ICOM 6115©Manuel Rodriguez-Martinez ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph.D. Lecture 16

ICOM 6115©Manuel Rodriguez-Martinez Lecture Objectives Look at Stop-and-Wait pseudo code –Sliding window comes on Tuesday Introduction to the Medium Access Control (MAC) Sublayer –Addressing –Channel allocation –IEEE 802 family of protocols Ethernet WiFi Broadband

ICOM 6115©Manuel Rodriguez-Martinez Multi-access Networks Issue –Same channel is shared by many host –Only one can be using it at any given time Problem –How to control access to the channel and be fair?

ICOM 6115©Manuel Rodriguez-Martinez Medium Access Control (MAC) Sublayer Lower part of the Data Link Layer for multi- access networks –e.g. Bus networks like Ethernet MAC has the protocols to control which host get access to the network Logical Link Control (LLC) –Actual Data Link Services Connectionless No ACKs Connectionless with ACKs Connection-oriented with ACKs

ICOM 6115©Manuel Rodriguez-Martinez A View of the Data Link Layer

ICOM 6115©Manuel Rodriguez-Martinez Why have a LLC? Split Data Link Layer complexity –MAC is specific to the physical network Various Flavors of Ethernet Various Flavors of Wireless (802.11a or b) –LLC is common to all Build just one version of this component –MAC is specific to interface with physical layer Permits backward compatibility or interoperability –802.11a y b share LLC

ICOM 6115©Manuel Rodriguez-Martinez Example LLC IEEE LLC receive packets from network layer Adds header with –Frame sequence number –Frame ACK number (if any) Call upon MAC sublayer to actually put data on wire At Receiver side, MAC delivers frames to LLC –LLC decides to ACK or not –Send data up to the network layer

ICOM 6115©Manuel Rodriguez-Martinez Example MAC sublayers IEEE – Ethernet IEEE – IBM Token Ring IEEE – Wireless LAN IEEE – Personal LAN (Bluetooth) IEEE – Broadband Wireless –Wireless Local Loop All these share a common LLC –IEEE 802.2

ICOM 6115©Manuel Rodriguez-Martinez Services provided by MAC sublayer Addressing –MAC Address Given to the network interface card Frame generation Checksum computation Error detection Arbitration for channel access –Conflict detection and resolution

ICOM 6115©Manuel Rodriguez-Martinez Static Channel Allocation Allocate each host a chance to send FDM or TDM can be used here –Reserve each host a part of the bandwidth But these do not work well in LANs –Bursty traffic means channel is poorly used Idle sender eats bandwidth that can be given to busy sender

ICOM 6115©Manuel Rodriguez-Martinez Dynamic Channel Allocation Distributed Algorithm is run to determine who gets to send –Get permission to send Token-based approach –IBM Token Ring or FDDI –Send first and then fix any problem Typical problem: Collisions –Two frames are send at the same time, thus distorting the signals

ICOM 6115©Manuel Rodriguez-Martinez Ethernet MAC Sublayer Invented at Xerox Standardized as IEEE Properties –Best effort delivery LLC must do re-transmission in case of timeout or CRC error –Carrier Sense with Collision Detection –Unique address for network Card –Support for multicasting and broadcasting

ICOM 6115©Manuel Rodriguez-Martinez Ethernet Types Cable type provides the type of Ethernet

ICOM 6115©Manuel Rodriguez-Martinez Connectivity of Ethernet styles

ICOM 6115©Manuel Rodriguez-Martinez Ethernet Frame Format Preamble – 64 bits long, each byte has pattern –Synchronize start of frame Dest addr – 48-bit destination address Src addr – 48-bit source address Type – 16-bit flag used to indicate which upper layer protocol gets the frame Body – variable length area to put the data from LLC CRC – 32-bit error detection code

ICOM 6115©Manuel Rodriguez-Martinez Ethernet Manchester Data Encoding Problem: How to discern an idle link from the signal for bit 0? Each bit period is divided into two sub-periods –1 is encoded as transition from high to low voltage –0 is endoced transition from low to high voltage

ICOM 6115©Manuel Rodriguez-Martinez Ethernet MAC Address Every Ethernet card has a unique MAC address –Stored (burned) in ROM –6 bytes long –Usually printed on card in hexadecimal digits, one digit per 4-bit group Example: –8:0:2B:E4:B1:2 represents MAC address –Can also be written as: 08:00:2B:E4:B1:02

ICOM 6115©Manuel Rodriguez-Martinez Ethernet Addressing Unicast –Frame is addressed to one network card Broadcast –Frame is addressed to all network cards –Special MAC Address for this Multicast –Frame is addressed to a group of cards –Each address has the first bit set to

ICOM 6115©Manuel Rodriguez-Martinez Transmitter Algorithm Ethernet is Carrier Sense system –Adaptor can “listen” to signal on wire If card has a frame to send it listens to wire –If signal is detected, then it waits and tries again later when channel is idle –Otherwise, frame is sent and card starts listening as it sends Send and listen simultaneously to detects collision

ICOM 6115©Manuel Rodriguez-Martinez Collision Detection If the card listens a different signal pattern than the one its sending, it assumes a collision Card waits a random amount of time t Card attempts to send the frame again This is called collision detection –Often the system is term CSMA/CD – Carrier Sense Multiple-Access with Collision Detection

ICOM 6115©Manuel Rodriguez-Martinez Collisions How much can it take to detect a collision? –1 RTT