Level Data Link Control common WAN data link LAPB : Link Access Protocol, Balanced DTE to DCE data link for X.25 Frame Relay framing for higher speed, reduce checking for higher speed PPP : Point to Point Protocol part of TCP/IP stack for WAN links

18 SLIP : Serial Line IP Method for encapsulation IP datagrams on serial line Popular for connecting home computer to Internet, via modem RFC 1005 de facto standard

19 SLIP Deficiency Need manual configuration of IP address both side (no negotiation mechanism ) Only one protocol can be used because there is no field to specify type of protocol No checksum, bad for protocol that depend on CRC Slow line make inefficient to carry only 1 byte data with 40 bytes overhead(IP + TCP header) –CSLIP (compress SLIP) reduces 40 bytes header to 3-5 bytes –smaller header greatly improve the interactive respond time

20 PPP : Point to Point Protocol Methods for encapsulation IP datagrams on serial line, correct all deficiency in SLIP Support either an asynchronous link with 8N1 or bit oriented synchronous link Two parts: –LCP :(Link Control Protocol ) to establish, configure and test connection –NCP :(Network Control Protocol) support different network layer protocols

21 PPP Advantage Support multiple protocols CRC for every frame Dynamic negotiation of the IP address for each end Link control protocol for negotiating data-link options

22 Loopback Interface Channel for client and server on the same host use to communicate Class A network ID 127 is reserve for the loopback interface Most system assigns IP with the name “ localhost ” Loopback interface appears as another link layer to the network layer

23 Summary The physical layer provides access to the medium of an internetwork The data link layer provides support for communications over several types of data links: –LAN (Ethernet, IEEE 802.X, FDDI) –Dedicated WAN (SDLC, HDLC,PPP,LAPB) –Switches LAN (X.25, Frame Relay, ISDN)