Bài giảng Mạng máy tính - Chapter 5: The data link layer (Part II) - Hồ Phương Đắc

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Summary of MAC protocols

 What do you do with a shared media?

 Channel Partitioning, by time, frequency or code

• Time Division,Code Division, Frequency Division

 Random partitioning (dynamic),

• ALOHA, S-ALOHA, CSMA, CSMA/CD

• carrier sensing: easy in some technoligies (wire), hard

in others (wireless)

• CSMA/CD used in Ethernet

 Taking Turns

• polling from a central cite, token passing

5: DataLink Layer

5a-1

LAN technologies

Data link layer so far:

 services, error detection/correction, multiple

access

Next: LAN technologies

 addressing

 Ethernet

 hubs, bridges, switches

 802.11

 PPP

 ATM

5: DataLink Layer

5a-2

LAN Addresses and ARP

32-bit IP address:

 network-layer address

 used to get datagram to destination network

(recall IP network definition)

LAN (or MAC or physical) 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

5a-3

LAN Addresses and ARP

Each adapter on LAN has unique LAN address

5: DataLink Layer

5a-4

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 network to which one attaches

5: DataLink Layer

5a-5

Recall earlier routing discussion

Starting at A, given IP

223.1.1.1

datagram addressed to B:

223.1.2.1

223.1.2.9

223.1.1.2

 look up net. address of B, find B

223.1.1.4

on same net. as A

 link layer send datagram to B

^223.1.2.2E

223.1.3.27

223.1.1.3

223.1.3.1

inside link-layer frame

223.1.3.2

frame source,

dest address

datagram source,

dest address

A’s IP

addr

B’s IP

addr

B’s MAC A’s MAC

IP payload

addr

datagram

frame

5: DataLink Layer

5a-6

ARP: Address Resolution Protocol

 Each IP node (Host,

Router) on LAN has

ARP module, table

Question: how to determine

MAC address of B

given B’s IP address?

 ARP Table: IP/MAC

address mappings for

some LAN nodes

< IP address; MAC address; TTL>

………………………….. >

 TTL (Time To Live): time

after which address

mapping will be forgotten

(typically 20 min)

5: DataLink Layer

5a-7

ARP protocol

 A knows B's IP address, wants to learn physical

address of B

 A broadcasts ARP query pkt, containing B's IP

address

 all machines on LAN receive ARP query

 B receives ARP packet, replies to A with its (B's)

physical layer address

 A caches (saves) IP-to-physical address pairs until

information becomes old (times out)

 soft state: information that times out (goes

away) unless refreshed

5: DataLink Layer

5a-8

Routing to another LAN

walkthrough: routing from A to B via R

5: DataLink Layer

5a-9

 A creates IP packet with source A, destination B

 A uses ARP to get R’s physical layer address for 111.111.111.110

 A creates Ethernet frame with R's physical address as dest,

Ethernet frame contains A-to-B IP datagram

 A’s data link layer sends Ethernet frame

 R’s data link layer receives Ethernet frame

 R removes IP datagram from Ethernet frame, sees its

destined to B

 R uses ARP to get B’s physical layer address

 R creates frame containing A-to-B IP datagram sends to B

5: DataLink Layer

5a-10

Ethernet

“dominant” LAN technology:

 cheap $5 for 100Mbs!

 first wildey used LAN technology

 Simpler, cheaper than token LANs and ATM

 Kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Etheret

sketch

5: DataLink Layer

5a-11

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

5a-12

Ethernet Frame Structure

(more)

 Addresses: 6 bytes, frame is received by all

adapters on a LAN and dropped if address does

not match

 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

5a-13

Ethernet: uses CSMA/CD

A: sense channel, if idle

then {

transmit and monitor the channel;

If detect another transmission

then {

abort and send jam signal;

update # collisions;

delay as required by exponential backoff algorithm;

goto A

}

else {done with the frame; set collisions to zero}

}

else {wait until ongoing transmission is over and goto A}

5: DataLink Layer

5a-14

Ethernet’s CSMA/CD (more)

Jam Signal: make sure all other transmitters are

aware of collision; 48 bits;

Exponential Backoff:

 Goal: adapt retransmission attemtps to estimated

current load

 heavy load: random wait will be longer

 first collision: choose K from {0,1}; delay is K x 512

bit transmission times

 after second collision: choose K from {0,1,2,3}…

 after ten or more collisions, choose K from

{0,1,2,3,4,…,1023}

5: DataLink Layer

5a-15

Ethernet Technologies: 10Base2

 10: 10Mbps; 2: under 200 meters max cable length

 thin coaxial cable in a bus topology

 repeaters used to connect up to multiple segments

 repeater repeats bits it hears on one interface to

its other interfaces: physical layer device only!

5: DataLink Layer

5a-16

10BaseT and 100BaseT

 10/100 Mbps rate; latter called “fast ethernet”

 T stands for Twisted Pair

 Hub to which nodes are connected by twisted pair,

thus “star topology”

 CSMA/CD implemented at hub

5: DataLink Layer

5a-17

10BaseT and 100BaseT (more)

 Max distance from node to Hub is 100 meters

 Hub can disconnect “jabbering adapter

 Hub can gather monitoring information, statistics

for display to LAN administrators

5: DataLink Layer

5a-18

Gbit Ethernet

 use 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 to be efficient

 uses hubs, called here “Buffered Distributors”

 Full-Duplex at 1 Gbps for point-to-point links

5: DataLink Layer

5a-19

Interconnecting LANs

Q: Why not just one big LAN?

 Limited amount of supportable traffic: on single

LAN, all stations must share bandwidth

 limited length: 802.3 specifies maximum cable

length

 large “collision domain” (can collide with many

stations)

 limited number of stations: 802.5 have token

passing delays at each station

5: DataLink Layer

5a-20

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