Ch 5. Link layer and Local Area Networks

from Ch. 5 of Computer Networking by Jim Kurose and Keith W. Ross, 2003.

Myungchul Kim

mckim@icu.ac.kr

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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

Data link layer

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Adaptors: network interface cards (NICs) Fig 5.3

The link interface is responsible for implementing the link-layer protocol

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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

Multiple access protocol

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Multiple access protocols: channel partitioning protocols, random access protocols, and taking-turns protocols.

Channel partitioning protocols– TDM, FDM– Fig 5.10

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Code Division Multiple Access (CDMA)– Assigns a different code to each node– Allows different nodes to transmit simultaneously and yet have t

heir 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.

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Fig 5.11

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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.

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CSMA– Listen before speaking: carrier sensing– If someone else begins talking at the same time, stop talking: col

lision detection.– CSMA vs CSMA/CD– The longer this propagation delay, the larger the chance that a c

arrier-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.

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Fig 5.15

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Fig 5.16

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

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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

LAN addresses and ARP

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Address resolution protocol (ARP): an IP address to a LAN address

Fig 5.19

DNS? ARP query within a broadcast message and plug-and-

play using ARP table (Fig 5.20)

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Sending a datagram to a node off the LAN Fig 5.21

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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

Ethernet

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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

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CSMA/CD mechanisms1. 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.

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10Base2, 10BaseT, 100BaseT, Gigabit Ethernet Repeater: a physical-layer device acts on individual bits

rather than on frames. Fig 5.25

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A hub is a repeater Fig 5.26

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The hub simply broadcasts the bit on all the other interfaces.

The same collision domain Fig 5.27

Hubs

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

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Layer-2 devices Isolated collision domain, different LAN, no limit on the

size of LAN Filtering and forwarding using bridge table Fig 5.28

Bridges

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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

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Fig 5.32

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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

Switches

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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

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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 a

nd use the same MAC protocol

Wireless Links

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

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

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No collision detection– Costly– A collision still occur at the receiver, why

Hidden terminal problem and fading– Fig 5.39

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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.

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– Fig 5.40

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Bluetooth– 2.45GHz– 721-64kbps– 10 – 100 meter– Replacement of cable– Cf. infrared technology

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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

PPP

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

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Byte stuffing– Forbid the upper-layer protocol from sending data containing the

flag field bit pattern.– Control escape byte, 01111101– Fig 5.43

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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

ATM

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Fig 5.44 and 5.45

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Fig 5.47

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IP over ATM– Dynamic vs Permanent virtual channel– pp. 503-504– Fig 5.52