Repeaters can Mix 802.3 PHY Standards

Repeaters can Mix 802.3 PHY Standards

Repeater Repeater

NICBNC

AUIConnectors

RJ-45

UTP DropCables

10Base5 Trunk Cable

10Base2

10Base-T

802.3 MAC Layer: Access Control• Media Access Control (MAC) Layer

– Control over when a station may transmit– Only one station may transmit at a time with a

hub– Otherwise, their signals would be scrambled

Hub Hub

802.3 MAC Layer: Access Control• Access Control in Ethernet: CSMA/CD

• Carrier Sense Multiple Access (CSMA)– Carrier sense = listen to traffic– Multiple access = control multiple stations

802.3 MAC Layer: Access Control• Access Control in Ethernet: CSMA/CD

• CSMA Operation– If no one else is transmitting, NIC may transmit– If anyone else is transmitting, NIC must wait

until nobody is transmitting

If IncomingTraffic, wait

If NoIncoming

Traffic, send

802.3 MAC Layer: Access Control

• CSMA/CD• Collision Detection (CD)

– If two stations transmit at the same time, each hears the other

– Both stop, wait random amounts of time

– Transmit after wait, but only if the line is free

802.3 MAC Layer: Access Control• CSMA/CD

• Collision Detection

– If there is another collision

– Stations back off a longer random time period

– After 16 collisions, discard the frame

802.3 MAC Layer: Access Control

• Switches Do Not Need CSMA/CD– No danger of collision– Can even work in full duplex (802.3x), with

NICs sending and receiving at the same time

• However, Ordinary NICs Can Work With Switches– Only hear other traffic if the traffic is directed

at them, so waits to transmit are rare and brief

802.3 Ethernet MAC Layer Frame• MAC Standard Also Defines 802.3 Ethernet

MAC Frame– Header– Data Field– Trailer

• Header Has Multiple Fields– Measure size in octets (bytes)

Trailer Data Field

Header Fields

Ethernet Frame

802.3 Ethernet MAC Layer Frame

• Preamble and Start of Frame Delimiter– To synchronize receiver’s clock– Preamble is 56-bit alternating 101010… pattern– SFD is 10101011 to end the synchronization– Together, 64-bit synchronizing pattern

PreSFDDASALenDataPADFCS

Ethernet 802.3 MAC Layer Frame

802.3 Ethernet MAC Layer Frame• Destination Address Field

– Address of destination device (receiver)

• Source Address Field– Address of source device (sender)

• 48-bit MAC Addresses– Must be unique– All NICs are sold with unique MAC addresses



• Source and Destination Addresses are Expressed in Hexadecimal Notation (hex)– Base 16– 48 bits are divided into twelve 4-bit units– Each unit is represented by a hex symbol (0-9, A-F)– Grouped in pairs of symbols, followed by a lower-case

h for Hex


A1-BD-23-0C-09-C3 h


• Hex SymbolsBits Hex

SymbolBits Hex

Symbol

0000 0 1000 8

0001 1 1001 9

0010 2 1010 A

0011 3 1011 B

0100 4 1100 C

0101 5 1101 D

0110 6 1110 E

0111 7 1111 F


• Length Field (2 Octets)

– Length of the Data Field, not of the entire frame

– Maximum data file size is 1500 octets

LenDataPAD


• Data Field– Frame of next higher layer,

LLC

• PAD Field

– 46-octet minimum size for MAC data field plus PAD

– If Data Field is smaller, add PAD field to bring data field plus PAD to 46 octets

LenDataPAD


• Frame Check Sequence Field (2 Octets)

– Error checking information

– Sending computer computes FCS number and places it in FCS field

– Uses cyclical redundancy check (CRC) method



• Frame Check Sequence (2 Octets)– Receiving NIC recomputes FCS number– If disagrees with transmitted FCS field,

discards the frame!– Does not ask for a retransmission– A higher layer must do this


802.3 Ethernet MAC Layer Frame• Tag Fields Being Added

– Added after address fields– To designate priority (frames with higher

priority go first if there is congestion)– To designate VLANs (Ch. 8)– 802.1Q standardizes overall structure– 802.1p standardizes priority levels

PreSFDDASALenDataPADFCS TPIDTCI

802.3 Ethernet MAC Layer Frame• Tag Protocol ID (TPID) (2 Octets)

– Located where length field normally goes

– Identifies frame as tagged

– If a length field, must be less than 1500, because the maximum length of the data field is 1500 octets

– TPID field is given the value 81-00 hex (33,024 decimal)



• Tag Control Information (TCI) (2 Octets)– Gives specific tagging information

– Three priority bits (000 to 111)

– Eight priority levels, with 111 being high

– 12-bit VLAN ID (see Chapter 8)

– One bit canonical form indicator (rarely used)


Processing an Incoming MAC Frame

• Receiving NIC reads Preamble and SFD– Synchronizes itself to the incoming bit stream

• Receiving NIC reads Source and Destination Address– Discards frame if destination address is not its

own– Saves addresses in RAM


• Reads Next two Octets– If Length field (values <= 1500), sets aside room in RAM

for data field

– If TPID, handles TCI information, then goes on and reads Length Field

– Note: reads next two octets; Not “the length field”

• Places Data Field in RAM

• Discards PAD if Present– Note: sender adds the PAD, not the receiver


• Examines Frame Check Sequence– Recomputes the Value based on bits in other fields

• If same value as transmitted, the frame is good– Passes deencapsulated data field to LLC layer

• If different value than transmitted, frame is bad– Discards the frame

– There is no error correction (retransmission)

Documents

Repeaters can Mix 802.3 PHY Standards