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【Chapter5】Link Layer and LANs


Chapter 5 Link Layer and LANs

Computer Networking: A Top Down Approach Featuring the Internet,
3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004.

5: DataLink Layer

5-1

Chapter 5: The Data Link Layer
Our goals:
? understand principles behind data link layer

services:
? ? ? ?

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

? instantiation and implementation of various link

layer technologies

5: DataLink Layer

5-2

Link Layer
? 5.1 Introduction and ?

?
? ?

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

? 5.6 Hubs and switches ? 5.7 PPP

5: DataLink Layer

5-3

Link Layer: Introduction
Some terminology:
? hosts and routers are nodes ? communication channels that

“link”

connect adjacent nodes along communication path are links
?

?
?

wired links wireless links LANs

? layer-2 packet is a frame,

encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5: DataLink Layer 5-4

Link layer: context
? Datagram transferred by

transportation analogy
? trip from Princeton to

different link protocols over different links:
?

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

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

? Each link protocol

? tourist = datagram

provides different services
?

? transport segment =

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

communication link ? transportation mode = link layer protocol ? travel agent = routing algorithm
5: DataLink Layer 5-5

Link Layer Services
? Framing, link access:
? ? ?

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!

? 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?
5: DataLink Layer 5-6

Link Layer Services (more)
? ?

Flow Control:
?

pacing between adjacent sending and receiving nodes errors caused by signal attenuation, noise. receiver detects presence of errors: ? signals sender for retransmission or drops frame

Error Detection:
?

?

? Error Correction:
?

?

Half-duplex and full-duplex
?

receiver identifies and corrects bit error(s) without resorting to retransmission
with half duplex, nodes at both ends of link can transmit, but not at same time
5: DataLink Layer 5-7

Adaptors(NIC) Communicating
datagram sending node link layer protocol

rcving node frame adapter

frame adapter

? link layer implemented in ? receiving side “adaptor” (aka NIC) ? looks for errors, rdt, flow control, etc ? Ethernet card, PCMCI ? extracts datagram, passes card, 802.11 card to rcving node ? sending side: ? adapter is semi? encapsulates datagram in autonomous a frame ? adds error checking bits, ? link & physical layers rdt, flow control, etc.
5: DataLink Layer 5-8

Link Layer
? 5.1 Introduction and ?

?
? ?

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

? 5.6 Hubs and switches ? 5.7 PPP

5: DataLink Layer

5-9

Error Detection
EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields
? Error detection not 100% reliable! ? protocol may miss some errors, but rarely ? larger EDC field yields better detection and correction

5: DataLink Layer

5-10

Parity Checking
Single Bit Parity:
Detect single bit errors

Two Dimensional Bit Parity:
Detect and correct single bit errors

0

0

5: DataLink Layer

5-11

Internet checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)
Sender:
? treat segment contents

Receiver:
? compute checksum of received

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

segment ? check if computed checksum equals checksum field value: ? NO - error detected ? YES - no error detected. But

maybe errors nonetheless?
More later ….

5: DataLink Layer

5-12

Checksumming: Cyclic Redundancy Check
? view data bits, D, as a binary number ? choose r+1 bit pattern (generator), G ? goal: choose r CRC bits, R, such that
? ? ?

<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

? widely used in practice (ATM, HDCL)

5: DataLink Layer

5-13

CRC Example
Want: D.2r XOR R = nG

equivalently:

equivalently:

D.2r = nG XOR R

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

R = remainder[

D.2r G

]

5: DataLink Layer

5-14

Link Layer
? 5.1 Introduction and ?

?
? ?

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

? 5.6 Hubs and switches ? 5.7 PPP

5: DataLink Layer

5-15

Multiple Access Links and Protocols
Two types of “links”:
? point-to-point ? PPP for dial-up access ? point-to-point link between Ethernet switch and host ? broadcast (shared wire or medium) ? traditional Ethernet ? upstream HFC ? 802.11 wireless LAN

5: DataLink Layer

5-16

Multiple Access protocols
? single shared broadcast channel ? two or more simultaneous transmissions by nodes:

interference
?

multiple access protocol

collision if node receives two or more signals at the same time

? distributed algorithm that determines how nodes

share channel, i.e., determine when node can transmit ? communication about channel sharing must use channel itself!
?

no out-of-band channel for coordination

5: DataLink Layer

5-17

Ideal Mulitple Access Protocol
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:
? ?

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

4. Simple

5: DataLink Layer

5-18

MAC Protocols: a taxonomy
Three broad classes: ? Channel Partitioning
?
?

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

? Random Access ? channel not divided, allow collisions ? “recover” from collisions ? “Taking turns” ? Nodes take turns, but nodes with more to send can take longer turns

5: DataLink Layer

5-19

Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
? access to channel in "rounds"

? each station gets fixed length slot (length = pkt

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

5: DataLink Layer

5-20

Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
? channel spectrum divided into frequency bands ? each station assigned fixed frequency band ? unused transmission time in frequency bands go idle ? example: 6-station LAN, 1,3,4 have pkt, frequency

bands 2,5,6 idle

frequency bands

5: DataLink Layer

5-21

Random Access Protocols
? When node has packet to send ? transmit at full channel data rate R. ? no a priori coordination among nodes ? two or more transmitting nodes ? “collision”,

? random access MAC protocol specifies: ? how to detect collisions ? how to recover from collisions (e.g., via delayed retransmissions) ? Examples of random access MAC protocols: ? slotted ALOHA ? ALOHA ? CSMA, CSMA/CD, CSMA/CA
5: DataLink Layer 5-22

Slotted ALOHA
Assumptions ? all frames same size ? time is divided into equal size slots, time to transmit 1 frame ? nodes start to transmit frames only at beginning of slots ? nodes are synchronized ? if 2 or more nodes transmit in slot, all nodes detect collision Operation ? when node obtains fresh frame, it transmits in next slot ? no collision, node can send new frame in next slot ? if collision, node retransmits frame in each subsequent slot with prob. p until success

5: DataLink Layer

5-23

Slotted ALOHA

Pros ? single active node can continuously transmit at full rate of channel ? highly decentralized: only slots in nodes need to be in sync ? simple

Cons ? collisions, wasting slots ? idle slots ? nodes may be able to detect collision in less than time to transmit packet ? clock synchronization
5: DataLink Layer 5-24

Slotted Aloha efficiency
Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send
? Suppose N nodes with ? For max efficiency

many frames to send, each transmits in slot with probability p ? prob that node 1 has success in a slot
= p(1-p)N-1

with N nodes, find p* that maximizes Np(1-p)N-1 ? For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37

At best: channel

? prob that any node has a success = Np(1-p)N-1

used for useful transmissions 37% of time!
5: DataLink Layer 5-25

Pure (unslotted) ALOHA
? unslotted Aloha: simpler, no synchronization ? when frame first arrives ? transmit immediately ? collision probability increases: ? frame sent at t0 collides with other frames sent in [t0-1,t0+1]

5: DataLink Layer

5-26

Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [p0-1,p0] = p . (1-p)N-1 . (1-p)N-1 = p . (1-p)2(N-1)
… choosing optimum p and then letting n -> infty ...

P(no other node transmits in [p0-1,p0] .

Even worse !

= 1/(2e) = .18

5: DataLink Layer

5-27

CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit: If channel sensed idle: transmit entire frame ? If channel sensed busy, defer transmission
? Human analogy: don’t interrupt others!

5: DataLink Layer

5-28

CSMA collisions
collisions can still occur:
propagation delay means two nodes may not hear each other’s transmission

spatial layout of nodes

collision: note:

entire packet transmission time wasted role of distance & propagation delay in determining collision probability

5: DataLink Layer

5-29

CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time ? colliding transmissions aborted, reducing channel wastage
?

? collision detection: ? easy in wired LANs: measure signal strengths, compare transmitted, received signals ? difficult in wireless LANs: receiver shut off while transmitting
? human analogy: the polite conversationalist
5: DataLink Layer 5-30

CSMA/CD collision detection

5: DataLink Layer

5-31

“Taking Turns” MAC protocols
channel partitioning MAC protocols: ? share channel efficiently and fairly at high load ? inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols ? efficient at low load: single node can fully utilize channel ? high load: collision overhead “taking turns” protocols look for best of both worlds!
5: DataLink Layer 5-32

“Taking Turns” MAC protocols
Token passing: Polling: ? control token passed from ? master node one node to next “invites” slave nodes sequentially. to transmit in turn ? token message ? concerns: ? concerns: ? polling overhead
? ?

latency single point of failure (master)

? ? ?

token overhead latency single point of failure (token)

5: DataLink Layer

5-33

Summary of MAC protocols
? What do you do with a shared media? ? Channel Partitioning, by time, frequency or code
? Time Division, Frequency Division
?

Random partitioning (dynamic),
? ALOHA, S-ALOHA, CSMA, CSMA/CD ? carrier sensing: easy in some technologies (wire), hard in others (wireless) ? CSMA/CD used in Ethernet ? CSMA/CA used in 802.11

?

Taking Turns
? polling from a central site, token passing
5: DataLink Layer 5-34

LAN technologies
Data link layer so far:
? services,

access

error detection/correction, multiple

Next: LAN technologies
? addressing

Ethernet ? hubs, switches ? PPP
?

5: DataLink Layer

5-35

Link Layer
? 5.1 Introduction and ?

?
? ?

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

? 5.6 Hubs and switches ? 5.7 PPP

5: DataLink Layer

5-36

MAC Addresses and ARP
? 32-bit IP address:
? ?

network-layer address
used to get datagram to destination IP subnet

? MAC (or LAN or physical or Ethernet)

address:
?

used to get datagram from one interface to another physically-connected interface (same network) ? 48 bit MAC address (for most LANs) burned in the adapter ROM
5: DataLink Layer 5-37

LAN Addresses and ARP
Each adapter on LAN has unique LAN address

1A-2F-BB-76-09-AD

Broadcast address = FF-FF-FF-FF-FF-FF

71-65-F7-2B-08-53

LAN (wired or wireless)
58-23-D7-FA-20-B0

= adapter

0C-C4-11-6F-E3-98

5: DataLink Layer

5-38

LAN Address (more)
? MAC address allocation administered by IEEE ? manufacturer buys portion of MAC address space

(to assure uniqueness) ? Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address ? MAC flat address ? portability
?

can move LAN card from one LAN to another

? IP hierarchical address NOT portable ? depends on IP subnet to which node is attached

5: DataLink Layer

5-39

ARP: Address Resolution Protocol
Question: how to determine MAC address of B knowing B’s IP address?
237.196.7.78

? Each IP node (Host,

1A-2F-BB-76-09-AD 237.196.7.23
237.196.7.14

Router) on LAN has ARP table ? ARP Table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL>
?

LAN
71-65-F7-2B-08-53

58-23-D7-FA-20-B0

TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

237.196.7.88

0C-C4-11-6F-E3-98

5: DataLink Layer

5-40

ARP protocol: Same LAN (network)
? A wants to send datagram

to B, and B’s MAC address not in A’s ARP table. ? A broadcasts ARP query packet, containing B's IP address ? Dest MAC address = FF-FF-FF-FF-FF-FF ? all machines on LAN receive ARP query ? B receives ARP packet, replies to A with its (B's) MAC address
?

? A caches (saves) IP-to-

MAC address pair in its ARP table until information becomes old (times out) ? soft state: information that times out (goes away) unless refreshed

frame sent to A’s MAC address (unicast)

? ARP is “plug-and-play”: ? nodes create their ARP tables without intervention from net administrator

5: DataLink Layer

5-41

Routing to another LAN
walkthrough: send datagram from A to B via R assume A know’s B IP address

A

R
? Two ARP tables in router R, one for each IP

B

network (LAN)

5: DataLink Layer

5-42

? A creates datagram with source A, destination B

? A uses ARP to get R’s MAC address for 111.111.111.110
? A creates link-layer frame with R's MAC address as dest, ?

?
? ?

?

frame contains A-to-B IP datagram A’s adapter sends frame R’s adapter receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B

A R

B
5: DataLink Layer 5-43

Link Layer
? 5.1 Introduction and ?

?
? ?

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

? 5.6 Hubs and switches ? 5.7 PPP

5: DataLink Layer

5-44

Ethernet
“dominant” wired LAN technology: ? cheap $20 for 100Mbs! ? first widely used LAN technology ? Simpler, cheaper than token LANs and ATM ? Kept up with speed race: 10 Mbps – 10 Gbps

Metcalfe’s Ethernet sketch

5: DataLink Layer

5-45

Star topology
? Bus topology popular through mid 90s ? Now star topology prevails

? Connection choices: hub or switch (more later)

hub or switch

5: DataLink Layer

5-46

Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble: ? 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 ? used to synchronize receiver, sender clock rates

5: DataLink Layer

5-47

Ethernet Frame Structure (more)
? Addresses: 6 bytes ? if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol ? otherwise, adapter discards frame ? Type: indicates the higher layer protocol (mostly

IP but others may be supported such as Novell IPX and AppleTalk) ? CRC: checked at receiver, if error is detected, the frame is simply dropped

5: DataLink Layer

5-48

Unreliable, connectionless service
? Connectionless: No handshaking between sending

and receiving adapter. ? Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter
?
? ?

stream of datagrams passed to network layer can have gaps gaps will be filled if app is using TCP otherwise, app will see the gaps

5: DataLink Layer

5-49

Ethernet uses CSMA/CD
? No slots ? adapter doesn’t transmit ? Before attempting a

if it senses that some other adapter is transmitting, that is, carrier sense ? transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection

retransmission, adapter waits a random time, that is, random access

5: DataLink Layer

5-50

Ethernet CSMA/CD algorithm
1. Adaptor receives 4. If adapter detects datagram from net layer & another transmission while creates frame transmitting, aborts and sends jam signal 2. If adapter senses channel idle, it starts to transmit 5. After aborting, adapter frame. If it senses enters exponential channel busy, waits until backoff: after the mth channel idle and then collision, adapter chooses transmits a K at random from {0,1,2,…,2m-1}. Adapter 3. If adapter transmits waits K?512 bit times and entire frame without returns to Step 2 detecting another transmission, the adapter is done with frame ! 5: DataLink Layer 5-51

Ethernet’s CSMA/CD (more)
Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time: .1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec Exponential Backoff: ? Goal: adapt retransmission attempts to estimated current load
?

heavy load: random wait will be longer

? first collision: choose K

See/interact with Java applet on AWL Web site: highly recommended !

from {0,1}; delay is K? 512 bit transmission times ? after second collision: choose K from {0,1,2,3}… ? after ten collisions, choose K from {0,1,2,3,4,…,1023}

5: DataLink Layer

5-52

CSMA/CD efficiency
? Tprop = max prop between 2 nodes in LAN ? ttrans = time to transmit max-size frame

efficiency?

1 1 ? 5t prop / ttrans

? Efficiency goes to 1 as tprop goes to 0

? Goes to 1 as ttrans goes to infinity

? Much better than ALOHA, but still decentralized,

simple, and cheap

5: DataLink Layer

5-53

10BaseT and 100BaseT
? 10/100 Mbps rate; latter called “fast ethernet”

? T stands for Twisted Pair
? Nodes connect to a hub: “star topology”; 100 m

max distance between nodes and hub

twisted pair

hub

5: DataLink Layer

5-54

Hubs
Hubs are essentially physical-layer repeaters: ? bits coming from one link go out all other links ? at the same rate ? no frame buffering ? no CSMA/CD at hub: adapters detect collisions ? provides net management functionality

twisted pair

hub

5: DataLink Layer

5-55

Manchester encoding

? Used in 10BaseT ? Each bit has a transition ? Allows clocks in sending and receiving nodes to

synchronize to each other
?

no need for a centralized, global clock among nodes!
5: DataLink Layer 5-56

? Hey, this is physical-layer stuff!

Gbit Ethernet
? uses standard Ethernet frame format ? allows for point-to-point links and shared ? ? ?

?

broadcast channels in shared mode, CSMA/CD is used; short distances between nodes required for efficiency uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now !

5: DataLink Layer

5-57

Link Layer
? 5.1 Introduction and ?

?
? ?

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

? 5.6 Interconnections:

Hubs and switches ? 5.7 PPP

5: DataLink Layer

5-58

Interconnecting with hubs
? Backbone hub interconnects LAN segments ? Extends max distance between nodes

? But individual segment collision domains become one

large collision domain ? Can’t interconnect 10BaseT & 100BaseT
hub

hub

hub

hub

5: DataLink Layer

5-59

Switch
? Link layer device

stores and forwards Ethernet frames ? examines frame header and selectively forwards frame based on MAC dest address ? when frame is to be forwarded on segment, uses CSMA/CD to access segment ? transparent ? hosts are unaware of presence of switches ? plug-and-play, self-learning ? switches do not need to be configured
?

5: DataLink Layer

5-60

Forwarding
1
2 switch 3

hub

hub

hub

? How do determine onto which LAN segment to forward frame? ? Looks like a routing problem...
5: DataLink Layer 5-61

Self learning
? A switch has a switch table
? entry in switch table:

(MAC Address, Interface, Time Stamp) ? stale entries in table dropped (TTL can be 60 min) ? switch learns which hosts can be reached through which interfaces ? when frame received, switch “learns” location of sender: incoming LAN segment ? records sender/location pair in switch table
?

5: DataLink Layer

5-62

Filtering/Forwarding
When switch receives a frame: index switch table using MAC dest address if entry found for destination then{ if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood

forward on all but the interface on which the frame arrived

5: DataLink Layer

5-63

Switch example
Suppose C sends frame to D
1 switch 2 3 hub D F hub I E address interface A B E G 1 1 2 3

A B C

hub

G

H

? Switch receives frame from from C ? notes in bridge table that C is on interface 1 ? because D is not in table, switch forwards frame into interfaces 2 and 3 ? frame received by D
5: DataLink Layer 5-64

Switch example
Suppose D replies back with frame to C.
switch
address interface A B E G C 1 1 2 3 1

A B C

hub D

hub F

hub I

E

G

H

? Switch receives frame from from D ? notes in bridge table that D is on interface 2 ? because C is in table, switch forwards frame only to interface 1 ? frame received by C
5: DataLink Layer 5-65

Switch: traffic isolation
? switch installation breaks subnet into LAN

segments ? switch filters packets: ? same-LAN-segment frames not usually forwarded onto other LAN segments ? segments become separate collision domains
switch collision domain hub hub hub

collision domain

collision domain

5: DataLink Layer

5-66

Switches: dedicated access
? Switch with many

interfaces ? Hosts have direct connection to switch ? No collisions; full duplex

A C’ B

switch

Switching: A-to-A’ and B-to-B’ simultaneously, no collisions

C B’ A’

5: DataLink Layer

5-67

More on Switches
? cut-through switching: frame forwarded

from input to output port without first collecting entire frame ? slight reduction in latency ? combinations of shared/dedicated, 10/100/1000 Mbps interfaces

5: DataLink Layer

5-68

Institutional network
to external network mail server router switch web server

IP subnet
hub

hub

hub

5: DataLink Layer

5-69

Switches vs. Routers
? both store-and-forward devices ? routers: network layer devices (examine network layer headers) ? switches are link layer devices ? routers maintain routing tables, implement routing

algorithms ? switches maintain switch tables, implement filtering, learning algorithms

5: DataLink Layer

5-70

Summary comparison
hubs traffic isolation plug & play optimal routing cut through no yes no yes routers yes no yes no switches yes yes no yes
5: DataLink Layer 5-71

Link Layer
? 5.1 Introduction and ?

?
? ?

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

? 5.6 Hubs and switches ? 5.7 PPP

5: DataLink Layer

5-72

Point to Point Data Link Control
? one sender, one receiver, one link: easier than

broadcast link: ? no Media Access Control ? no need for explicit MAC addressing ? e.g., dialup link, ISDN line ? popular point-to-point DLC protocols: ? PPP (point-to-point protocol) ? HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!

5: DataLink Layer

5-73

PPP Design Requirements [RFC 1557]
? packet framing: encapsulation of network-layer

? ? ? ?

datagram in data link frame ? carry network layer data of any network layer protocol (not just IP) at same time ? ability to demultiplex upwards bit transparency: must carry any bit pattern in the data field error detection (no correction) connection liveness: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address
5: DataLink Layer 5-74

PPP non-requirements
? no error correction/recovery
? no flow control ? out of order delivery OK ? no need to support multipoint links (e.g., polling)

Error recovery, flow control, data re-ordering all relegated to higher layers!

5: DataLink Layer

5-75

PPP Data Frame
? Flag: delimiter (framing) ? Address: does nothing (only one option) ? Control: does nothing; in the future possible

multiple control fields ? Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

5: DataLink Layer

5-76

PPP Data Frame
? info: upper layer data being carried ? check: cyclic redundancy check for error

detection

5: DataLink Layer

5-77

Byte Stuffing
? “data transparency” requirement: data field must be allowed to include flag pattern <01111110> ? Q: is received <01111110> data or flag?

? Sender: adds (“stuffs”) extra < 01111110> byte

after each < 01111110> data byte ? Receiver: ? two 01111110 bytes in a row: discard first byte, continue data reception ? single 01111110: flag byte
5: DataLink Layer 5-78

Byte Stuffing
flag byte pattern in data to send

flag byte pattern plus stuffed byte in transmitted data
5: DataLink Layer 5-79

PPP Data Control Protocol
Before exchanging networklayer data, data link peers must ? configure PPP link (max. frame length, authentication) ? learn/configure network layer information ? for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address
5: DataLink Layer 5-80

Chapter 5: Summary
? principles behind data link layer services: ? error detection, correction ? sharing a broadcast channel: multiple access ? link layer addressing ? instantiation and implementation of various link

layer technologies ? Ethernet ? switched LANS ? PPP

5: DataLink Layer

5-81


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5) The data unit of data link layer is ___...8) Wireless LANs use the 802.__ LAN standard....And further, please given.. what their ...
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5页 免费 SEMESTER 1 Chapter 7KEY 暂无评价 4页 免费 SEMESTER 1 Chapter 6...the Logical Link Control (LLC) sublayer and of the Data Link Layer? the...
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