Repeaters can Mix PHY Standards Repeater NIC BNC AUI Connectors RJ-45 UTPDrop Cables 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

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 Incoming Traffic, wait If No Incoming 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 Ethernet MAC Frame –Header –Data Field –Trailer Header Has Multiple Fields –Measure size in octets (bytes) TrailerData 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 … pattern –SFD is to end the synchronization –Together, 64-bit synchronizing pattern PreSFDDASALenDataPADFCS Ethernet 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 PreSFDDASALenDataPADFCS

802.3 Ethernet MAC Layer Frame 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 PreSFDDASALenDataPADFCS A1-BD-23-0C-09-C3 h

802.3 Ethernet MAC Layer Frame Hex Symbols BitsHex Symbol BitsHex Symbol A B C D E F

802.3 Ethernet MAC Layer Frame Length Field (2 Octets) –Length of the Data Field, not of the entire frame –Maximum data file size is 1500 octets LenDataPAD

802.3 Ethernet MAC Layer Frame 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

802.3 Ethernet MAC Layer Frame 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 PreSFDDASALenDataPADFCS

802.3 Ethernet MAC Layer Frame 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 PreSFDDASALenDataPADFCS

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 PreSFDDASALenDataPADFCSTPIDTCI

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 hex (33,024 decimal) PreSFDDASALenDataPADFCSTPIDTCI

802.3 Ethernet MAC Layer Frame 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) PreSFDDASALenDataPADFCSTPIDTCI

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

Processing an Incoming MAC Frame 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

Processing an Incoming MAC Frame 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)