The Medium Accesscontrol Sublayer
The Medium Accesscontrol Sublayer
The Medium Accesscontrol Sublayer
SUBLAYER
• The protocols used to determine who goes
next on a multi-access channel belong to a
sublayer ofthe data link layer called the MAC
(Medium Access Control)sublayer.
• The MAC sublayer is especially important in
LANs, particularly wireless ones because
wireless is naturally a broadcast channel.
• WANs, in contrast, use point-to-point links,
except for satellite networks.
THE CHANNEL ALLOCATION PROBLEM
• Static ChannelAllocation
• If there are N users, the bandwidth is divided into
N equal-sized portions, with each user being
assigned one portion. Since each user has a
private frequency band, there is now no
interference among users.
• When there is only a small and constant number
of users, each of which has a steady stream or a
heavy load of traffic, this division is a simple and
efficient allocation mechanism.
• A wireless example is FM radio stations. Each
station gets a portion of the FM band and usesit
most of the time to broadcast its signal.
• when the number of senders is large and
varying or the traffic is suddenly
changing(burst of data), FDM presents some
problems.
• If the spectrum is cut up into N regions and
fewer than Nusers are currently interested in
communicating, a large piece of valuable
spectrum will be wasted. And ifmore than N
users want to communicate, some of them
will be denied permission for lack of
bandwidth, even if some of the users who
have been assigned a frequency band hardly
ever transmit or receiveanything.
• A static allocation is a poor fit to most
computer systems, in which data traffic is
extremely bursty, often with peak traffic to
mean traffic ratios of 1000:1. Consequently,
most of the channels will be idle most of the
time.
• The poor performance of static FDM can easily be seen
with a simple queuing theory calculation. Let us start by
finding the mean time delay, T,to send a frame onto a
channel of capacity Cbps.
• We assume that the frames arrive randomly with an
average arrival rate of λ frames/sec, and that the frames
vary in length with an average length of 1/μ bits. With these
parameters, the service rate of the channel is μC
frames/sec.
• A standard queueing theory resultis
• T = 1/(μC − λ)
• Now let us divide the single channel into N independent
subchannels, each with capacity C /N bps. The meaninput
rate on each of the subchannels will now be λ/N.
Recomputing T,weget
TN = 1/ (μ(C /N) − (λ/N)) = N/(μC − λ) = NT
• Ex: if C is 100 Mbps, the mean frame length,
1/μ, is 10,000 bits, and the frame arrivalrate,
λ, is 5000 frames/sec, then
• T = 1/(μC − λ) T = 200 μsec.
• Now if 100-Mbps network with10 networks of
10 Mbps each and statically allocate each user
to one of them, the mean delay would jump
from 200 μsec to 2msec.
• TN = 1/ (μ(C /N) − (λ/N)) = N/(μC − λ) =
NT=200*10=2000 μsec=2msec.
• The mean delay for the divided channel isN
times worse than that of without dividing.
(TN = NT)
• This same result says that a bank lobby full of
ATM machines is better off having a single
queue feeding all the machines than a
separate queue in front of eachmachine.
Assumptions for Dynamic Channel
Allocation
• There are the following five keyassumptions:
• Independent Traffic
• Single Channel.
• Observable Collisions.
• Continuous or SlottedTime.
• Carrier Sense or No CarrierSense
1. The model consists of Nindependent stations each with a
program or user that generates frames for transmission.
Once a frame has been generated, the station is blocked
and does nothing until the frame has been successfully
transmitted.
2. A single channel is available for all communication.All
stations can transmit on it and all can receive from it.
3. All stations can detect that a collision has occurred.A
collided frame must be transmitted againlater.
4. Time may be assumed continuous, frame transmission can
begin at any instant. Alternatively, time may be slotted or
divided into discrete intervals (called slots). Frame
transmissions must then begin at the start of a slot.
5. With the carrier sense assumption, stations can tell if the
channel is in use before trying to use it. Station will
transmit only when channel isfree.
MULTIPLE ACCESS PROTOCOLS
ALOHA
• Abramson’s work, called the ALOHAsystem,
used ground based radio broadcasting. The
basic idea is applicable to any system in which
uncoordinated users are competing for the
use of a single shared channel.
• Two versions of ALOHA: pure and slotted.
They differ w r t whether time is continuous,
as in the pure version, or divided intodiscrete
slots into which all frames must fit.(slotted
aloha)
Pure ALOHA
• In the ALOHAsystem, after each station has
sent its frame to the central computer, this
computer rebroadcasts the frame to all of the
stations. A sending station can thus listen for
the broadcast from the hub to see if its frame
has received properly.
• In other systems, such as wired LANs, the
sender might be able to listen forcollisions
while transmitting.
• If the frame was destroyed, the sender just
waits a random amount of time and sends it
again. The waiting time must be random or
the same frames will collide over and over,in
lockstep.
• Systems in which multiple users share a
common channel in a way that can lead to
conflicts are known as contention systems.
PUREALOHA
Comparison of the channel utilization versus load for various random accessprotocols.
CSMA with CollisionDetection
• CSMA/CD is the basis of theclassic Ethernet LAN. It
is important to realize that collision detection is an
analog process.
• The station’s hardware must listen to the channel
while it is transmitting. If the signal it reads back is
different from the signal it is sending, it knows that a
collision is occurring.
• The implications are that a received signal must not
be tiny compared to the transmitted signal (which is
difficult for wireless, as received signals may be
1,000,000 times weaker than transmitted signals)
and that the modulation must be chosen to allow
collisions to bedetected.
• CSMA/CD consist of alternating contention and
transmission periods, with idle periodsoccurring
when all stations arequiet.
The 802 working groups. The important ones are marked with *. The ones
marked with are hibernating. The one marked with † gave up.
IEEE802.3
• IEEE802.3 is a working group and a collection of
IEEEstandards produced by the working group
defining the physical layer and data link layer's
media access control (MAC) of wired Ethernet.
• This is generally a local area network technology
with some wide area network applications.
Physical connections are made between nodes
and/or infrastructure devices (hubs, switches,
routers) by various types ofcopper or fiber cable.
• 802.3 is a technology that supports the IEEE
802.1 network architecture.
• 802.3 also defines LAN access method using
CSMA/CD
IEEE802.4
• Token bus is a network implementing the token ring protocol over a
"virtual ring" on a coaxialcable.
• A token is passed around the network nodes and only the node
possessing the token may transmit. If a node doesn't have anything to
send, the token is passed on to the next node on the virtual ring. Each
node must know the address of its neighbor in the ring, so a special
protocol is needed to notify the other nodes of connections to, and
disconnections from, the ring.
• Token bus was standardized by IEEEstandard 802.4. It is mainly used
for industrial applications. Token bus was used by General Motors for
their Manufacturing Automation Protocol (MAP) standardization
effort. This is an application of the concepts used in token ring
networks. The main difference is that the endpoints of the bus do
not meet to form a physicalring.
• Due to difficulties handling device failures and adding new stationsto
a network, token bus gained a reputation for being unreliable and
difficult to upgrade.
IEEE802.5
• Token Ring is a LAN protocol defined in the IEEE802.5 where all
stations are connected in a ring and each station candirectly
hear transmissions only from its immediate neighbor.
Permission to transmit is granted by a message (token) that
circulates around the ring.
• Token-passing networks move a small frame, called a token,
around the network.
• Possession of the token grants the right to transmit. If a node
receiving the token has no information to send, it seizes the
token, alters 1 bit of the token, appends the information that it
wants to transmit, and sends this information to the next
station on the ring.
• While the information frame is circling the ring, no token is on
the network, which means that other stations wanting to
transmit must wait. Therefore, collisions cannot occur inToken
Ring networks.
IEEE802.6
• IEEE802.6 is a standard governed by the ANSI for
Metropolitan Area Networks (MAN). It is an improvementof
an older standard (also created by ANSI) which used the
Fiber distributed data interface (FDDI) network structure.
• The FDDI-based standard failed due to its expensive
implementation and lack of compatibility with current LAN
standards. The IEEE802.6 standard uses the Distributed
Queue Dual Bus (DQDB) network form.
• This standard has also failed, mostly for the same reasons
that the FDDI standard failed. Most MANs now use
Synchronous Optical Network (SONET) or Asynchronous
Transfer Mode (ATM) network designs, with recentdesigns
using native Ethernet.
IEEE802.11
• IEEE802.11 is a set of medium access control
(MAC) and physical layer (PHY) specifications
for implementing wireless local area network
(WLAN) computer communication in the 2.4,
3.6, 5 and 60 GHz frequency bands. They are
created and maintained by the IEEELAN/MAN
Standards Committee (IEEE 802). These
standards provide the basis for wireless
network products using the Wi-Fibrand.
• IEEE802.11 family consist of a series of half-
duplex over-the-air modulation techniques that
use the same basic protocol.
• The most popular are those defined by the
802.11b and 802.11g protocols, 802.11a was the
first widely accepted one, followed by 802.11b
and 802.11g. 802.11n is a new multi-streaming
modulation technique.
• Other standards in the family (c–f, h, j) are service
amendments and extensions or corrections to the
previous specifications.
IEEE802.15
• IEEE802.15 is a working group of the Institute of Electrical
and Electronics Engineers (IEEE) IEEE802 standards
committee which specifies Wireless Personal AreaNetwork
(WPAN) standards. It includes seven task groups.
frame control
start delimiter Ending delimiter
Frame Status