Ch 5. Link layer and Local Area Networks from Ch. 5 of Computer Networking by Jim Kurose and Keith W. Ross, Myungchul Kim

2 o A transmitting node encapsulates the datagram in a link-layer frame and transmits the frame into the link; and a receiving node receives the frame and extracts the datagram. o Error detection, retransmission, flow control, and random access o A single link in the path o A link-layer protocol includes – Framing – Link access: multiple access problem – Reliable delivery – Flow control: frame buffering capacity – Error detection – Error correction – Half-duplex and full-duplex Data link layer

3 o Adaptors: network interface cards (NICs) o Fig 5.3 o The link interface is responsible for implementing the link-layer protocol

4 o Point-to-point link: PPP, HDLC o Broadcast link: multiple sending and receiving nodes all connected to the same, single, shared broadcast channel. o Fig 5.9 Multiple access protocol

5 o Multiple access protocols: channel partitioning protocols, random access protocols, and taking-turns protocols. o Channel partitioning protocols – TDM, FDM – Fig 5.10

6 o Code Division Multiple Access (CDMA) – Assigns a different code to each node – Allows different nodes to transmit simultaneously and yet have their respective receivers correctly receive a sender’s encoded data bits in spite of interfering transmissions by other node. – Partitions the codespace – Issues: 1. codes must be carefully chosen, 2. the received signal strengths from various senders at a receiver are the same.

7 o Fig 5.11

8 o Random access protocols: slotted ALOHA, ALOHA, CSMA o Slotted ALOHA – Page 440. – Fig 5.13 – At best only 37 percent of the slots do useful work.

9 o CSMA – Listen before speaking: carrier sensing – If someone else begins talking at the same time, stop talking: collision detection. – CSMA vs CSMA/CD – The longer this propagation delay, the larger the chance that a carrier-sensing node is not yet able to sense a transmission that has already begun at another node in the network. – When a node performs collision detection, it will cease transmission as soon as it detects a collision.

10 o Fig 5.15

11 o Fig 5.16

12 o Taking-turns protocol – Polling protocol – Token-passing protocol

13 o LAN address, physical address, Ethernet address, MAC address: six bytes in hexadecimal notation o Fig 5.18 o LAN broadcast address: FF-FF-FF-FF-FF-FF LAN addresses and ARP

14 o Address resolution protocol (ARP): an IP address to a LAN address o Fig 5.19 o DNS? o ARP query within a broadcast message and plug-and- play using ARP table (Fig 5.20)

15 o Sending a datagram to a node off the LAN o Fig 5.21

16 o Reasons for success – Deployed early – Simple and cheap – Producing competent versions o Use the same frame structure o Fig 5.23 o Data field (46 to 1500 bytes): carries the IP datagram, MTU Ethernet

17 o Type fields (2bytes): IP, Novell IPX, AppleTalk, ARP,.. o CRC: detect errors o Preamble (8bytes): (7times) and o Unreliable connectionless service vs IP, TCP, UDP o Baseband transmission o Manchester encoding (physical layer) o Fig 5.24

18 o CSMA/CD mechanisms 1. An adaptor may begin to transmit at any time; that is, no slots are used. 2. An adaptor never transmits a frame when it senses that some other adaptor is transmitting; that is, it uses carrier sensing. 3. A transmitting adapter aborts its transmission as soon as it detects that another adaptor is also transmitting; that is, it uses collision detection. 4. Before attempting a retransmission, an adaptor waits a random time that is typically small compared with the time to transmit a frame.

19 o 10Base2, 10BaseT, 100BaseT, Gigabit Ethernet o Repeater: a physical-layer device acts on individual bits rather than on frames. o Fig 5.25

20 o A hub is a repeater o Fig 5.26

21 o The hub simply broadcasts the bit on all the other interfaces. o The same collision domain o Fig 5.27 Hubs

22 o Limitations – Larger collision domain – Same Ethernet technologies – Max allowable number of nodes in a collision domain

23 o Layer-2 devices o Isolated collision domain, different LAN, no limit on the size of LAN o Filtering and forwarding using bridge table o Fig 5.28 Bridges

24 o Self-learning: a bridge table is build automatically. o Plug-and-play device o Bridges vs Routers – Plug-and-play or not – Layer 2 or 3 – Broadcast – Flat vs hierarchical addressing – Network size

25 o Fig 5.32

26 o Bridge (a small number of interfaces) switches (dozens of interfaces) o Full-duplex mode o Fig 5.34 o Neither collision detection nor carrier sending o No medium-access protocol Switches

27 o Cut-through switching: if the buffer becomes empty before the entire packet has arrived, the switch can start to transmit the front of the packet while the back of the packet continues to arrive. o Table 5.1

28 o Ubiquitous computing o IEEE b: wireless Ethernet, Wi-Fi – 2.4 GHz – 11 Mbps – Physical layer: Direct Sequence Spread Spectrum (DSSS) – MAC layer o a: 5-6GHz, 54Mbps o g: 2.4GHZ, 54Mbps o All of the standards have the same architecture and use the same MAC protocol Wireless Links

29 o Basic service set (BSS): a cell, Access point (AP) o ad hoc network o Fig 5.36

30 o Media access protocol – An explicit ack from back to the sender – Fig 5.38

31 o No collision detection – Costly – A collision still occur at the receiver, why o Hidden terminal problem and fading – Fig 5.39

32 o To avoid collisions (CSMA/CA) – A duration field indicating the length of time that its frame will be transmitting on the channel, network allocation vector (NAV) – RTS and CTS to reserve access to the channel  CTS frame helps avoid both the hidden station problem and the fading problem  The RTS and CTS frames are short.

33 – Fig 5.40

34 o Bluetooth – 2.45GHz – kbps – 10 – 100 meter – Replacement of cable – Cf. infrared technology

35 o Data link layer protocol – Packet framing – Transparency – Multiple network-layer protocols – Multiple types of links – Error detection – Connection liveness – Network-layer address negotiation – Simplicity o Not required to implement – Error correction – Flow control – Sequencing – Multipoint links PPP

36 o PPP data framing – Address and control fields not used – Fig 5.41

37 o Byte stuffing – Forbid the upper-layer protocol from sending data containing the flag field bit pattern. – Control escape byte, – Fig 5.43

38 o characteristics – From an application-level API to the physical layer – CBR, VBR, ABR and UBR – Cell: bytes – Virtual circuits: virtual channel identifier (VCI) – No retransmission on a link-by-link basis – Congestion control only within the ATM ABR – Run over any physical layer ATM

39 o Fig 5.44 and 5.45

40 o Fig 5.47

41 o IP over ATM – Dynamic vs Permanent virtual channel – pp – Fig 5.52