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Title: mac sublayer description
Description: It gives an over all view about various wired and wireless protocols
Description: It gives an over all view about various wired and wireless protocols
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Unit – II
Chapter - 2
Medium Access Control
Sub layer
• In broadcast networks, How the channel is divided
between competing users?
• What is Medium Access Control (MAC)?
• What protocols are used for allocating a
multiple access channel ?
◦ The need for determine who gets to use the
channel
- When there are two or more users trying to
use a shared single channel there should be an
algorithm to control this access
...
MAC
-
Medium Access Control (MAC) is a sublayer
of the Data-link layer
...
-
MAC is important in LAN which use a
multiaccess channel as the basis for
communication
...
2) Dynamic Channel Allocation
Computer Networks
4
Static Channel Allocation:
In this scheme a Frequency Division Multiplexing
(FDM) is used for allocating a single channel
among competing users
...
++ FDM is a simple and efficient allocation
mechanism
...
Computer Networks
5
Dynamic Channel Allocation:
Before the discussion of algorithms used for
dynamic allocation we need to consider the
following assumptions
...
(Generate
>Block >Transmission)
2) Single channel Assumption: Single channel is
available for all communication
...
- there are two versions of ALOHA:
1) Pure ALOHA; 2) Slotted ALOHA
The basic difference with respect to timing
is:
Pure ALOHA does not require global time
synchronization;
Slotted ALOHA does
Computer Networks
7
• Pure ALOHA
ALOHA is a system proposed for solving the channel allocation
problem
...
The basic idea of an ALOHA system is simple: let users
transmit whenever they have data to be sent
...
If
the frame was destroyed, the sender just waits a random amount of
time and sends it again
...
-If listening while transmitting is not possible for some reason,
acknowledgements are needed
...
Pure ALOHA
The system is working as follows:
1- let users transmit whenever they have data to be sent
...
3- the collided frames will be destroyed
...
5- retransmit the destroyed frame
...
This is expected due to the feature that all users
transmit whenever they want
...
In Slotted ALOHA, a computer can not send anytime, instead it is
required to wait for the beginning of the time slot
...
Computer Networks
11
Slotted ALOHA
There is a limit for the best channel utilization
using Slotted ALOHA
...
In LAN networks this is possible, therefore they
can achieve better utilization than Slotted
ALOHA
...
(i
...
, a
transmission) and act accordingly
...
- if the channel is busy, the station waits until it becomes
idle
...
- If a collision occurs, the station waits a random amount
of time and starts all over again
...
2) There is a chance when two stations start transmission
at the same time
...
- After the first attempt, if the channel is idle, it
sends, however, if the channel is already in use,
it waits a random period of time and repeats
the algorithm
...
Computer Networks
15
P-persistent CSMA Protocol:
This is used for Slotted Systems
...
•
Step 1: If the medium is idle, transmit with probability p, and defers
until the next slot with a probability q=1-p
•
Step 2: If the medium is busy, continue to listen until medium
becomes idle, then go to Step 1
•
Step 3: If transmission is delayed by one time slot, continue with
Step 1
– A good tradeoff between nonpersistent and 1-persistent CSMA
The main advantage of persistent and non-persistent over
ALOHA is that they ensure no station begins to transmit
when it senses the channel busy
...
- This protocol is called CSMA/CD
...
- It is the basis of the popular Ethernet LAN
...
ii) A Binary Count down
A station wanting to use the channel now broadcasts
its address as a binary bit string, starting with the
high-order bit
...
The bits in each address position from
different stations are BOOLEAN ORed together
...
For example, if stations 0010, 0100, 1001, and 1010 are all
trying to get the channel
...
These are ORed together to form a 1
...
Stations 1001 and 1010
continue
...
The next bit
is 1, so station 1001 gives up
...
After winning the
bidding, it may now transmit a frame, after which another
bidding cycle starts
...
Limited contention protocols:
◦ collision based protocols (ALOHA,CSMA/CD)
are good when the network load is low
...
◦ On combining the advantages
...
under light load, every one can try for each slot like aloha
under heavy load, only a small group can try for each slot
how do we do it
treat stations as the leaf of a binary tree
...
if no conflict, fine
...
(depth first search)
The tree for eight stations
Wavelength Division Multiple Access
•To allow multiple transmissions at the same time, the spectrum
is divided into channels (wavelength bands)
...
A
narrow channel is provided as a control channel to signal the
station, and a wide channel is provided so the station can
output data frames
...
The MACA protocol
...
(b) B responding
with a CTS to A
...
This short frame (30
bytes) contains the length of the data frame that will
eventually follow
...
The CTS frame contains the data length (copied from the
RTS frame)
...
MACAW (MACA for Wireless)
Add ACK packet after data success
Carrier Sensing on RTS
Congestion information exchange between stations
...
Most Computers are connected to each other through
LAN
...
IEEE 802
...
11 – Wireless LAN
IEEE 802
...
16 – Wireless MAN
Ethernet Cabling
Four types of cables are commonly used
...
With Manchester encoding, each bit period is divided into
two equal intervals
...
A binary 0 is just the reverse: first low and
then high
...
For example, to
send data at 10 Mbps, the signal has to change 20 million
times/sec
...
A 0 bit is indicated by the
presence of a transition at the start of the interval
...
The Ethernet MAC Sub layer Protocol
Frame formats
...
(b) IEEE 802
...
Preamble:
i) Let the receiving computer know a frame is coming so that the frame
can be synchronizes
...
Destination Address
Each computer has a network interface card(NIC)
...
Each NIC has a ROM chip which is configured so that it
has a unique address
...
This series of 48 assigned to the NIC is called
the Media Access control address or MAC address
...
Source Address
When the destination computer successfully receives the
frame, it sends an ACK to the source computer that the
frame arrived at its intended destination
...
Type
The type field specifies which process to give the frame
to in the recipient's computer
...
Ex: Word doc’s, information from a sensor,
etc
...
If the
data portion of the frame is smaller than it needs to be,
the sending NIC adds extra bits before transmission to
bring the length up to 64 bytes
...
Checksum
To detect errors, a checksum algorithm (CRC) is used to perform
error detection
...
The first one was to
reduce the preamble to 7 bytes and use the last byte for a start of
frame(SOF) delimiter
...
Length: The frame has a segment that indicates the length or no: of
bytes, that make up the data segment so that the destination system
knows how many are coming
...
• After a collision, time is divided into discrete slots whose
length is equal to 2τ, where τ is the maximum propagation
delay in the network
...
• The stations involved in the collision randomly pick an integer
from the set {0,1}
...
If
the sources collide again because they picked the same integer,
the contention window size is doubled and it becomes
{0,1,2,3}
...
Before they try to
transmit, they listen to the channel and transmit only if the
channel is idle
...
•
•
In general, after collisions, a random
number between 0 and is chosen
...
•
In Ethernet, the doubling of the contention
window stops after 10 collisions and the
contention window remains {0,1,
...
•
After 16 collisions , the process is aborted
and the source stops trying
...
11)
Protocol Stack
The infrared method uses much the same technology as television
remote controls do
...
11 Physical Layer
FHSS (Frequency Hopping Spread Spectrum) uses 79 channels, each 1MHz wide, starting at the low end of the 2
...
A
pseudorandom number generator is used to produce the sequence of
frequencies hopped to
...
IEEE 802
...
OFDM is same as FDM with
one difference i
...
, all the sub bands are used by one source at a given
time
...
IEEE 802
...
4 GHz band
...
CCK encodes 4 or
8 bits to one CCK symbol
...
11g OFDM uses OFDM with a 2
...
The
complex modulation technique achieves a 54 Mbps data rate
...
Broad band Wireless (IEEE 802
...
16 Protocol Stack
802
...
e
...
The transmission is propagated by three techniques
...
The points give
the legal amplitude and phase combinations of each symbol
...
The phase of a dot is
indicated by the angle a line from it to the origin makes with the
positive x-axis
...
It has four valid combinations and can be used to transmit 2
bits per symbol
...
This modulation scheme can be used to
transmit 4 bits per symbol
...
iii) Quadrature Amplitude
(QAM- 64)
It allows 64 different combinations, so 6 bits can be transmitted per
symbol
...
Higher-order QAMs also are used
...
(b) QAM-16
...
802
...
MAC common sub layer describes about channel management i
...
, It
can schedule the downlink ( base station to subscriber) channels
very efficiently and plays a major role in managing the uplink (
subscriber to base) channels as well
...
The base station
simply decides what to put in which sub frame
...
Its allocation is tied
closely to the quality-of-service issue
...
It is
accommodated by dedicating certain time slots to each connection
of this type
...
ii) Real time available bit rate service: It is accommodated by the base
station polling the subscriber at a fixed interval to ask how much
bandwidth is needed this time
...
A constant bit rate customer can set a bit in one of its
frames requesting a poll in order to send additional (variable bit rate)
traffic
...
Requests
for bandwidth are done in time slots marked in the up link map as
available for contention
...
If it is not successful, unsuccessful
subscribers have to try again later
...
The Type field
identifies the frame type, mostly telling whether packing and
fragmentation are present
...
The EK field tells which of the
encryption keys is being used (if any)
...
The Connection
identifier tells which connection this frame belongs to
...
Figure: (a) A generic frame
...
Bluetooth ( IEEE 802
...
Multiple Piconets can exist in the same (large) room and
can even be connected via a bridge node
...
Bluetooth Applications
Bluetooth protocol Stack
Bluetooth Radio Layer: The radio layer moves the bits from master to
slave, or vice versa
...
4-GHz ISM band
...
Bluetooth Base band Layer: Each frame is transmitted over a logical
channel, called a link, between the master and a slave
...
The first is the ACL (Asynchronous Connection-Less) link,
which is used for packet-switched data available at irregular intervals
...
This type of channel is
allocated a fixed slot in each direction
...
Bluetooth L2CAP Layer: The L2CAP layer has three major functions
...
At the far end, the frames
are reassembled into packets again
...
Third, L2CAP handles the quality of service requirements (error
rates, bandwidth, throughput, transmission delay etc
...
RFcomm (Radio Frequency communication) is the protocol that
emulates the standard serial port found on PCs for connecting the
keyboard, mouse, and modem, among other devices
...
It also manages call setup and termination
...
The top layer is where the applications and profiles are located
...
Each application has its own dedicated subset of the protocols
...
11 to 802
...
Assume host n is transmitting a
frame F with unknown destination
...
Bridge B sees F on LAN 2 (with Bridge A
unknown destination), and copies
the frame back to LAN 1
Bridge A does the same
...
1 has an algorithm that builds and maintains a spanning tree
in a dynamic environment
...
To build the spanning tree, first the bridges have to choose one bridge
to be the root of the tree
...
The bridge with the lowest serial
number becomes the root
...
This tree is the spanning
tree
...
Remote Bridge
A common use of bridges is to connect two or more distant LAN’s
and there is a wide area connecting the two LAN’s
A remote bridge takes the frame before it leaves the first LAN and
encapsulates the WAN headers and trailers
...
g
...
Title: mac sublayer description
Description: It gives an over all view about various wired and wireless protocols
Description: It gives an over all view about various wired and wireless protocols