EEL 5718 Computer Communications Medium Access Control (MAC) 6:55 PM

Outline Duplexing Channelization MAC protocols Deterministic Random Hybrid 6:55 PM

Multiple Access Control (MAC) Data link layer = Logical Link Control + MAC Logical link control (e.g., ARQ) hides the physical connection from higher layers, while MAC is the part interacting with physical characteristics If link is dedicated, MAC may not be necessary What if a link is broadcast in nature? E.g., daily conversation Need coordination---MAC 6:55 PM

MAC WLAN 6:55 PM

MAC MAC: Medium Access Control or Multiaccess Control or Multiple Access Control MAC: protocols coordinating the use of shared resource (“channel”) Classification centralized vs distributed deterministic vs random static vs dynamic reservation-based vs demand-based QoS-based or fair sharing 6:55 PM

Duplexing Control transmissions in both directions Forward (downlink) Reverse (uplink) Time Division Duplexing (TDD) forward and reserse use the same channel but at different time assignment time slots in a frame divided into uplink and downlink Frequency Division Duplexing (FDD) forward and reverse transmissions use different frequency channels 6:55 PM

Channelization (Deterministic MAC) Resource available in the network will be shared time space frequency code Classification Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Space Division Multiple Access (SDMA) 6:55 PM

FDMA Assigns individual channels to individual users each assigned channel is exclusively used by that user only Frequency 1 Guard bands 2 … M-1 M Time 6:55 PM

TDMA Divide the radio spectrum into time slots, in each slot, only one user is allowed to either transmit or receive (guard times and preambles will be used) Frequency Synchronization intervals … W 1 2 3 M 1 Time 1 Frame 6:55 PM

TDMA Transmission will use “frame” (do not be confused with frame in DLC) 6:55 PM

CDMA Instead of slicing time or frequency, different orthogonal codes can be used, e.g., different languages are used in a party Use all the spectrum: transmitting power is spread over the whole frequency band Hide the transmitted signal behind the noise floor Developed in military applications 6:55 PM

CDMA Each user will use a unique code (orthogonal to other users) for communications (pseudo-random sequence is used to generate codes) 6:55 PM

Transmitter from one user Binary Information R1 bps W1 Hz Radio Antenna X X R >> R1bps W >> W1 Hz Unique User Binary Random Sequence Digital Modulation Signal plus residual interference Signals from all transmitters X X Binary Information Correlate to User Binary Random Sequence Digital Demodulation

CDMA Rough mathematical illustration: Transmitted signal: si(t) for user i, obtained from PN code ci(t) orthogonal At receiver (BS) s(t) = s1(t)+s2(t)+…+ sk(t) BS can recovered the signal from user i using correlation: s(t)ci(t) due to  ci(t)sj(t) dt -> 0 6:55 PM

CDMA However, it has near-far problem: a user near a BS may overtake a far user Power control must be done: transmitting powers are controlled in such a way that all received power at a BS will be the same Simple interpretation: draw a comparison between the modulated signal by cos(fct) and a spread signal by a PN signal c(t), all signal magnitudes are decreased over a wider band! c(t) can be regarded as a signal with rich components 6:55 PM

SDMA Control the radiated power/energy for each user in space using spot beams (using the same frequency in different space) Adaptive antennas 6:55 PM

Random Access (Distributed) Coordination of the same shared medium (channel) Easy to implement and does not need a central controller (distributed in nature) Major random MACs Pure Aloha Slotted Aloha CSMA, CSMA/CD, CSMA/CA Tree splitting algorithm 6:55 PM

Pure Aloha In 1970s, Norm Abramson at Univ. of Hawaii devised random access scheme for packet radio: link library among a few islands via radio Coordination rule: no coordination! Whenever there is data, transmit at will, hope for the best, if collide, delay for random length of time, retransmit 6:55 PM

Throughput Throughput: the percentage of successful transmissions among all transmission attempts Basic assumptions Frame size is constant with transmission time t Merged traffic (new attempts+retransmits) is Poisson with parameter G G: frame arrivals per frame-time t traffic load or offered traffic Collision is assumed as long as there are overlaps between transmitted frames 6:55 PM

Aloha Transmissions 6:55 PM

Time Diagram t t0-X t0 t0+X t0+X+2tprop t0+X+2tprop First transmission Retransmission t t0-X t0 t0+X t0+X+2tprop t0+X+2tprop Vulnerable period Time-out Backoff period Retransmission if necessary 6:55 PM

Vulnerable Period 6:55 PM

Formula 6:55 PM

Slotted Aloha Coordination rule: a little bit coordination, “let’s transmit only at certain time instants” In 1972, Roberts proposed a simple scheme to double the throughput: divide time into fixed time interval (time slot), each user transmits only at the beginning of each slot; if collide, delay fixed number of slots, retransmits Vulnerable period is reduced to one frame-time: as long as there is no arrival in a frame period, there is no collision 6:55 PM

Time Diagram for S-Aloha kX (k+1)X t0 +X+2tprop t0 +X+2tprop+B Time-out Backoff period Vulnerable period Retransmission if necessary 6:55 PM

Throughput Aloha: Smax=18.4% Slotted Aloha: Smax=36.8% 6:55 PM

Aloha vs Slotted Aloha Maximum throughput for Aloha is 18.4%, while maximum throughput for slotted Aloha is 36.8%, slotted Aloha doubles the throughput of pure aloha Catch: slotted Aloha needs synchronization of all users, a global clock! 6:55 PM

Carrier Sense Multiple Access CSMA Coordination rule: sense before transmit, or listen before you speak How to sense: detect whether there is any transmission in the channel Many operating modes non-persistent 1-persistent p-persistent 6:55 PM

Non-persistent CSMA A station senses the channel If the channel is busy, it will not continue sensing the channel, instead, it delays a random period of time, repeats the same procedure If channel is idle, it transmits If a collision occurs, it will delay a random period of time, starts all over again 6:55 PM

1-persistent CSMA A station (a user, a transmitter) senses the channel If the channel is busy, it will wait until the channel is idle (sensing all the time) Whenever it senses the channel is idle, it will transmit If a collision occurs, it will delay a random period of time, start it all over again 6:55 PM

p-persistent CSMA Observation: if you sense idle, others too, collision is surely occur p-persist CSMA applies to slotted channels A station senses the channel If it senses idle channel, it transmits with probability p, with probability q=1-p, it defers to next slot, repeats the same procedure If it senses busy channel, it delays a random number of slots, starts all over 6:55 PM

Comparison 6:55 PM

CSMA with Collision Detection CSMA/CD, widely used in LAN (IEEE 802.3) If we know there is a collision, why continue? New improvement: stop transmission if collision is detected Efficient when frame is long How collision detection is made? Detect power increase at the receiver How soon (late) a collision is detected? 2t - double of the propagation time between two farthest stations 6:55 PM

CSMA/CD Time Diagram Time consists of idle period, contention period and transmission period 6:55 PM

Performance Throughput frame contention Pmax Psuccess is maximized at p=1/n: 6:55 PM n

Performance Compute contention interval: the average number of minislots that elapsed until a station capture the channel: J is the random number 6:55 PM

Binary Exponential Backoff Algorithm for delayed transmission: random period of time Binary exponential backoff algorithm: first time collision, delay either 0 or 1 slot with probability 1/2 i-th time collision, delay either 0, 1, 2,…, or 2^i-1 slots with equal probability Usually, when i=16, i.e., after 16 consecutive collisions, the controller will throw the towel or wave the white flag 6:55 PM

CSMA/CA CSMA/CD does not work in wireless channel difficult to detect collisions in a radio environment difficult to control the wireless channel hidden and exposed terminal problems CSMA/CA is used in wireless environments sense before transmit gain the access right via contention (slotted Aloha) wait for channel grant of usage busy-tone may be used IEEE 802.11 standard 6:55 PM

Tree Splitting Algorithm Capetanakis (1979) developed a tree splitting algorithm, which can improve throughput to 43% Idea: when a collision occurs, say in the k-th slot, all nodes not involved in the collision go into a waiting mode, and all those involved in the collision split into two groups. In the next slot, only one group is allowed to transmit, if collision continues, further splitting is done until no collision occurs, then the second group will allowed to transmit in the similar manner, etc. 6:55 PM

Tree Splitting Algorithm Splitting methods flip a coin, or 50/50 at each node splitting time of arrivals into half How to handle new arrivals delay all new arrivals until collision is resolved allow new arrivals at end of each collision resolution period Refer to Bertsekas and Gallager’s Data Networks 6:55 PM

Centralized MAC Scheduling approach, a type of central coordinator has to make a decision on channel access Classification deterministic TDMA, TDMA, CDMA reservation systems polling token-passing protocols 6:55 PM

Reservation-based MAC Any transmission must make reservation A reservation interval is divided into M minislots, stations use their corresponding minislots to announce their intention to transmit and make reservation accordingly By listening the reservation interval, stations can determine the order of transmissions Variable-length packets can be handled if the reservation message packet-length information 6:55 PM

Reservation-based MAC Operations Reservation interval Data Transmissions r d d d r d d d time 1 frame 1 frame Each station has own minislot for making reservations r = 1 2 3 M 6:55 PM

Variations A station can reserve more than one slot per packet transmission per minislot Random access-reservation based protocol Instead of assigning minislots to stations, using random access such as slotted Aloha to contend for access right on each minislots, then whoever wins at the minislot will transmit accordingly 6:55 PM

Polling Used in wired fixed networks Nodes take turns for transmission A central node is the coordinator Operations the central node sends a poll message to node asking whether there is anything to transmit if yes, transmit in response to the poll if no, the central node polls the next node 6:55 PM

(a) Shared inbound line Outbound line Central Controller (b) (c) Central Controller

Token Passing Protocols (Distributed Polling) Used in ring architecture Tokens (a short packet) is circling around the ring Operations whoever seizes the token has the sole right to transmit when a node finishes its transmission, it will regenerate the token and reinsert it on the ring 6:55 PM

Token-Passing Ring listen mode transmit mode input from ring output to delay delay to device from device 6:55 PM

Hybrid MAC Some kinds of combinations of random access and reservation MACs can create new classed of MAC protocols contention + contention-free random access + reservation MAC + channel condition MAC + QoS more 6:55 PM

Further Reading Textbook: Chapter 6 Tanenbaum’s book: Chapter 4 6:55 PM