10/11/2007 EETS The Medium Access Control Sublayer Chapter 4

10/11/2007 EETS MAC Layer LAN WAN router Manch MAC PPP QAM IP Manch MAC IP QAM PPP IP

10/11/2007 EETS The Channel Allocation Problem Static AllocationDynamic Allocation Tc = Ts/(1 –  )      Tn = (Ts/N)/(1 –  ) Tn Ts/N N Tc Ts Example T1 carrier: N = 24, 6400 bits/frame. Ts = 6400/64000 = 0.1 s. For  = 0.8 Tc = 0.5 s, Tn = 0.5/24 s for the same traffic.

10/11/2007 EETS Dynamic Channel Allocation in LANs Station Model: N independent stations sending messages according to Poisson distribution. Single Channel: A single channel is available to all stations to transmit to and receive from. Carrier Sense: Stations sense if the channel is in use and wait vs. stations cannot tell if the channel is in use. Collision Assumption: If stations transmit at the same time, frames will collide and garbled. All stations can detect collision and retransmit frame later. Frame beginning: Pure Aloha (frame can start transmission at any time) vs. slotted Aloha (frame can start transmission only at a given time instance).

10/11/2007 EETS Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Wavelength Division Multiple Access Protocols

10/11/2007 EETS Pure ALOHA (1970) In pure ALOHA, frames are transmitted at completely arbitrary times.

10/11/2007 EETS Collision conditions in pure Aloha period is 2t If another frame starts here we will have collision All frames are of the same size. Frame transmission can start at any time instant.

10/11/2007 EETS Throughput versus offered traffic for ALOHA systems. Max throughputs: 18% at G = 0.5 for pure 37% at G = 1 for slotted. (throughput per packet time)

10/11/2007 EETS (1-p) carr ->random back off CSMA: Persistent and Non-persistent no carr -> transmit carr -> check after random time nonpersistent no carr -> transmit with prob p. p-persistent is slotted carr ->check next slot

10/11/2007 EETS CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle. Minimum contention slot is 2  where  ( 5  s/km) is the propagation delay between the two most remote stations.

10/11/2007 EETS Reservation protocols: basic bit-map protocol

10/11/2007 EETS The binary countdown protocol

Wavelength Division Multiple Access 10/11/2007 EETS Station A Station B tunable control transmit fixed data transmit tunable data receive fixed control receive A wants to send data to B 1. A tunes to listen B status S. S tells B control slots #3 is free. 2. A tunes transmit control and sends to B “data for you in my slot 4” in B control slot #3. 3. B tunes to A output and receives data from slot 4. Fiber LAN implementation Channel allocations per station

10/11/2007 EETS Ethernet Ethernet Cabling Manchester Encoding The Ethernet MAC Sublayer Protocol The Binary Exponential Backoff Algorithm Ethernet Performance Switched Ethernet Fast Ethernet Gigabit Ethernet IEEE 802.2: Logical Link Control Retrospective on Ethernet

10/11/2007 EETS Ethernet MHz Segment length in hundredths meters Base band Vampire taps T conn.

10/11/2007 EETS Ethernet Cabling Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

10/11/2007 EETS Ethernet coding (+/ V) (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

10/11/2007 EETS Ethernet MAC Sublayer Protocol Frame formats. (a) DIX (DEC, Intel Xerox) (b) IEEE Address bit 46 determines local or global address. Min frame is 64 bytes from dest. address to checksum.

Ethernet Preamble (PRE). The PRE is an alternating pattern of ones and zeros. Start-of-frame delimiter (SOF). The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits. Destination address (DA). Source addresses (SA). Length/Type indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format. Data n bytes (46=< n =<1500) of any value. (The total frame minimum is 64bytes.) Frame check sequence (FCS) 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames. FCDataLength/typeSADASOFPre

10/11/2007 EETS Collision detection can take as long as 2  This is to sense a collision before end of the frame reach far end. In 10 Mbps LAN 1 bit is 100 nsec, and max segment 2500 m round trip delay is 2  = 50 mksec = 500 bits = 64 bytes.

10/11/2007 EETS Binary exponential backoff Simplified version with persistance p: If k stations contend for a channel the probability that any of k gets a channel is: A = kp(1 – p) k-1 p = 1/k gives A max -> 1/e for k -> inf. Average number of contention slots =    jA(1 – A) j-1 = 1/A. Therefore, channel efficiency = P/(P + 2  /A) Frame first collision second collision for i-th collision station randomly chooses between 0 and 2 i - 1 contention slots until P sec

10/11/2007 EETS Efficiency of Ethernet at 10 Mbps with 512- bit slot times

10/11/2007 EETS Switched Ethernet Plug-in card handles collision domain either usual way or w/out collision. Frames destined outside plug-in domain are switched over backplane to the destined plug-in card.

10/11/2007 EETS Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet.

10/11/2007 EETS Gigabit Ethernet cabling

10/11/2007 EETS IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats. When reliable service is required the LLC (Logical Link Control) layer is added on the top of MAC layer. LLC is HDLC based (frame sequencing etc.).

10/11/2007 EETS Wireless LANs (WiFi) The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure Services

10/11/2007 EETS Part of the protocol stack FHSS - Frequency Hopping Spread Spectrum DSSS - Direct Sequence Spread Spectrum OFDM - Orthogonal Frequency Division Multiplexing HR-DSSS - High Rate DSSS MAC: PCF - Point Coordin. Funct.: Base Station Polls mobiles within its cell. DCF - Distributed Coordination Function further discussed.

10/11/2007 EETS Wireless LAN (bandwidth is 11 to 54 MHz) (a) A transmitting: C is out of range and cannot hear A. (b) B transmitting: C hears B and falsely concludes that it cannot transmit to D. Before transmitting the transmitting station wants to know whether is any activity around receiver. CSMA tells only activity around transmitter. Solution is that sender stimulate receiver to transmit short frame.

10/11/2007 EETS DCF: wireless MACA (Multiple Access with Collision Avoidance) C hears RTS A to B (30 bytes) frame with the length of the frame to follow. D hears B responding to A with a CTS (copying the length of the next frame). A starts transmitting.

10/11/2007 EETS The use of virtual channel sensing using CSMA/CA NAV - Network Allocation Vector

10/11/2007 EETS A fragment burst Due to the noisy wireless channels data packets are split into short fragments.

10/11/2007 EETS Interframe spacing in

10/11/2007 EETS The Data Frame Structure Frames must process in order Encrypted using WEP More frames to follow Toggle bit to put receiver to sleep Retransmitted frame Data, Control, Mngmnt. RTS, CTS Frame out of Cell More fragments to follow For NAV For Base Station 12 bit frame, 4 bits fragment

10/11/2007 EETS Distribution Services Association (Attach) Disassociation (Detach) Reassociation (Handoff) Distribution (Frame routing) Integration (Protocol conversion)

10/11/2007 EETS Intracell Services Authentication (of mobile) Deauthentication (on leave) Privacy (encryption) Data Delivery (data exchange)

10/11/2007 EETS Broadband Wireless (WiMax) Comparison of and The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure

vs environment indooroutdoor mobility mobilefixed access point-to-multipointpoint-to-point distance meterskm spectrum ISM (900 MHz-5.7 GHz) GHz (mm waves) QoS internetmultimedia (con orient) errors detection correction (Hamming) securityoptionalmandatory bit rate10 Mbps100 Mbps

10/11/2007 EETS The Protocol Stack

10/11/2007 EETS The Physical Layer

10/11/2007 EETS The Physical Layer (2) Frames and time slots for time division duplexing.

10/11/2007 EETS The MAC Sublayer Protocol Service Classes Constant bit rate service (for T1) Real-time variable bit rate service (for multimedia) Non-real-time variable bit rate service (file transfer) Best efforts service (for contended upstream traffic)

10/11/2007 EETS The Frame Structure (a) A generic frame. (b) A bandwidth request frame.