Multiple Access An Engineering Approach to Computer Networking An Engineering Approach to Computer Networking
What is it all about? Consider an audioconference where Consider an audioconference where if one person speaks, all can hear if one person speaks, all can hear if more than one person speaks at the same time, both voices are if more than one person speaks at the same time, both voices are garbled garbled How should participants coordinate actions so that How should participants coordinate actions so that the number of messages exchanged per second is maximized the number of messages exchanged per second is maximized time spent waiting for a chance to speak is minimized time spent waiting for a chance to speak is minimized This is the multiple access problem multiple access problem This is the
Some simple solutions Use a moderator Use a moderator a speaker must wait for moderator to call on him or her, even if no a speaker must wait for moderator to call on him or her, even if no one else wants to speak one else wants to speak what if the moderator what if the moderator ʼ ʼ s connection breaks? s connection breaks? Distributed solution Distributed solution speak if no one else is speaking speak if no one else is speaking but if two speakers are waiting for a third to finish, guarantee but if two speakers are waiting for a third to finish, guarantee collision collision Designing good schemes is surprisingly hard! Designing good schemes is surprisingly hard!
Outline Contexts for the problem Contexts for the problem Choices and constraints Choices and constraints Performance metrics Performance metrics Base technologies Base technologies Centralized schemes Centralized schemes Distributed schemes Distributed schemes
Contexts for the multiple access problem Broadcast transmission medium Broadcast transmission medium message from any transmitter is received by all receivers message from any transmitter is received by all receivers Colliding messages are garbled Colliding messages are garbled Goal Goal maximize message throughput maximize message throughput minimize mean waiting time minimize mean waiting time Shows up in five main contexts Shows up in five main contexts
Contexts
Contexts
Solving the problem First, choose a base technology First, choose a base technology to isolate traffic from different stations to isolate traffic from different stations can be in time domain or frequency domain can be in time domain or frequency domain Then, choose how to allocate a limited number of transmission Then, choose how to allocate a limited number of transmission resources to a larger set of contending users resources to a larger set of contending users
Outline Contexts for the problem Contexts for the problem Choices and constraints Choices and constraints Performance metrics Performance metrics Base technologies Base technologies Centralized schemes Centralized schemes Distributed schemes Distributed schemes
Choices Centralized vs. distributed design Centralized vs. distributed design is there a moderator or not? is there a moderator or not? in a centralized solution one of the stations is a in a centralized solution one of the stations is a master master and the and the others are slaves slaves others are master->slave = downlink master->slave = downlink slave->master = uplink slave->master = uplink in a distributed solution, all stations are peers in a distributed solution, all stations are peers Circuit-mode vs. packet-mode Circuit-mode vs. packet-mode do stations send steady streams or bursts of packets? do stations send steady streams or bursts of packets? with streams, doesn with streams, doesn ʼ ʼ t make sense to contend for every packet t make sense to contend for every packet allocate resources to streams allocate resources to streams with packets, makes sense to contend for every packet to avoid with packets, makes sense to contend for every packet to avoid wasting bandwidth wasting bandwidth
Constraints Spectrum scarcity Spectrum scarcity radio spectrum is hard to come by radio spectrum is hard to come by only a few frequencies available for long-distance communication only a few frequencies available for long-distance communication multiple access schemes must be careful not to waste bandwidth multiple access schemes must be careful not to waste bandwidth Radio link properties Radio link properties radio links are error prone radio links are error prone fading fading multipath interference multipath interference hidden terminals hidden terminals transmitter heard only by a subset of receivers transmitter heard only by a subset of receivers capture capture on collision, station with higher power overpowers the other on collision, station with higher power overpowers the other lower powered station may never get a chance to be heard lower powered station may never get a chance to be heard
The parameter ʻ a ʼ The number of packets sent by a source before the farthest The number of packets sent by a source before the farthest station receives the first bit station receives the first bit
Outline Contexts for the problem Contexts for the problem Choices and constraints Choices and constraints Performance metrics Performance metrics Base technologies Base technologies Centralized schemes Centralized schemes Distributed schemes Distributed schemes
Performance metrics Normalized throughput Normalized throughput fraction of link capacity used to carry non-retransmitted packets fraction of link capacity used to carry non-retransmitted packets example example with no collisions, 1000 packets/sec with no collisions, 1000 packets/sec with a particular scheme and workload, 250 packets/sec with a particular scheme and workload, 250 packets/sec => goodput = 0.25 => goodput = 0.25 Mean delay Mean delay amount of time a station has to wait before it successfully transmits amount of time a station has to wait before it successfully transmits a packet a packet depends on the load and the characteristics of the medium depends on the load and the characteristics of the medium
Performance metrics Stability Stability with heavy load, is all the time spent on resolving contentions? with heavy load, is all the time spent on resolving contentions? => unstable => unstable with a stable algorithm, throughput does not decrease with offered with a stable algorithm, throughput does not decrease with offered load load if infinite number of uncontrolled stations share a link, then if infinite number of uncontrolled stations share a link, then instability is guaranteed instability is guaranteed but if sources reduce load when overload is detected, can achieve but if sources reduce load when overload is detected, can achieve stability stability Fairness Fairness no single definition no single definition ʻ ʻ no-starvation no-starvation ʼ ʼ : source eventually gets a chance to send : source eventually gets a chance to send max-min fair share: will study later max-min fair share: will study later
Outline Contexts for the problem Contexts for the problem Choices and constraints Choices and constraints Performance metrics Performance metrics Base technologies Base technologies Centralized schemes Centralized schemes Distributed schemes Distributed schemes
Base technologies Isolates data from different sources Isolates data from different sources Three basic choices Three basic choices Frequency division multiple access (FDMA) Frequency division multiple access (FDMA) Time division multiple access (TDMA) Time division multiple access (TDMA) Code division multiple access (CDMA) Code division multiple access (CDMA)
FDMA Simplest Simplest Best suited for analog links Best suited for analog links Each station has its own frequency band, separated by guard Each station has its own frequency band, separated by guard bands bands Receivers tune to the right frequency Receivers tune to the right frequency Number of frequencies is limited Number of frequencies is limited reduce transmitter power; reuse frequencies in non-adjacent cells reduce transmitter power; reuse frequencies in non-adjacent cells example: voice channel = 30 KHz example: voice channel = 30 KHz 833 channels in 25 MHz band 833 channels in 25 MHz band with hexagonal cells, partition into 118 channels each with hexagonal cells, partition into 118 channels each but with N cells in a city, can get 118N calls => win if N > 7 but with N cells in a city, can get 118N calls => win if N > 7
Recommend
More recommend
