Ch 5. Link layer and Local Area Networks 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 Packet collisions: 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 senders 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 Baseband transmission o Manchester encoding (physical layer) o Fig 5.24 18 o CSMA/CD mechanisms (pp. 460) o 10Base2, 10BaseT, 100BaseT, Gigabit Ethernet o Repeater: a physical-layer device acts on individual bits rather than on frames. o Fig 5.25 19 o A hub is a repeater o Fig 5.26 20 o The hub simply broadcasts the bit on all the other interfaces. o The same collision domain o Fig 5.27 Hubs 21 o Limitations Larger collision domain Same Ethernet technologies Max allowable number of nodes in a collision domain 22 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 23 o Self-learning (pp. 471) 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 24 o Fig 5.32 25 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 26 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 27 o Ubiquitous 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 28 o Basic service set (BSS): a cell, Access point (AP), ad hoc network o Fig 5.36 29 o Media access protocol An explicit ack from back to the sender Fig 5.38 30 o No collision detection Costly A collision still occur at the receiver, why o Hidden terminal problem and fading Fig 5.39 31 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. 32 Fig 5.40 33 o Bluetooth 2.45GHz kbps 10 100 meter Replacement of cable Cf. infrared technology 34 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 35 o PPP data framing Address and control fields not used Fig 5.41 36 o Byte stuffing Forbid the upper-layer protocol from sending data containing the flag field bit pattern. Control escape byte, Fig 5.43 37 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 38 o Fig 5.44 and 5.45 39 o Fig 5.47 40 o IP over ATM Permanent virtual channel pp Fig 5.52