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Transcript
Ch 5. Link layer and Local Area Networks from Ch. 5 of Computer Networking by Jim Kurose and Keith W. Ross, 2003. Myungchul Kim [email protected] Data link layer 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. Error detection, retransmission, flow control, and random access A single link in the path 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 2 Adaptors: network interface cards (NICs) Fig 5.3 The link interface is responsible for implementing the link-layer protocol 3 Multiple access protocol Point-to-point link: PPP, HDLC Broadcast link: multiple sending and receiving nodes all connected to the same, single, shared broadcast channel. Fig 5.9 4 Multiple access protocols: channel partitioning protocols, random access protocols, and taking-turns protocols. Channel partitioning protocols – – TDM, FDM Fig 5.10 5 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. 6 Fig 5.11 7 Random access protocols: slotted ALOHA, ALOHA, CSMA Slotted ALOHA – Page 440. Fig 5.13 – At best only 37 percent of the slots do useful work. – 8 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. 9 Fig 5.15 10 Fig 5.16 11 Taking-turns protocol – – Polling protocol Token-passing protocol 12 LAN addresses and ARP LAN address, physical address, Ethernet address, MAC address: six bytes in hexadecimal notation Fig 5.18 LAN broadcast address: FF-FF-FF-FF-FF-FF 13 Address resolution protocol (ARP): an IP address to a LAN address Fig 5.19 DNS? ARP query within a broadcast message and plug-andplay using ARP table (Fig 5.20) 14 Sending a datagram to a node off the LAN Fig 5.21 15 Ethernet Reasons for success – – – Deployed early Simple and cheap Producing competent versions Use the same frame structure Fig 5.23 Data field (46 to 1500 bytes): carries the IP datagram, MTU 16 Type fields (2bytes): IP, Novell IPX, AppleTalk, ARP, .. CRC: detect errors Preamble (8bytes): 10101010(7times) and 10101011 Unreliable connectionless service vs IP, TCP, UDP Baseband transmission Manchester encoding (physical layer) Fig 5.24 17 CSMA/CD mechanisms 1. 2. 3. 4. An adaptor may begin to transmit at any time; that is, no slots are used. An adaptor never transmits a frame when it senses that some other adaptor is transmitting; that is, it uses carrier sensing. A transmitting adapter aborts its transmission as soon as it detects that another adaptor is also transmitting; that is, it uses collision detection. Before attempting a retransmission, an adaptor waits a random time that is typically small compared with the time to transmit a frame. 18 10Base2, 10BaseT, 100BaseT, Gigabit Ethernet Repeater: a physical-layer device acts on individual bits rather than on frames. Fig 5.25 19 A hub is a repeater Fig 5.26 20 Hubs The hub simply broadcasts the bit on all the other interfaces. The same collision domain Fig 5.27 21 Limitations – – – Larger collision domain Same Ethernet technologies Max allowable number of nodes in a collision domain 22 Bridges Layer-2 devices Isolated collision domain, different LAN, no limit on the size of LAN Filtering and forwarding using bridge table Fig 5.28 23 Self-learning: a bridge table is build automatically. Plug-and-play device Bridges vs Routers – – – – – Plug-and-play or not Layer 2 or 3 Broadcast Flat vs hierarchical addressing Network size 24 Fig 5.32 25 Switches Bridge (a small number of interfaces) switches (dozens of interfaces) Full-duplex mode Fig 5.34 Neither collision detection nor carrier sending No medium-access protocol 26 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. Table 5.1 27 Wireless Links Ubiquitous computing IEEE 802.11b: wireless Ethernet, Wi-Fi – – – – 2.4 GHz 11 Mbps Physical layer: Direct Sequence Spread Spectrum (DSSS) MAC layer 802.11a: 5-6GHz, 54Mbps 802.11g: 2.4GHZ, 54Mbps All of the 802.11 standards have the same architecture and use the same MAC protocol 28 Basic service set (BSS): a cell, Access point (AP) ad hoc network Fig 5.36 29 802.11 Media access protocol – – An explicit ack from back to the sender Fig 5.38 30 No collision detection – – Costly A collision still occur at the receiver, why Hidden terminal problem and fading – Fig 5.39 31 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 Bluetooth – – – – – 2.45GHz 721-64kbps 10 – 100 meter Replacement of cable Cf. infrared technology 34 PPP Data link layer protocol – – – – – – – – Packet framing Transparency Multiple network-layer protocols Multiple types of links Error detection Connection liveness Network-layer address negotiation Simplicity Not required to implement – – – – Error correction Flow control Sequencing Multipoint links 35 PPP data framing – – Address and control fields not used Fig 5.41 36 Byte stuffing – – – Forbid the upper-layer protocol from sending data containing the flag field bit pattern. Control escape byte, 01111101 Fig 5.43 37 ATM characteristics – – – – – – – From an application-level API to the physical layer CBR, VBR, ABR and UBR Cell: 5 + 48 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 38 Fig 5.44 and 5.45 39 Fig 5.47 40 IP over ATM – – – Dynamic vs Permanent virtual channel pp. 503-504 Fig 5.52 41