Medium Access Control Nick Feamster CS 4251 Computer Networking II Spring 2008.
Medium Access Control NWEN302 Computer Network Design.
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Transcript of Medium Access Control NWEN302 Computer Network Design.
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Medium Access Control
NWEN302 Computer Network Design
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Multiple Access Links
Two types of “links”:• point-to-point
– dial-up access– point-to-point link between
Ethernet switch/hub and host• broadcast (shared)
– shared wire, e.g. Ethernet, (upstream) HFC, Token Ring/Bus
– shared wireless, e.g. 802.11 WLAN, HiperLAN, WiMAX
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shared wire (e.g., cabled Ethernet)
shared RF (e.g., 802.11 WiFi)
humans at a party (shared air, acoustical)
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Medium Access Control (MAC)• Distributed algorithm that determines how
nodes share channel, i.e., determine when node can transmit
• Communication about channel sharing must use channel itself!
no out-of-band (separate) channel for coordination
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MAC Protocols: a taxonomy• Channel Partitioning
– divide channel based on time, frequency, code– allocate portion to node for exclusive use
• Random Access– channel not divided, allow collisions– “recover” from collisions
• Ordered Access– nodes take turns– nodes with more to send can take longer turns– nodes with higher priority get more turns
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Ideal Multiple Access Protocol
Broadcast channel of rate R bps1. when one node wants to transmit, it can send at
rate R.
2. when M nodes want to transmit, each can send at average rate R/M
3. fully decentralized:– no special node to coordinate transmissions– no synchronization of clocks, slots
4. simple
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Channel Partitioning MACs
TDMA: time division multiple access • access to channel in "rounds" • each station gets fixed length slot (length =
packet transmission time) in each round • unused slots go idle • E.g: 6-station LAN, Stations1,3,4 have packets,
slots 2,5,6 idle
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1 3 4 1 3 4
6-slotframe
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Channel Partitioning MACs
FDMA: frequency division multiple access • channel spectrum divided into frequency bands• each station assigned fixed frequency band• unused transmission time in frequency bands go idle • E.g. 6-station LAN, Stations 1,3,4 have packets to
send, frequency bands 2,5,6 idle
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frequency
bands
time
FDM cable
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3
4
5
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Channel Partitioning MACs
Code Division Multiple Access (CDMA)• Unique “code” assigned to each user; i.e., code set
partitioning• all users use same frequency but each user has
own “chipping” sequence (code) to encode data• encoded signal = (original data) X (chipping
sequence)• decoding: inner-product of encoded signal and
chipping sequence• Imagine people talking in different languages!
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Random Access Protocols• When a node has a frame to send,
– it transmits at the full channel data rate R– no a priori coordination among nodes
• When two or more nodes transmit frames simultaneously
interference
collision!
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Random Access Protocols• A Random Access MAC protocol specifies:
– how to detect collisions– how to recover from collisions (e.g., via delayed
retransmissions)
• Examples of random access MAC protocols:– ALOHA (developed at Univ of Hawaii in 1970s)– Slotted ALOHA– Carrier Sense Multiple Access (CSMA)– CSMA with Collision Detection (CSMA/CD)– CSMA with Collision Avoidance (CSMA/CA)
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ALOHA Network
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• Connect central time-sharing
computer (on main Oahu campus)
with terminals elsewhere• 2-channel star configuration
–Users-to-computer; computer-to-users
• Random access communication for user transmissions; why random access? computer and user data are bursty…
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ALOHA Protocol• Basic idea is simple let users transmit
whenever they have data to be sent.• If two or more users send their frames at the
same time, a collision occurs.• If there is a collision,
– sender waits a random amount of time and sends it again.
• Waiting time must be random; otherwise, the same frames will collide again.
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A Sketch of Frame Generation
Note that all frames have the same length because the throughput of ALOHA systems is maximized by having a uniform frame size.
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Throughput• Throughput:
– Number of frames successfully transmitted through the channel per frame time.
• Throughput of an ALOHA network can be determined through a simple performance analysis
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Assumptions• Infinite population of users
• New frames are generated according to a Poisson distribution with mean S frames per frame transmission time. – Probability that k frames are generated
during a given frame transmission duration:
http://homepage.stat.uiowa.edu/~mbognar/applets/pois.html!
]Pr[k
eSk
Sk
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frame
S new frames arriving
time
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Observation on S• If S > 1, frames are generated at a higher
rate than the channel can handle.
• Therefore, we expect:
0 < S < 1
• If the channel can handle all the frames, then S is the throughput.
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Packet Retransmission• In addition to the new frames, the stations
also generate retransmissions of frames that previously collided.
• Assume that distribution of frames (new + retransmitted) generated is also Poisson with mean G per frame time.
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!]Pr[
k
eGk
Gk
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Relation between G and S• Clearly, G ≥ S
• At low load, few collisions: G S
• At high load, many collisions: G > S
• Under all loads,
S = GP0
where P0 is the probability that a frame does not suffer a collision.
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Vulnerable Period• Under what conditions will the shaded frame
arrive undamaged?
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Throughput
• Vulnerable period: from t0 to t0+2t
• Probability that no other frame is generated during the vulnerable period is:
P0 = e-2G
• Using S = GP0, we get
S = Ge-2G
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Slotted ALOHAAssumptions:• all frames are of the same size• time divided into equal size slots (time to
transmit 1 frame)• nodes start to transmit only at beginning of a
slot• nodes are synchronized• if two or more nodes transmit in slot, all
nodes will detect the collision
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Slotted ALOHAOperation:• when a node obtains a new frame, it
transmits it in next slot– if no collision: node can send a new frame in
next slot– if collision: node retransmits the frame in
each subsequent slot with probability p until it is successful
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Slotted ALOHA
Pros• single active node can
continuously transmit at full rate of channel
• highly decentralized: only slots in nodes need to be in sync
• simple
Cons• collisions, wasting slots• idle slots• nodes may be able to
detect collision in less than time to transmit packet
• clock synchronization
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Slotted Aloha Throughput• Assume there are N nodes with many frames to
send, and each transmits in a slot with probability, p• Probability that a given node is successful in
transmitting a frame in a slot p(1-p)N-1
• Probability that any node is successful in transmitting a frame in a slot
Np(1-p)N-1
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Slotted Aloha Throughput• Maximum throughput:
find p* that maximizes Np(1-p)N-1
• For an arbitrary number of nodes, take limit of Np*(1-p*)N-1 as N infinity, we get:
Max efficiency = 1/e = .37• Channel used for useful transmissions only
37% of time!
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Slotted ALOHA Throughput
Vulnerable period: from t0+t to t0+2t• Since transmission time is divided into
discrete intervals, only the slot in which a frame is being transmitted is vulnerable.
• Probability that no other frame is generated during the vulnerable period is:
P0 = e-G • Hence, S = Ge-G
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Relation between G and S
Aloha: Max throughput occurs at G=0.5, with S=1/(2e)=0.184.
Slotted Aloha: Max throughput occurs at G=1.0, with S=1/e=0.37.
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Carrier Sense Multiple Access• Start: listen/sense the channel
• If idle: – transmit entire frame, and wait for
acknowledgement (ACK) if no ACK received after specified duration, assume there was a collision.
• If busy:– defer transmission don’t transmit!
• Vulnerable period: one tprop
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CSMA – 1-Persistent• Start:
– sense channel
• if busy,– go back to Start:
• if idle, – send immediately– if collision is detected (i.e. no ACK)
• wait a random amount of time• go back to Start:
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CSMA Non-Persistent• Start:
– sense channel• if busy,
– wait a random amount of time – go back to Start:
• if idle, – send immediately (if collision detected, same
as 1-persistent mode)• Not retrying immediately less collisions• Drawback: more delay
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CSMA p-Persistent• Start:
– sense channel• if busy,
– go back to Start:• if idle,
– with probability p, transmit packet– with probability 1-p, wait tprop; go to Start:– if collision detected, same as 1-persistent
• Reduced idle channel time (1-persistent) & Reduced collisions (non-persistent)
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CSMA collisions
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Collisions can still occur:•propagation delay means two nodes may not hear each other’s transmission
•entire packet transmission time wasted
Vulnerable period: tprop
•tprop : maximum one-way propagation delay between any two nodes
•distance & propagation delay crucial in determining collision probability
spatial layout of nodes
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CSMA/CD (Collision Detection)• 1-persistent approach• Continue channel sensing as
frame is being transmitted• If collision detected,
– stop frame transmission immediately;
– transmit a brief jamming signal
• Wait for a random time, and restart sequence.
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Collision Detection• Easy in wired LANs:
– measure signal strengths, compare transmitted, received signals
– CSMA/CD is used in Ethernet – the dominant wired LAN technology
• Difficult in wireless LANs: – received signal strength overwhelmed by local
transmission strength– alternative approach needed avoidance– more later…
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CSMA/CD efficiency• tprop = maximum propagation delay between 2 nodes
• ttrans = time to transmit maximum-size frame
• Efficiency/throughput goes to 1 – as tprop goes to 0
– as ttrans goes to infinity
• better performance than ALOHA, and simple, cheap,
decentralized!
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transprop /ttefficiency
51
1
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Channel Utilization
Comparison of the channel utilization versus load for various random access protocols.
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Summary of MAC protocols• channel partitioning
– TDMA, FDMA and CDMA
• random access (dynamic), – ALOHA– Slotted ALOHA– CSMA– CSMA/CD used in Ethernet / 802.3
CD (collision detection) easy in some technologies (wire)hard in others (wireless)
– CSMA/CA used in 802.11
• taking turns– polling from central site, token passing– Bluetooth, FDDI, IBM Token Ring
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