mạng máy tính nâng cao nguyễn đức thái chương 5 link layer and lan sinhvienzone com

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Chapter 5
Link Layer and LANs

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A note on the use of these ppt slides:

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We’re making these slides freely available to all (faculty, students, readers).
They’re in PowerPoint form so you can add, modify, and delete slides
(including this one) and slide content to suit your needs. They obviously
represent a lot of work on our part. In return for use, we only ask the
following:
 If you use these slides (e.g., in a class) in substantially unaltered form,
that you mention their source (after all, we’d like people to use our book!)
 If you post any slides in substantially unaltered form on a www site, that
you note that they are adapted from (or perhaps identical to) our slides, and
note our copyright of this material.

Computer Networking:

A Top Down Approach

4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.

Thanks and enjoy! JFK/KWR
All material copyright 1996-2007
J.F Kurose and K.W. Ross, All Rights Reserved
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5: DataLink Layer

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Chapter 5: The Data Link Layer

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Our goals:

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 understand principles behind data link layer



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

error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
reliable data transfer, flow control: done!

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services:

 instantiation and implementation of various link

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layer technologies

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Link Layer
 5.1 Introduction and


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 5.8 Link virtualization:

ATM, MPLS

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

 5.7 PPP

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

services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet

 5.6 Link-layer switches

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Link Layer: Introduction

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Some terminology:
 hosts and routers are nodes

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wired links
wireless links
LANs

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connect adjacent nodes along
communication path are links

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 communication channels that

 layer-2 packet is a frame,

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encapsulates datagram

data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link
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Link layer: context

e.g., Ethernet on first link,
frame relay on
intermediate links, 802.11
on last link

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

 trip from Princeton to

 each link protocol

provides different
services

e.g., may or may not
provide rdt over link

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

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Lausanne
 limo: Princeton to JFK
 plane: JFK to Geneva
 train: Geneva to Lausanne

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different link protocols
over different links:

transportation analogy

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 datagram transferred by

 tourist = datagram
 transport segment =

communication link
 transportation mode =
link layer protocol
 travel agent = routing
algorithm
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Link Layer Services


reliable delivery between adjacent nodes




we learned how to do this already (chapter 3)!
seldom used on low bit-error link (fiber, some twisted
pair)
wireless links: high error rates
• Q: why both link-level and end-end reliability?

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

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encapsulate datagram into frame, adding header, trailer
channel access if shared medium
“MAC” addresses used in frame headers to identify
source, dest
• different from IP address!

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framing, link access:

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

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5: DataLink Layer

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flow control:

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error detection:

errors caused by signal attenuation, noise.
receiver detects presence of errors:
• signals sender for retransmission or drops frame

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pacing between adjacent sending and receiving nodes

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Link Layer Services (more)

 error correction:

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receiver identifies and corrects bit error(s) without
resorting to retransmission

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

half-duplex and full-duplex


with half duplex, nodes at both ends of link can transmit,
but not at same time
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Where is the link layer implemented?

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 in each and every host



Ethernet card, PCMCI
card, 802.11 card
implements link, physical
layer

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 attaches into host’s

application
transport
network
link

cpu

memory

controller
link
physical

host
bus
(e.g., PCI)

physical
transmission

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system buses
 combination of
hardware, software,
firmware

host schematic

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“adaptor” (aka network
interface card NIC)

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 link layer implemented in

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network adapter
card

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5-8


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Adaptors Communicating
datagram

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datagram

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sending host

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controller

controller

receiving host

datagram

frame

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 sending side:
 encapsulates datagram in
frame
 adds error checking bits,
rdt, flow control, etc.
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 receiving side
 looks for errors, rdt, flow
control, etc
 extracts datagram, passes
to upper layer at receiving
side

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Link Layer
 5.1 Introduction and

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 5.8 Link Virtualization:

ATM. MPLS

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

 5.7 PPP

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

services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet

 5.6 Link-layer switches

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Error Detection

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EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields

otherwise

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• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction

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Parity Checking

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Detect and correct single bit errors

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Detect single bit errors

Two Dimensional Bit Parity:

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Single Bit Parity:

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0

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Internet checksum (review)

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Goal: detect “errors” (e.g., flipped bits) in transmitted
packet (note: used at transport layer only)

 treat segment contents

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as sequence of 16-bit
integers
 checksum: addition (1’s
complement sum) of
segment contents
 sender puts checksum
value into UDP checksum

field
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Receiver:
 compute checksum of
received segment
 check if computed checksum
equals checksum field value:
 NO - error detected
 YES - no error detected.

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Sender:

But maybe errors
nonetheless?

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 view data bits, D, as a binary number
 choose r+1 bit pattern (generator), G

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 goal: choose r CRC bits, R, such that

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Checksumming: Cyclic Redundancy Check

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<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder:
error detected!
can detect all burst errors less than r+1 bits

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 widely used in practice (Ethernet, 802.11 WiFi, ATM)

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CRC Example

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Want:
D.2r XOR R = nG

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equivalently:
equivalently:

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if we divide D.2r by
G, want remainder R

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D.2r = nG XOR R

R = remainder[

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D.2r
G

]

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Link Layer
 5.1 Introduction and

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 5.8 Link Virtualization:

ATM, MPLS

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

 5.7 PPP

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

services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet

 5.6 Link-layer switches

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Multiple Access Links and Protocols

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Two types of “links”:

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 point-to-point
 PPP for dial-up access
 point-to-point link between Ethernet switch and host

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 broadcast (shared wire or medium)

 old-fashioned Ethernet
 upstream HFC
 802.11 wireless LAN

shared wire (e.g.,
cabled Ethernet)
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shared RF
(e.g., 802.11 WiFi)

shared RF
(satellite)

humans at a
cocktail party
(shared air, acoustical)
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Multiple Access protocols

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 single shared broadcast channel

interference

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 two or more simultaneous transmissions by nodes:
collision if node receives two or more signals at the same time

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multiple access protocol

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 distributed algorithm that determines how nodes

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share channel, i.e., determine when node can transmit
 communication about channel sharing must use channel
itself!
no out-of-band channel for coordination

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Ideal Multiple Access Protocol

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no special node to coordinate transmissions
no synchronization of clocks, slots

4. simple

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

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Broadcast channel of rate R bps
1. when one node wants to transmit, it can send at
rate R.
2. when M nodes want to transmit, each can send at
average rate R/M
3. fully decentralized:

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MAC Protocols: a taxonomy

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divide channel into smaller “pieces” (time slots,
frequency, code)
allocate piece to node for exclusive use

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Three broad classes:
 Channel Partitioning

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 Random Access
 channel not divided, allow collisions
 “recover” from collisions

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 “Taking turns”
 nodes take turns, but nodes with more to send can take
longer turns

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Channel Partitioning MAC protocols: TDMA

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TDMA: time division multiple access

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 access to channel in "rounds"

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 each station gets fixed length slot (length = pkt

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trans time) in each round
 unused slots go idle
 example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6
idle

1

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6-slot
frame

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3

4

1

3

4

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Channel Partitioning MAC protocols: FDMA

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FDMA: frequency division multiple access

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 channel spectrum divided into frequency bands

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 each station assigned fixed frequency band

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 unused transmission time in frequency bands go idle
 example: 6-station LAN, 1,3,4 have pkt, frequency

FDM cable
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frequency bands

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bands 2,5,6 idle

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Random Access Protocols

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 When node has packet to send
 transmit at full channel data rate R.
 no a priori coordination among nodes

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 two or more transmitting nodes ➜ “collision”,

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 random access MAC protocol specifies:
 how to detect collisions
 how to recover from collisions (e.g., via delayed
retransmissions)

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 Examples of random access MAC protocols:
 slotted ALOHA
 ALOHA
 CSMA, CSMA/CD, CSMA/CA
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Operation:
 when node obtains fresh
frame, transmits in next
slot
 if no collision: node can
send new frame in next
slot
 if collision: node
retransmits frame in
each subsequent slot
with prob. p until
success

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Assumptions:
 all frames same size
 time divided into equal
size slots (time to
transmit 1 frame)
 nodes start to transmit
only slot beginning
 nodes are synchronized
 if 2 or more nodes
transmit in slot, all
nodes detect collision

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Slotted ALOHA

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Slotted ALOHA

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Pros
 single active node can
continuously transmit
at full rate of channel
 highly decentralized:
only slots in nodes
need to be in sync
 simple
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Cons
 collisions, wasting slots
 idle slots
 nodes may be able to
detect collision in less
than time to transmit
packet
 clock synchronization
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