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Physical and Data Link Layer

Physical and Data Link Layer. Computer Network System Sirak Kaewjamnong. Network Devices. A. B. Circuit Switching. Source. Destination. It ’ s the method used by the telephone network. A call has three phases: Establish circuit from end-to-end ( “ dialing ” ), Communicate,

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Physical and Data Link Layer

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  1. Physical and Data Link Layer Computer Network System Sirak Kaewjamnong

  2. Network Devices

  3. A B Circuit Switching Source Destination • It’s the method used by the telephone network. • A call has three phases: • Establish circuit from end-to-end (“dialing”), • Communicate, • 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. Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000 calls. Circuit Switching : Telephone Network Destination “Callee” Source “Caller” Central Office “C.O.” Central Office “C.O.” Trunk Exchange

  5. A B Packet Switching R2 Source Destination R1 R3 R4 • 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 Link 1, ingress Link 1, egress Link 2 Link 1 Link 3 R1 Link 2, ingress Link 2, egress Link 4 Link 3, ingress Link 3, egress Link 4, ingress Link 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. B A Packet Switching R2 Source Destination R1 R3 R4 Host A TRANSP1 “Store-and-Forward” at each Router TRANSP2 R1 PROP1 TRANSP3 R2 PROP2 TRANSP4 R3 PROP3 Host B PROP4

  9. Packet SwitchingWhy not send the entire message in one packet? M/R M/R Host A Host A R1 R1 R2 R2 R3 R3 Host B Host B 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.

  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 802.3 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 802.5 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 127.0.0.1 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)

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