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Chap 4 Multiaccess Communication (Part 1). Ling-Jyh Chen. Overview. Ethernet and Wi-Fi are both “multi-access” technologies Broadcast medium, shared by many hosts Simultaneous transmissions will result in collisions Media Access Control (MAC) protocol required Rules on how to share medium. - PowerPoint PPT Presentation
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Chap 4 Multiaccess Communication(Part 1)
Ling-Jyh Chen
Overview
Ethernet and Wi-Fi are both “multi-access” technologies Broadcast medium, shared by many hosts Simultaneous transmissions will result in collisions
Media Access Control (MAC) protocol required Rules on how to share medium
Media Access Control Protocols
Channel partitioning Divide channel into smaller “pieces” (e.g., time slots,
frequency) Allocate a piece to node for exclusive use E.g. Time-Division-Multi-Access (TDMA) cellular network
Taking-turns Tightly coordinate shared access to avoid collisions E.g. Token ring network
Contention Allow collisions “recover” from collisions E.g. Ethernet, Wi-Fi
Contention Media Access Control Goals
Share medium If two users send at the same time, collision results in no
packet being received (interference) If no users send, channel goes idle Thus, want to have only one user send at a time
Want high network utilization TDMA doesn’t give high utilization
Want simple distributed algorithm no fancy token-passing schemes that avoid collisions
Evolution of Contention Protocols
Developed in the 1970s for a packet radio networkAloha
SlottedAloha
Improvement: Start transmission only at fixed times (slots)
CSMA
CSMA = Carrier Sense Multiple AccessImprovement: Start transmission only if no transmission is ongoing
CD = Collision Detection
Improvement: Stop ongoing transmission if a collision is detected (e.g. Ethernet)
CSMA/CD
4.2 Idealized slotted multiaccess model
m transmitting nodes and one receiver
1. Slotted systema) packets are of the same length
b) each packet requires one time unit for transmission
c) the reception of each packet starts at an integer time and ends before the next integer time
2. Poisson Arrivalsoverall arrival rate of the system: λindividual rate of each node: λ/m
3. Collision or Perfect Receptiona) If just one node sends a packet in a given
slot, the packet is correctly received.b) If two or more nodes send a packet in a
given time slot, then there is a collision and the receiver obtains no information about the contents or the source of the transmitted packets.
4. 0,1,e Immediate Feedback Assuming each node obtains feedback from
the receiver at the end of each slot
5. Retransmission of Collisions Assuming each packet involved in a collision
must be retransmitted in some later slot. A node with a packet that must be
retransmitted is said to be backlogged.
6. Two addition assumptionsa. No buffering
If one packet at a node is currently waiting for transmission or colliding with another packet during transmission, new arrivals at that node are discarded and never transmitted.
This assumption provides the lower bound to the delay for systems with buffering and flow control!
b. Infinite set of nodes (m=∞): This assumption provides the upper bound!
Slotted ALOHA
The basic idea: Each unbacklogged node simply transmit a
newly arriving packet in the first slot after packet arrival.
Slotted ALOHA risks occasional collisions but achieves very small delay if collisions are rare.
Contrast to TDM systems, which avoids collisions at the expense of large delays.
Collisions in S-ALOHA
1.1 1.2
TransmissionDelay
Station 1
2.1Station 2
3.1 3.2
Station 3
Broadcastchannel
2.2
1.3
CompleteCollision
Slotted ALOHA (cont.)
When a collision occurs, each node sending one of the colliding packets discovers the collision at the end of the slot and becomes backlogged.
Such nodes wait for some random number of slots before retransmitting.
Slotted ALOHA (cont.)
Using infinite-node assumption, the total number of retx and tx in a given slot is a Poisson random variable with parameter G, where G> λ.
The prob. of a successful transmission in a slot is
In equilibrium, the arrival rate, λ, should be the same as the departure rate, Ge-G.
GG
GeeG
nP
!1
]1[1
Slotted ALOHA (cont.)
Using GNUPlotset xr [0:5]plot x*exp(-x)
Slotted ALOHA (cont.)
The MAX departure rate occurs at G=1 and is 1/e ≈ 0.368.
If G<1, too many idle slots are generated. If G>1, too many collisions are generated.
Slotted ALOHA (cont.)
Markov Chain for Slotted ALOHA State: the number of backlogged packets Increases by the number of new arrivals
transmitted by unbacklogged nodes Decreases by one each time if a packet is
transmitted successfully.
Slotted ALOHA (cont.)
qr: the prob. of a backlogged node retx in the next slot i.e., the number of slots from a collision until a
given node involved in the collision retx is a geometric R.V. having value i>1 with prob. qr(1-qr)i-1
qa: the prob. of an unbacklogged node transmits a packet in the given slot i.e. qa=1-e-λ/m
Slotted ALOHA (cont.)
Qa(i, n): the prob. that i unbacklogged nodes transmit packets in a given slot
Qr(i, n): the prob. that i backlogged nodes transmit.
ir
inrr
ia
inmaa
qqi
nniQ
qqi
nmniQ
)1(),(
)1(),(
Slotted ALOHA (cont.)
1),,1(),0(
0)],,1(1)[,0(),0(),1(
1)],,0(1)[,1(
)(2),,(
,
inQnQ
inQnQnQnQ
inQnQ
nminiQ
P
ra
rara
ra
a
inn
),1(),0(),0(),1( nQnQnQnQP rarasucc
Slotted ALOHA (cont.)
Dn: “drift” in state n, i.e. the expected change in backlog over one slot time
G(n): the expected number of attempted transmissions in a slot
If qa and qr are small, )()( nGsucc enGP
Slotted ALOHA (cont.)
The “drift” is the difference between the throughput curve (Ge-G) and the straight line:
Slotted ALOHA (cont.)
Using infinite-node assumption:
Using no-buffering assumption:
4.2.3 (optional)
rnqnG )(
ra nqqnmnG )()(
Unslotted ALOHA
Unslotted ALOHA (a.k.a. Pure ALOHA) was the precursor to slotted ALOHA.
In Pure ALOHA, each node transmits a new packet immediately upon receiving, rather than waiting for a slot boundary.
If a packet is involved in a collision, it is retransmitted after a random delay.
Collisions in (Pure) ALOHA
1.1 1.2
TransmissionTime
(F)
Station 1
2.1Station 2
3.1 3.2Station 3
Broadcastchannel
2.2
1.3
CompleteCollision
PartialCollision
Unslotted ALOHA (cont.)
Frame which collideswith start of red frame
Frame
t0-F t0 t0+F
VulnerablePeriod of red frame
Time
Frame which collideswith end of red frame
A frame (red frame) will be in a collision if and only if another transmission begins in the vulnerable period of the frame
Vulnerable period has the length of 2 frame times
Unslotted ALOHA (cont.)
Since arrivals are independent, Psucc=e-2G
Since attempted transmissions occur at rate G(n), the throughput = Ge-2G
The MAX throughput of a Pure ALOHA system = 1/(2e), achieved when G=0.5.
If λ is very small and the mean retx time is very large, the system can be expected to run for long periods w/o major backlog buildup.
The main adv. of pure ALOHA is that it can be used with variable-length packets.
Comparison of ALOHA and S-ALOHA
0 0.5 1 1.5 2 2.5 30
0.1
0.2
0.3
0.4
0.5
R
Thr
ough
put
(ALO
HA
)
Slotted ALOHA: Re-R
Pure ALOHA: Re-2R
Ideal (no collisions): R