cs 453 computer networks
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CS 453 Computer Networks. Lecture 13 Medium Access Control Sublayer. MAC Sublayer. So, lets add another complication… So far we have discussed protocols with the assumption that the lower physical layer medium is wire-like – But it a different sense than serial throughput - PowerPoint PPT PresentationTRANSCRIPT
CS 453CS 453Computer NetworksComputer Networks
Lecture 13Lecture 13
Medium Access Control Medium Access Control
SublayerSublayer
MAC Sublayer
So, lets add another complication…
So far we have discussed protocols with the assumption that the lower physical layer medium is wire-like –
But it a different sense than serial throughput That is, it is point to point , and… It is single channel (this is not completely true) We did not need to consider how the communicating
stations (ends) get access to the medium
MAC Sublayer
But media are not point-to-point
We call these Broadcast media
These include – Radio Cable – I’ll explain later Satellite Ethernet
These have a diffused, wide area footprint
And….
MAC Sublayer
There is simultaneous potential for access by multiple stations – possibly many stations
Not only simultaneous potential access to the medium, but to a single channel
So, the problem is – How to grant that access And enable communications
MAC Sublayer
We have already looked at a few ways grant or allocate access to single channels- Frequency Division Multiplexing (FDM)
Divide the frequency bandwidth of a channel into multiple smaller channels, and…
Allocate the smaller channels as if they were point-to-point channels
MAC Sublayer
FDM Simple and, in principle, very efficient However, with bursty or irregular data traffic… Some channels may be heavily used… While other channel little or not used at all Unused bandwidth can not usually be
reallocated, so Very inefficient is this scenario
MAC Sublayer
Time Division Multiplexing (TDM) Instead of dividing up the frequency bandwidth of the
channel… We can divide up time on the channel, and … Allocate slices of time to alternative pairs of
communicating stations
TDM has similar problems to FDM
With Statistical Time Division Multiplexing (STDM) we can achieve some improvement in efficiency
MAC Sublayer
With broadcast media like radio, wireless, satellite and cable, we have a new set of problems…
We can’t use static allocations schemes like FDM and TDM
N stations potentially accessing the medium
N might be unknown
N might be variable
We might not know the location of the stations
MAC Sublayer
Broadcast media We have a single communication media Potentially many stations competing for
access and use of the media Multiple stations trying to access the medium
at the same time is know as contention We need a way to resolve contention if any
communications is going to get done And we need to do this dynamically
MAC Sublayer
Before going on – a few assumptions Single channel – our problem (for now)
concerns the allocation of a single communications channel
Collisions – any time two or more stations try to or do transmit at the same time, all transmitted frames are mangled and consider destroyed
Collision detection – all stations can detect collisions
MAC Sublayer
A side note – To a large extent the broadcast medium issues
discussed here concern local area network (LAN) and metropolitan area networks (MAN)
Wide Area Networks (WAN) typically use point-to-point links
Satellites are notable exception to the previous statement…
… and the broadcast medium allocation issues apply
MAC Sublayer
Multiple Access Protocols There are many Multiple Access Protocols Date back to the early 1970s Some have come and gone Some have come and gone and come back
again
MAC SublayerAloha Alohanet – work in 1970s at University of
HawaiiBy Abramson and colleaguesTo establish communications links among Hawaiian islandsUsed radioRadio is a broadcast mediumThey had a multistation contention problemDeveloped an access protocol called (you guessed it) Aloha
MAC SublayerAloha
Frames are fixed size Any station can transmit a frame anytime it wants
Free-for-allNo waitingNo clock
Since the transmission is broadcast the sender can listen to what it sent
If there was another overlapping transmission by another station (collision)…
Each sending/listening station would here a garbled version of what it sent…
And declare its transmission to be in error... …wait a random period and.. retransmit
MAC Sublayer
Aloha So how well does it work, or how much data
gets through Frame time is the amount of time the channel
is used to transmit one frame If two frames are transmitted in the same
frame time there is a collision
MAC SublayerAloha N = mean number of frames transmitted per
frame time (N is a Poisson distribution) If N>1 nearly every frame has a collision 0<N<1 for Aloha to work with any efficiency For every frame that has a collision that frame
is retransmitted G = N (number of frames transmitted per
frame time) + number of retransmissions due to collisions
MAC Sublayer
Aloha S (throughput) = GP where P = Probability of
frame avoiding a collision
S = Ge-2G
MAC Sublayer
Aloha So, S maxs out with G = 0.5 -- S = 1/2e or about 18%
efficiency With G> 0.5
collision retries begin to swamp the network
Aloha Throughput
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
G - Frame attempts per frame time
Su
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sfu
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ram
es p
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Aloha
MAC SublayerSlotted Aloha
Remember – Aloha was a free-for-allAny station any time
Slotted Aloha –Create time slotsAny station can transmitBut only at the start of a time slotIf a station does not have anything to transmit at the start of the time slot, …It must wait until the next time slotSo now, all collisions are complete collisions…No overlap collisionsSynchronization? - Aloha – broadcast a clock tick
MAC Sublayer
Slotted Aloha S (throughput) = GP where P = Probability of
frame avoiding a collision now…
S = Ge-G
MAC SublayerSlotted Aloha
With Slotted Aloha, S maxs out
with G = 1 S = 1/e or about 37%
efficiency With G> 1 collision
retries begin to swamp the network,…
… collisions increase exponentially
Throughput
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
G frame attemps per frame time
S T
hro
ug
hp
ut
Aloha
Slotted Aloha S
MAC Sublayer
Carrier Sense Multiple Access With Slotted Aloha the best we can do for
throughput is 1/e What if each station could, before transmitting
a frame, listen to see if the channel is idle or busy…
…and only if idle transmit a frame This is Carrier Sense Multiple Access or CSMA
MAC Sublayer
Persistent Carrier Sense Multiple Access A greedy protocol When a station wants to transmit it listen to
the channel. If busy it continues to listen until the channel
is idle…Sort of like eaves-dropping on a party-line
Then transmits its frame
MAC Sublayer
Persistent Carrier Sense Multiple Access Can still have collisions
Two stations listen and find the channel busy
Both continue listening and …
when the transmission finishes (hangs up)..
Both immediately start transmitting their frame
MAC Sublayer
Persistent Carrier Sense Multiple Access Propagation delay problems
Station 1 listens and finds the channel idle
But it takes a millisecond or so to start transmission
In the millisecond, Station 2 listens and…
Find the channel seemingly idle because of the propagation delay in Station 1…
So Station 2 proceed to transmit, even though Station 1 has already started…
=Collision
MAC SublayerNon-persistent Carrier Sense Multiple Access In non-persistent CSMA Stations listen – If they find the channel busy, They don’t continue to listen,… but backoff and wait a random wait-time… then listen again Nearly eliminates collisions Still can have some --- how?
MAC Sublayer
Peak efficiency Aloha ~17% Slotted Aloha ~37% 1-persistent CSMA ~55% Non-persistent CSMA ~90%
MAC Sublayer
CSMA-CD CSMA with Carrier Detection Up to now the protocols could only tell there
was a collision after the transmission completed
So the mangle frames arrive Are determined to be irrepairable and Trashed… But…
MAC Sublayer
CSMA-CD What if tranmitting stations could sense a
collision as soon as it occurs? This is CSMA-CD As soon as a collision is detected both senders
terminate the transmission… Wait a random wait-period.. And reinitiate the carrier-sense-tranmit
algorithm..
MAC Sublayer
CSMA/CD Collision Detection does not reduce the
number of collisions.. But it does stop transmissions as soon as the
collision is detected… This frees up bandwidth that would have been
waste by continuing to transmit mangled frames
MAC Sublayer
CSMA/CD CSMA/CD is commonly used as MAC sublayer
protocol on LANs CSMA/CD is the basis for Ethernet
MAC Sublayer
Collision Free Protocols
It is possible to have protocols that avoid collisions altogether…
At the expense, usual of some overhead
MAC Sublayer
Collision Free Protocols Reservation Protocols In short a station needing to send a frame makes a reservation
to to do so Bit-map scheme There are N stations A contention frame is broadcast, The contention frame has a bit position for each of the N stations Each station can transmit in order, but only if it has made a
reservation It does this by setting its bit in the contention slot to 1 Then if its bit is 1, it transmits its frame after any (if any) previous
stations that made a reservation
MAC Sublayer
Collision Free Protocols
Reservation Protocols
0 1 0 0 1 1 0 0 Frame 1 Frame 4 Frame 5
MAC Sublayer
Collision Free Protocols Token Passing A small (low overhead) token is passed from station to
stationCan be physically or numerically a ring
A station can transmit a frame on the medium… When and only when, it has the token If the station has nothing to transmit it passes the
token to the next station If the station has something to transmit, it transmits its
frame of data, then passes its frame to the next station