Ch 5. The Link Layer and Local Area Networks Myungchul Kim

2 – Frame – Error detection, retransmission, flow control, random access – Ethernet, IEEE wireless LAN (WiFi), Token Ring, PPP, ATM – Services offered by a link-layer protocol  Framing  Link access: Medium Access Control (MAC) protocol  Reliable delivery  Flow control  Error detection  Error correction  Half-duplex and full-duplex – The services by the link layer vs the transport layer

3

4 – The link layer is implemented in a network adaptor known as a network interface card (NIC) – LAN-on-motherboard configuration

5 Error-Detection and –Correction Techniques

6 o Parirty checks

7 – Forward error correction (FEC) o Checksumming methods – The receiver checks the checksum by taking the 1s complement of the sum of the received data (including the checksum). – Why checksumming at the Transport layer and CRC at the link layer?

8 o Cyclic Redundancy Check (CRC) - generator: The sender and receiver must agree on an r+1 bit pattern - D * 2 r XOR R = nG - D * 2 r = nG XOR R - R = remainder D * 2 r / G - The CRC standards can detect burst errors of fewer than r+1 bits.

9

10 Multiple access protocols – Point-to-point link – Broadcast link: medium access problem – Multiple access protocols – Channel partitioning protocols, random access rptocols and taking-turns protocols

11 Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: – no special node to coordinate transmissions – no synchronization of clocks, slots 4. simple

12 o Channel partitioning protocols – Code Division Multiple Access (CDMA)

13 – ALOHA  All frames L bits  Time slots of size L/R seconds  The nodes are synchronized so that each node knows when the slots begin.  If two or more frames collide in a slot, then all the nodes detect the collision event before the slot ends.  Max efficiency of the protocol 1/e = 0.37

14 – ALOHA  Max efficiency of the protocol 1/2e

15 – Carrier Sense Multiple Access (CSMA)  Listen before speaking: carrier sensing  If someone else begins talking at the same time, stop talking: collision detection  CSMA, CSMA/CD

16

17

18 o Taking-Turns protocols – Polling protocols: a master node – Token-passing protocol: no master node o Local Area Networks (LANs) – Ethernet: IEEE – Token Ring: IEEE – Fiber Distribution Data Interface (FDDI)

19 Link-layer addressing o MAC address – Link-layer address, LAN address, MAC address – 6 bytes long = 2 48 possible MAC addresses

20 – No two adaptors have the same address – A flat structure o Address Resolution Protocol (ARP) – To send a datagram, the source node must give its adapter not only the IP datagram but also the MAC address for destination node. – An ARP module in the sending node takes any IP address on the same LAN as input, and returns the corresponding MAC address. – DNS vs ARP – ARP query and response

21

22 – Sending a datagram to a node off the subnet

23 o A creates IP datagram with source A, destination B  A uses ARP to get R ’ s MAC address for o A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram  A ’ s NIC sends frame  R ’ s NIC receives frame o R removes IP datagram from Ethernet frame, sees its destined to B  R uses ARP to get B ’ s MAC address o R creates frame containing A-to-B IP datagram sends to B R 1A-23-F9-CD-06-9B E6-E BB-4B CC-49-DE-D0-AB-7D A C-E8-FF B2-2F-54-1A-0F B BD-D2-C7-56-2A

24 Ethernet – Success factors  First widely deployed  Simple and cheap  Kept up with speed race – Hub: whenever a hub receives a bit from one of its interfaces, it sends a copy of the bit to all of its other interfaces.

25 – Ethernet frame structure  Data field (46 to 1,500 bytes): MTU  Destination address (6bytes)  Source address (6 bytes)  Type field (2 bytes): IP and other network layer protocols  CRC (4 bytes)  Preamble (8 bytes) – Connectionless service – Unreliable service

26 o CSMA/CD: Ethernet’s Multiple Access Protocol 1. An adapter may begin to transmit at any time. 2. An adapter never transmits a frame when it senses that some other adapter is transmitting. (carrier sensing) 3. A transmitting adapter aborts its transmission as soon as it detects that another adapter is also transmitting. (collision detection) 4. Before attempting a retransmission, and adapter waits a random time that is typically samll compared with the time to transmit a frame. (exponential backoff)

27 – Ethernet efficiency = 1 / (1 + 5 d prop / d trans ) where d prop = max time to propagate between any two adapters and d trans = the time to transmit a maximum-size Ethernet fame. o Ethernet Technologies – Repeater – Point-to-point linkes use switches (no collision, no MAC protocol) while broadcast channels use hubs

28 Link-layer switches – The role of the switch is to receive incoming link-layer frames and forward them onto outgoing links. – Filtering – Forwarding – Switch table

29

30 – Suppose a frame arrives at the switch on interface x, the switch does  If there is no entry for the destination address, the switch broadcasts the frame.  There is an entry in the table with interface x, the switch performs the filtering by discarding the frame.  There is an entry in the table with interface y ≠ x, the switch performs its forwarding. – As long as the switch table is complete and accurate, the switch forwards frames towards destination without any broadcasting.

31 o Self-Learning – The switch table is initially empty. – For each incoming frame received on an interface, the switch stores in its table … – Aging time – Plug-and-play devices

32 o Properties of link-layer switching – Elimination of collisions – Heterogeneous links – Management o Switches vs routers – Switch: layer 2, router: layer 3 – Switch: plug-and-play, a spanning tree, hundreds hosts – Router: hierachical, thousands of hosts

33 PPP: The Point-to-Point Protocol – Error detection – No flow control – No sequencing – Single point link – Information: max length 1,500 bytes

34

35 Link Virtualization: A Network as a Link Layer – ATM, MPLS: packet-switched, virtual-circuit networks o Asynchronous Transfer Mode (ATM) networks

36 – An ATM inferface will have an IP address and an ATM address.

37 o Mulitprotocol Label Switching (MPLS) – Like a switched LAN or ATM network, the MPLS-capable routers do without ever touching the IP header of a packet. – Traffic engineering – Virtual private network

38