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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
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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
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Framing
Link access: multiple access problem
Reliable delivery
Flow control: frame buffering capacity
Error detection
Error correction
Half-duplex and full-duplex
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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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Multiple access 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
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Multiple access protocols: channel partitioning protocols, random
access protocols, and taking-turns protocols.
Channel partitioning protocols
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TDM, FDM
Fig 5.10
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Code Division Multiple Access (CDMA)
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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.
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Fig 5.11
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Random access protocols: slotted ALOHA, ALOHA,
CSMA
Slotted ALOHA
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Page 440.
Fig 5.13
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At best only 37 percent of the slots do useful work.
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CSMA
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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.
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Fig 5.15
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Fig 5.16
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Taking-turns protocol
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Polling protocol
Token-passing protocol
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LAN addresses and ARP
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LAN address, physical address, Ethernet address, MAC
address: six bytes in hexadecimal notation
Fig 5.18
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LAN broadcast address: FF-FF-FF-FF-FF-FF
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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-andplay 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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Ethernet
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Reasons for success
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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
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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 mechanisms
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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.
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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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Hubs
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The hub simply broadcasts the bit on all the other
interfaces.
The same collision domain
Fig 5.27
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Limitations
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Larger collision domain
Same Ethernet technologies
Max allowable number of nodes in a collision domain
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Bridges
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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
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Self-learning: a bridge table is build automatically.
Plug-and-play device
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Bridges vs Routers
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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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Switches
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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
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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.
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Table 5.1
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Wireless Links
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Ubiquitous computing
IEEE 802.11b: wireless Ethernet, Wi-Fi
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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
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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
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An explicit ack from back to the sender
Fig 5.38
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No collision detection
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Costly
A collision still occur at the receiver, why
Hidden terminal problem and fading
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Fig 5.39
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To avoid collisions (CSMA/CA)
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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
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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
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2.45GHz
721-64kbps
10 – 100 meter
Replacement of cable
Cf. infrared technology
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PPP
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Data link layer protocol
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Packet framing
Transparency
Multiple network-layer protocols
Multiple types of links
Error detection
Connection liveness
Network-layer address negotiation
Simplicity
Not required to implement
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Error correction
Flow control
Sequencing
Multipoint links
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PPP data framing
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Address and control fields not used
Fig 5.41
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Byte stuffing
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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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ATM
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characteristics
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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
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Fig 5.44 and 5.45
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Fig 5.47
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IP over ATM
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Dynamic vs Permanent virtual channel
pp. 503-504
Fig 5.52
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