Repeaters can Mix 802.3 PHY Standards

1. Repeaters can Mix 802.3 PHY Standards Repeater Repeater 10Base2 RJ-45 AUI Connectors BNC NIC 10Base-T UTP Drop Cables 10Base5 Trunk Cable

2. 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

3. 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

4. 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 NoIncoming Traffic, send If Incoming Traffic, wait

5. 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

6. 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

7. 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

8. 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) Header Fields Trailer Data Field Ethernet Frame

9. 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 FCS PAD Data Len SA DA SFD Pre Ethernet 802.3 MAC Layer Frame

10. 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 FCS PAD Data Len SA DA SFD Pre

11. 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 A1-BD-23-0C-09-C3 h FCS PAD Data Len SA DA SFD Pre

12. 802.3 Ethernet MAC Layer Frame • Hex Symbols

13. 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 PAD Data Len

14. 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 PAD Data Len

15. 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 FCS PAD Data Len SA DA SFD Pre

16. 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 FCS PAD Data Len SA DA SFD Pre

17. 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 FCS PAD Data Len TCI TPID SA DA SFD Pre

18. 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) FCS PAD Data Len TCI TPID SA DA SFD Pre

19. 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) FCS PAD Data Len TCI TPID SA DA SFD Pre

20. 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

21. 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

22. 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)