Computer Network MAC Layer Notes as per RGPV syllabus

Unit-III/ Computer Networking Truba College of Science & Tech. Bhopal
Prepared By: Ms. Nandini Sharma (CSE Deptt.) Page 1
Medium Access Control: In any broadcast network, the key issue is how to determine who
gets to use the channel deals when there is competition for it. To make this point clearer,
consider a conference call in which six people, on six different telephones, are all connected so
that each one can hear and talk to all the others. It is very likely that when one of them stops
speaking, two or more will start talking start once, leading to chaos. In a face-to-face meeting,
chaos is avoided by external means, for example, at a meeting; people raise their hands to
request permission to speak. Where only a single channel is available, determining who should
go next is much harder. So, broadcast channel are sometimes referred to as multi-access
channels or random access channels.
LAN and WAN
i) Static Channel Allocation in LAN and MAN
ii) Dynamic Channel Allocation in LAN and MAN
As the data link layer is overloaded, it is split into MAC and LLC sub layers. MAC sub-layer
is the bottom part of the data link layer. Medium access control is often used as a synonym
to multiple access protocol, since the MAC sub layer provides the protocol and control
mechanisms that are required for a certain channel access method. This unit deals with
broadcast networks and their protocols.
In any broadcast network, the key issue is how to determine who gets to use the channel when
there is a competition. When only one single channel is available, determining who should get
access to the channel for transmission is a very complex task. Many protocols for solving the
problem are known and they form the contents of this unit.
Thus unit provides an insight of a channel access control mechanism that makes it possible for
several terminals or network nodes to communicate within a multipoint network. The MAC
layer is essentially important in local area networks(LAN’s), many of which use a multi-access
channel as the basis for communication. WAN’s in contrast use a point to point networks.
To get a head start, let us define LANs and MANs.
Definition: A Local Area Network (LAN) is a network of systems spread over small
geographical area, for example a network of computers within a building or small campus.
The owner of a LAN may be the same organization within which the LAN network is set up. It
has higher data rates i.e. in scales of Mbps (Rates at which the data are transferred from one
system to another) because the systems to be spanned are very close to each other in proximity.
Definition: A WAN (Wide Area Network) typically spans a set of countries that have data
rates less than 1Mbps, because of the distance criteria.
The LANs may be owned by multiple organizations since the spanned distance is spread over
some countries.
i) Static Channel Allocation in LAN and MAN

Before going for the exact theory behind the methods of channel allocations, we need to
understand the base behind this theory, which is given below:
The channel allocation problem
We can classify the channels as static and dynamic. The static channel is where the number of
users are stable and the traffic is not bursty. When the number of users using the channel keeps
on varying the channel is considered as a dynamic channel. The traffic on these dynamic
channels also keeps on varying. For example: In most computer systems, the data traffic is
extremely bursty. We see that in this system, the peak traffic to mean traffic ratios of 1000:1
are common.
· Static channel allocation
The usual way of allocating a single channel among the multiple users is frequency division
multiplexing (FDM). If there are N users, the bandwidth allocated is split into N equal sized
portions. FDM is simple and efficient technique for small number of users. However when the
number of senders is large and continuously varying or the traffic is bursty, FDM is not
suitable.
The same arguments that apply to FDM also apply to TDM. Thus none of the static channels
allocation methods work well with bursty traffic we explore the dynamic channels.
· Dynamic channels allocation in LAN’s and MAN’s
Before discussing the channel allocation problems that is multiple access methods we will see
the assumptions that we are using so that the analysis will become simple.
Assumptions:
1. The Station Model:
The model consists of N users or independent stations. Stations are sometimes called
terminals. The probability of frame being generated in an interval of length ∆t is λ.Δt, where λ
is a constant and defines the arrival rate of new frames. Once the frame has been generated, the
station is blocked and does nothing until the frame has been successfully transmitted.
2. Single Channel Assumption:
A single channel is available for all communication. All stations can transmit using this single
channel. All can receive from this channel. As far as the hard is concerned, all stations are
equivalent. It is possible the software or the protocols used may assign the priorities to them.
3. Collisions:
If two frames are transmitted simultaneously, they overlap in time and the resulting signal is
distorted or garbled. This event or situation is called a collision. We assume that all stations
can detect collisions. A collided frame must be retransmitted again later. Here we consider no
other errors for retransmission other than those generated because of collisions.

4. Continuous Time
For a continuous time assumption we mean, that the frame transmission on the channel can
begin any instant of time. There is no master clock dividing the time into discrete intervals.
5. Slotted Time
In case of slotted time assumption, the time is divided into discrete slots or intervals. The
frame transmission on the channel begins only at the start of a slot. A slot may contain 0, 1, or
more frames. The 0 frame transmission corresponds to idle slot, 1 frame transmission
corresponds to successful transmission, and more frame transmission corresponds to a
collision.
6. Carrier Sense
Using this facility the users can sense the channel. i.e. the stations can tell if the channel is in
use before trying to use it. If the channel is sensed as busy, no station will attempt to transmit
on the channel unless and until it goes idle.
7. No Carrier Sense:
This assumption implies that this facility is not available to the stations. i.e. the stations cannot
tell if the channel is in use before trying to use it. They just go ahead and transmit. It is only
after transmission of the frame they determine whether the transmission was successful or not.
The first assumption states that the station is independent and work is generated at a constant
rate. It also assumes that each station has only one program or one user. Thus when the station
is blocked no new work is generated. The single channel assumption is the heart of this station
model and this unit. The collision assumption is also very basic. Two alternate assumptions
about time are discussed. For a given system only one assumption about time holds good, i.e.
either the channel is considered to be continuous time based or slotted time based. Also a
channel can be sensed or not sensed by the stations. Generally LANs can sense the channel but
wireless networks cannot sense the channel effectively. Also stations on wired carrier sense
networks can terminate their transmission prematurely if they discover collision. But in case of
wireless networks collision detection is rarely done.
ALOHA Protocols
In 1970s, Norman Abramson and his colleagues at University of Hawaii devised a new and
elegant method to solve the channel allocation problem. Their work has been extended by
many researchers since then. His work is called the ALOHA system which uses ground-based
radio broadcasting. This basic idea is applicable to any system in which uncoordinated users
are competing for the use of a shared channel.
Pure or Un-slotted Aloha
The ALOHA network was created at the University of Hawaii in 1970 under the leadership of
Norman Abramson. The Aloha protocol is an OSI layer 2 protocol for LAN networks
with broadcast topology.

The first version of the protocol was basic:
· If you have data to send, send the data
· If the message collides with another transmission, try resending it later
Figure 3.1: Pure ALOHA
Figure 3.2: Vulnerable period for the node: frame
The Aloha protocol is an OSI layer 2 protocols used for LAN. A user is assumed to be always
in two states: typing or waiting. The station transmits a frame and checks the channel to see if
it was successful. If so the user sees the reply and continues to type. If the frame transmission
is not successful, the user waits and retransmits the frame over and over until it has been
successfully sent.
Let the frame time denote the amount of time needed to transmit the standard fixed length
frame. We assume the there are infinite users and generate the new frames according Poisson
distribution with the mean N frames per frame time.
· If N>1 the users are generating the frames at higher rate than the channel can handle. Hence
all frames will suffer collision.
· Hence the range for N is
0<N<1

· If N>1 there are collisions and hence retransmission frames are also added with the new
frames for transmissions.
Let us consider the probability of k transmission attempts per frame time. Here the
transmission of frames includes the new frames as well as the frames that are given for
retransmission. This total traffic is also poisoned with the mean G per frame time. That is G ≥
N
· At low load: N is approximately =0, there will be few collisions. Hence few retransmissions
that is G=N
· At high load: N >>1, many retransmissions and hence G>N.
· Under all loads: throughput S is just the offered load G times the probability of successful
transmission P0
S = G*P0
The probability that k frames are generated during a given frame time is given by Poisson
distribution
P[k]= Gke-G / K!
So the probability of zero frames is just e-G. The basic throughput calculation follows a
Poisson distribution with an average number of arrivals of 2G arrivals per two frame time.
Therefore, the lambda parameter in the Poisson distribution becomes 2G.
Hence P0 = e-2G
Hence the throughput S = GP0 = Ge-2G
We get for G = 0.5 resulting in a maximum throughput of 0.184, i.e. 18.4%.
Pure Aloha had a maximum throughput of about 18.4%. This means that about 81.6% of the
total available bandwidth was essentially wasted due to losses from packet collisions.
Slotted or Impure ALOHA
An improvement to the original Aloha protocol was Slotted Aloha. It is in 1972 Roberts
published a method to double the throughput of a pure ALOHA by using discrete time-slots.
His proposal was to divide the time into discrete slots corresponding to one frame time. This
approach requires the users to agree to the frame boundaries. To achieve synchronization one
special station emits a pip at the start of each interval similar to a clock. Thus the capacity of
slotted ALOHA increased to the maximum throughput of 36.8%.
The throughput for pure and slotted ALOHA system is shown in figure 3.3. A station can send
only at the beginning of a timeslot and thus collisions are reduced. In this case, the average
number of aggregate arrivals is G arrivals per 2X seconds. This leverages the lambda
parameter to be G. The maximum throughput is reached for G = 1.

Figure 3.3: Throughput versus offered load traffic
With Slotted Aloha, a centralized clock sends out small clock tick packets to the outlying
stations. Outlying stations are allowed to send their packets immediately after receiving a clock
tick. If there is only one station with a packet to send, this guarantees that there will never be a
collision for that packet. On the other hand if there are two stations with packets to send, this
algorithm guarantees that there will be a collision, and the whole of the slot period up to the
next clock tick is wasted. With some mathematics, it is possible to demonstrate that this
protocol does improve the overall channel utilization, by reducing the probability of collisions
by a half.
It should be noted that Aloha’s characteristics are still not much different from those
experienced today by Wi - Fi, and similar contention-based systems that have no carrier sense
capability. There is a certain amount of inherent inefficiency in these systems. It is typical to
see these types of networks’ throughput break down significantly as the number of users and
message burstiness increase. For these reasons, applications which need highly deterministic
load behavior often use token-passing schemes (such as token ring) instead of contention
systems.
For instance ARCNET is very popular in embedded applications. Nonetheless, contention
based systems also have significant advantages, including ease of management and speed in
initial communication. Slotted Aloha is used on low bandwidth tactical Satellite
communications networks by the US Military, subscriber based Satellite communications
networks, and contact less RFID technologies.
3.3 LAN Protocols
With slotted ALOHA, the best channel utilization that can be achieved is 1 / e. This is hardly
surprising since with stations transmitting at will, without paying attention to what other
stations are doing, there are bound to be many collisions. In LANs, it is possible to detect what
other stations are doing, and adapt their behavior accordingly. These networks can achieve a
better utilization than 1 / e.
CSMA Protocols:
Protocols in which stations listen for a carrier (a transmission) and act accordingly are
called Carrier Sense Protocols. "Multiple Access" describes the fact that multiple nodes
send and receive on the medium. Transmissions by one node are generally received by all

other nodes using the medium. Carrier Sense Multiple Access (CSMA) is
a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of
other traffic before transmitting on a shared physical medium, such as an electrical bus, or a
band of electromagnetic spectrum.
The following three protocols discuss the various implementations of the above discussed
concepts:
i) Protocol 1. 1-persistent CSMA:
When a station has data to send, it first listens to the channel to see if any one else is
transmitting. If the channel is busy, the station waits until it becomes idle. When the station
detects an idle channel, it transmits a frame. If a collision occurs, the station waits a random
amount of time and starts retransmission.
The protocol is so called because the station transmits with a probability of a whenever it finds
the channel idle.
ii) Protocol 2. Non-persistent CSMA:
In this protocol, a conscious attempt is made to be less greedy than in the
1-persistent CSMA protocol. Before sending a station senses the channel. If no one else is
sending, the station begins doing so itself. However, if the channel is already in use, the station
does not continuously sense the channel for the purpose of seizing it immediately upon
detecting the end of previous transmission. Instead, it waits for a random period of time and
then repeats the algorithm. Intuitively, this algorithm should lead to better channel utilization
and longer delays than 1-persistent CSMA.
iii) Protocol 3. p - persistent CSMA
It applies to slotted channels and the working of this protocol is given below:
When a station becomes ready to send, it senses the channel. If it is idle, it transmits with a
probability p. With a probability of q = 1 – p, it defers until the next slot. If that slot is also
idle, it either transmits or defers again, with probabilities p and q. This process is repeated until
either the frame has been transmitted or another station has begun transmitting. In the latter
case, it acts as if there had been a collision. If the station initially senses the channel busy, it
waits until the next slot and applies the above algorithm.
CSMA/CD Protocol
In computer networking, Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
is a network control protocol in which a carrier sensing scheme is used. A
transmitting data station that detects another signal while transmitting a frame, stops
transmitting that frame, transmits a jam signal, and then waits for a random time interval. The
random time interval also known as "backoff delay" is determined using the truncated binary
exponential backoff algorithm. This delay is used before trying to send that frame

again. CSMA/CD is a modification of pure Carrier Sense Multiple Access (CSMA).
Collision detection is used to improve CSMA performance by terminating transmission as
soon as a collision is detected, and reducing the probability of a second collision on retry.
Methods for collision detection are media dependent, but on an electrical bus such as Ethernet,
collisions can be detected by comparing transmitted data with received data. If they differ,
another transmitter is overlaying the first transmitter’s signal (a collision), and transmission
terminates immediately. Here the collision recovery algorithm is nothing but an binary
exponential algorithm that determines the waiting time for retransmission. If the number of
collisions for the frame hits 16 then the frame is considered as not recoverable.
CSMA/CD can be in anyone of the following three states as shown in figure 3.4.
1. Contention period
2. transmission period
3. Idle period
Figure 3.4: States of CSMA / CD: Contention, Transmission, or Idle
A jam signal is sent which will cause all transmitters to back off by random intervals, reducing
the probability of a collision when the first retry is attempted. CSMA/CD is a layer 2 protocol
in the OSI model. Ethernet is the classic CSMA/CD protocol.
Collision Free Protocols
Although collisions do not occur with CSMA/CD once a station has unambiguously seized the
channel, they can still occur during the contention period. These collisions adversely affect the
system performance especially when the cable is long and the frames are short. And also
CSMA/CD is not universally applicable. In this section, we examine some protocols that
resolve the contention for the channel without any collisions at all, not even during the
contention period.
In the protocols to be described, we assume that there exists exactly N stations, each with a
unique address from 0 to N-1 “wired” into it. We assume that the propagation delay is
negligible.

i) A Bit Map Protocol
In this method, each contention period consists of exactly N slots. If station 0 has a frame to
send, it transmits a 1 bit during the zeroth slot. No other station is allowed to transmit during
this slot. Regardless of what station 0 is doing, station 1 gets the opportunity to transmit a 1
during slot 1, but only if it has a frame queued. In general, station j may announce that it has a
frame to send by inserting a 1 bit into slot j. After all N stations have passed by, each station
has complete knowledge of which stations wish to transmit. At that point, they begin
transmitting in a numerical order.
Since everyone agrees on who goes next, there will never be any collisions. After the last ready
station has transmitted its frame, an event all stations can monitor, another N bit contention
period is begun. If a station becomes ready just after its bit slot has passed by, it is out of luck
and must remain silent until every station has had a chance and the bit map has come around
again.
Protocols like this in which the desire to transmit is broadcast before the actual transmission
are called Reservation Protocols.
ii) Binary Countdown
A problem with basic bit map protocol is that the overhead is 1 bit per station, so it odes not
scale well to networks with thousands of stations. We can do better by using binary station
address.
A station wanting to use the channel now broadcasts its address as a binary bit string, starting
with the high-order bit. All addresses are assumed to be of the same length. The bits in each
address position from different stations are Boolean ORed together. We call this protocol
as Binary countdown, which was used in Datakit. It implicitly assumes that the transmission
delays are negligible so that all stations see asserted bits essentially simultaneously.
To avoid conflicts, an arbitration rule must be applied: as soon as a station sees that a high-
order bit position that is 0 in its address has been overwritten with a 1, it gives up.
Example: If stations 0010, 0100, 1001, and 1010 are all trying to get the channel for
transmission, these are ORed together to form a 1. Stations 0010 and 0100 see the 1 and know
that a higher numbered station is competing for the channel, so they give up for the current
round. Stations 1001 and 1010 continue.
The next bit is 0, and both stations continue. The next bit is 1, so station 1001 gives up. The
winner is station 1010 because t has the highest address. After winning the bidding, it may now
transmit a frame, after which another bidding cycle starts.
This protocol has the property that higher numbered stations have a higher priority than lower
numbered stations, which may be either good or bad depending on the context.
iii) Limited Contention Protocols
Until now we have considered two basic strategies for channel acquisition in a cable network:

Contention as in CSMA, and collision – free methods. Each strategy can be rated as to how
well it does with respect to the two important performance measures, delay at low load,
and channel efficiency at high load.
Under conditions of light load, contention (i.e. pure or slotted ALOHA) is preferable due to its
low delay. As the load increases, contention becomes increasingly less attractive, because the
overhead associated with channel arbitration becomes greater. Just the reverse is true for
collision free protocols. At low load, they have high delay, but as the load increases, the
channel efficiency improves.
It would be more beneficial if we could combine the best features of contention and collision
free protocols and arrive at a protocol that uses contention at low load to provide low delay,
but uses a collision free technique at high load to provide good channel efficiency. Such
protocols can be called Limited Contention protocols.
iv) Adaptive Tree Walk Protocol
A simple way of performing the necessary channel assignment is to use the algorithm devised
by US army for testing soldiers for syphilis during World War II. The Army took a blood
sample from N soldiers. A portion of each sample was poured into a single test tube. This
mixed sample was then tested for antibodies. If none were found, all the soldiers in the group
were declared healthy. If antibodies were present, two new mixed samples were prepared, one
from soldiers 1 through N/2 and one from the rest. The process was repeated recursively until
the infected soldiers were detected.
For the computerized version of this algorithm, let us assume that stations are arranged as the
leaves of a binary tree as shown in figure 3.4 below:
Figure 3.5: A tree for four stations
In the first contention slot following a successful frame transmission, slot 0, all stations are
permitted to acquire the channel. If one of them does, so fine. If there is a collision, then during
slot 1 only stations falling under node 2 in the tree may compete. If one of them acquires the
channel, the slot following the frame is reserved for those stations under node 3. If on the other
hand, two or more stations under node 2 want to transmit, there will be a collisions during slot

1, in which case it is node 4’s turn during slot 2.
In essence, if a collision occurs during slot 0, the entire tree is searched, depth first to locate all
ready stations. Each bit slot is associated with some particular node in a tree. If a collision
occurs, the search continues recursively with the node’s left and right children. If a bit slot is
idle or if only one station transmits in it, the searching of its node can stop because all ready
stations have been located.
When the load on the system is heavy, it is hardly worth the effort to dedicate slot 0 to node 1,
because that makes sense only in the unlikely event that precisely one station has a frame to
send.
At what level in the tree should the search begin? Clearly, the heavier the load, the farther
down the tree the search should begin.
3.4 IEEE 802 standards for LANs
IEEE has standardized a number of LAN’s and MAN’s under the name of IEEE 802. Few of
the standards are listed in figure 3.6. The most important of the survivor’s are 802.3 (Ethernet)
and 802.11 (wireless LAN). Both these two standards have different physical layers and
different MAC sub layers but converge on the same logical link control sub layer so they have
same interface to the network layer.
IEEE No Name Title
802.3 Ethernet CSMA/CD Networks (Ethernet)
802.4 Token Bus Networks
802.5 Token Ring Networks
802.6 Metropolitan Area Networks
802.11 WiFi Wireless Local Area Networks
802.15.1 Bluetooth Wireless Personal Area Networks
802.15.4 ZigBee Wireless Sensor Networks
802.16 WiMa Wireless Metropolitan Area Networks
Figure 3.6: List of IEEE 802 Standards for LAN and MAN
Ethernets
Ethernet was originally based on the idea of computers communicating over a shared coaxial
cable acting as a broadcast transmission medium. The methods used show some similarities to
radio systems, although there are major differences, such as the fact that it is much easier to
detect collisions in a cable broadcast system than a radio broadcast. The common cable
providing the communication channel was likened to the ether and it was from this reference
the name "Ethernet" was derived.
From this early and comparatively simple concept, Ethernet evolved into the complex
networking technology that today powers the vast majority of local computer networks. The
coaxial cable was later replaced with point-to-point links connected together by hubs and/or

switches in order to reduce installation costs, increase reliability, and enable point-to-point
management and troubleshooting. Star LAN was the first step in the evolution of Ethernet
from a coaxial cable bus to a hub-managed, twisted-pair network.
Above the physical layer, Ethernet stations communicate by sending each other data packets,
small blocks of data that are individually sent and delivered. As with other IEEE 802 LANs,
each Ethernet station is given a single 48-bit MAC address, which is used both to specify the
destination and the source of each data packet. Network interface cards (NICs) or chips
normally do not accept packets addressed to other Ethernet stations. Adapters generally come
programmed with a globally unique address, but this can be overridden, either to avoid an
address change when an adapter is replaced, or to use locally administered addresses.
The most kinds of Ethernets used were with the data rate of 10Mbps. The table 3.1 gives the
details of the medium used, number of nodes per segment and distance it supported, along with
the application.
Table 3.1 Different 10Mbps Ethernets used
Name Cable
Type
Max
Segment
Length
Nodes per
Segment
Advantages
10Base5 Thick coax 500 m 100 Original Cable; Now
Obsolete
10Base2 Thin coax 185 m 30 No hub needed
10Base-T Twisted
Pair
100 m 1024 Cheapest system
10Base-F Fibre
Optics
2000 m 1024 Best between
buildings
Fast Ethernet
Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the
nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the 100
megabit Ethernet standards 100baseTX is by far the most common and is supported by the vast
majority of Ethernet hardware currently produced. Full duplex fast Ethernet is sometimes
referred to as "200 Mbit/s" though this is somewhat misleading as that level of improvement
will only be achieved if traffic patterns are symmetrical. Fast Ethernet was introduced in 1995
and remained the fastest version of Ethernet for three years before being superseded by gigabit
Ethernet.
A fast Ethernet adaptor can be logically divided into a medium access controller (MAC) which
deals with the higher level issues of medium availability and a physical layer interface (PHY).
The MAC may be linked to the PHY by a 4 bit 25 MHz synchronous parallel interface known
as MII. Repeaters (hubs) are also allowed and connect to multiple PHYs for their different
interfaces.
· 100BASE-T is any of several Fast Ethernet standards for twisted pair cables.

· 100BASE-TX (100 Mbit/s over two-pair Cat5 or better cable),
· 100BASE-T4 (100 Mbit/s over four-pair Cat3 or better cable, defunct),
· 100BASE-T2 (100 Mbit/s over two-pair Cat3 or better cable, also defunct).
The segment length for a 100BASE-T cable is limited to 100 meters. Most networks had to be
rewired for 100-megabit speed whether or not they had supposedly been CAT3 or CAT5 cable
plants. The vast majority of common implementations or installations of 100BASE-T are done
with 100BASE-TX.
100BASE-TX is the predominant form of Fast Ethernet, and runs over two pairs of category
5 or above cable. A typical category 5 cable contains 4 pairs and can therefore support two
100BASE-TX links. Each network segment can have a maximum distance of 100 metres. In its
typical configuration, 100BASE-TX uses one pair of twisted wires in each direction, providing
100 Mbit/s of throughput in each direction (full-duplex).
The configuration of 100BASE-TX networks is very similar to 10BASE-T. When used to
build a local area network, the devices on the network are typically connected to
a hub or switch, creating a star network. Alternatively it is possible to connect two devices
directly using a crossover cable.
In 100BASE-T2, the data is transmitted over two copper pairs, 4 bits per symbol. First, a 4 bit
symbol is expanded into two 3-bit symbols through a non-trivial scrambling procedure based
on a linear feedback shift register.
100BASE-FX is a version of Fast Ethernet over optical fibre. It uses two strands of multi-mode
optical fibre for receive (RX) and transmit (TX). Maximum length is 400 metres for half-
duplex connections or 2 kilometers for full-duplex.
100BASE-SX is a version of Fast Ethernet over optical fibre. It uses two strands of multi-mode
optical fibre for receive and transmit. It is a lower cost alternative to using 100BASE-FX,
because it uses short wavelength optics which are significantly less expensive than the long
wavelength optics used in 100BASE-FX. 100BASE-SX can operate at distances up to 300
meters.
100BASE-BX is a version of Fast Ethernet over a single strand of optical fibre (unlike
100BASE-FX, which uses a pair of fibres). Single-mode fibre is used, along with a special
multiplexer which splits the signal into transmit and receive wavelengths.
Gigabit Ethernet
Gigabit Ethernet (GbE or 1 GigE) is a term describing various technologies for
transmitting Ethernet packets at a rate of a gigabit per second, as defined by the IEEE 802.3-
2005 standard. Half duplex gigabit links connected through hubs are allowed by the
specification but in the marketplace full duplex with switches is the norm.
Gigabit Ethernet was the next iteration, increasing the speed to 1000 Mbit/s. The initial
standard for gigabit Ethernet was standardized by the IEEE in June 1998 as IEEE 802.3z.

802.3z is commonly referred to as 1000BASE-X (where -X refers to either -CX, -SX, -LX, or -
ZX).
IEEE 802.3ab, ratified in 1999, defines gigabit Ethernet transmission over unshielded twisted
pair (UTP) category 5, 5e, or 6 cabling and became known as 1000BASE-T. With the
ratification of 802.3ab, gigabit Ethernet became a desktop technology as organizations could
utilize their existing copper cabling infrastructure.
Initially, gigabit Ethernet was deployed in high-capacity backbone network links (for instance,
on a high-capacity campus network). Fibre gigabit Ethernet has recently been overtaken by 10
gigabit Ethernet which was ratified by the IEEE in 2002 and provided data rates 10 times that
of gigabit Ethernet. Work on copper 10 gigabit Ethernet over twisted pair has been completed,
but as of July 2006, the only currently available adapters for 10 gigabit Ethernet over copper
requires specialized cabling.
InfiniBand connectors and is limited to 15 m. However, the 10GBASE-T standard specifies
use of the traditional RJ-45 connectors and longer maximum cable length. Different gigabits
Ethernet are listed in table 3.2.
Table 3.2 Different Gigabit Ethernets
Name Medium
1000BASE-T unshielded twisted pair
1000BASE-SX multi-mode
1000BASE-LX single-mode
1000BASE-CX balanced copper cabling
1000BASE-ZX single-mode
IEEE 802.3 Frame format
Preamble SOF Destination
Address
Source
Address
Length Data Pad Checksum
Figure 3.7: Frame format of IEEE 802.3
· Preamble field
Each frame starts with a preamble of 8 bytes, each containing bit patterns “10101010”.
Preamble is encoded using Manchester encoding. Thus the bit patterns produce a 10MHz
square wave for 6.4 micro sec to allow the receiver’s clock to synchronize with the sender’s
clock.
· Address field
The frame contains two addresses, one for the destination and another for the sender. The
length of address field is 6 bytes. The MSB of destination address is ‘0’ for ordinary addresses
and ‘1’ for group addresses. Group addresses allow multiple stations to listen to a single

address. When a frame is sent to a group of users, all stations in that group receive it. This type
of transmission is referred to as multicasting. The address consisting of all ‘1’ bits is reserved
for broadcasting.
· SOF: This field is 1 byte long and is used to indicate the start of the frame.
· Length:
This field is of 2 bytes long. It is used to specify the length of the data in terms of bytes that is
present in the frame. Thus the combination of the SOF and the length field is used to mark the
end of the frame.
· Data :
The length of this field ranges from zero to a maximum of 1500 bytes. This is the place where
the actual message bits are to be placed.
· Pad:
When a transceiver detects a collision, it truncates the current frame, which means the stray
bits and pieces of frames appear on the cable all the time. To make it easier to distinguish valid
frames from garbage, Ethernet specifies that valid frame must be at least 64 bytes long, from
the destination address to the checksum, including both. That means the data field come must
be of 46 bytes. But if there is no data to be transmitted and only some acknowledgement is to
be transmitted then the length of the frame is less than what is specified for the valid frame.
Hence these pad fields are provided, i.e. if the data field is less than 46 bytes then the pad field
comes into picture such that the total data and pad field must be equal to 46 bytes minimum. If
the data field is greater than 46 bytes then pad field is not used.
· Checksum:
It is 4 byte long. It uses a 32-bit hash code of the data. If some data bits are in error, then the
checksum will be wrong and the error will be detected. It uses CRC method and it is used only
for error detection and not for forward error correction.
Fibre Optic Networks
Fibre optics is becoming increasingly important, not only for wide area point-to-point links,
but also for MANs and LANs. Fibre has high bandwidth, is thin and lightweight, is not
affected by electromagnetic interference from heavy machinery, power surges or lightning, and
has excellent security because it is nearly impossible to wiretap without detection.
FDDI (Fibre Distributed Data Interface)
It is a high performance fibre optic token ring LAN running at 100 Mbps over distances up to
200 km with up to 1000 stations connected. It can be used in the same way as any of the 802
LANs, but with its high bandwidth, another common use is as a backbone to connect copper
LANs.

FDDI – II is a successor of FDDI modified to handle synchronous circuit switched PCM data
for voice or ISDN traffic, in addition to ordinary data.
FDDI uses multimode fibres. It also uses LEDs rather than lasers because FDDI may
sometimes be used to connect directly to workstations.
The FDDI cabling consists of two fibre rings, one transmitting clockwise and the other
transmitting counter clockwise. If any one breaks, the other can be used as a backup.
FDDI defines two classes of stations A and B. Class A stations connect to both rings. The
cheaper class B stations only connect to one of the rings. Depending on how important fault
tolerance is, an installation can choose class A or class B stations, or some of each.
Wireless LANs
Although Ethernet is widely used, it is about to get some competition. Wireless LANs are
increasingly popular, and more and more office buildings, airports and other public places are
being outfitted with them. Wireless LANs can operate in one of two configurations, a base
station and without a base station. Consequently, the 802.11 LAN standards take this into
account and makes provision for both arrangements.
802.11 Protocol Stack
The protocols used by all the 802 variants, including Ethernet, have a certain commonality of
structure. A partial view of the 802.11 protocol stack is given in diagram. The physical layer
corresponds to the OSI physical layer fairly well, but the data link layer in all the 802 protocols
is split into two or more sub layers. In 802.11, the MAC sub layer determines how the channel
is allocated, that is, who gets to transmit next. Above it is the LLC sublayer, whose job it is to
hide the differences between the different 802 variants and make them indistinguishable as far
as the network layer is concerned.
In 1977 802.11 standard specifies three transmission techniques allowed in the physical layer.
The infrared method uses much the same technology as television remote controls do. The
other two use short-range radio, using techniques called FHSS and DSSS. Both of these use a
part of the spectrum that does not require licensing. Radio-controlled garage door openers also
use this piece of the spectrum, so your notebook computer may find itself in competition with
your garage door. Cordless telephones and microwave oven also use this band.
Diagram: Part of the 802.11 protocol stack

802.11 Physical Layer
The purpose of this document is to explain the basic ideas laying in the foundation of the
technologies adopted by IEEE 802.11 standards for wireless communications at the physical
layer. It is designed for audience working with or administrating the devices complying to the
named standards, and willing to know their principles of operation believing that such
knowledge can help to make educated decisions regarding the related equipment, choose and
utilize the available hardware more efficiently.
Using Radio Waves For Data Transmission
Designing a wireless high speed data exchange system is not a trivial task to do. Neither is the
development of the standard for wireless local area networks. The major problems at the
physical layer here caused by the nature of the chosen media are:
 bandwidth allocation;
 external interference;
 reflection.
802.11 First Standard For Wireless
LANs
The Institute of Electronic and
Electrical Engineers (IEEE) has
released IEEE 802.11 in June 1997.
The standard defined physical and
MAC layers of wireless local area
networks (WLANs).
The physical layer of the original
802.11 standardized three wireless
data exchange techniques:

 Infrared (IR);
 Frequency hopping spread spectrum (FHSS);
 Direct sequence spread spectrum (DSSS).
The 802.11 radio WLANs operate in the 2.4GHz (2.4 to 2.483 GHz) unlicensed Radio
Frequency (RF) band. The maximum isotropic transmission power in this band allowed by
FCC in US is 1Wt, but 802.11 devices are usually limited to the 100mWt value.
The physical layer in 802.11 is split into Physical Layer Convergence Protocol (PLCP) and the
Physical Medium Dependent (PMD) sub layers. The PLCP prepares/parses data units
transmitted/received using various 802.11 media access techniques. The PMD performs the
data transmission/reception and modulation/demodulation directly accessing air under the
guidance of the PLCP. The 802.11 MAC layer to the great extend is affected by the nature of
the media. For example, it implements a relatively complex for the second layer fragmentation
of PDUs.
802.11 MAC Sub layer Protocol
It is quite different from that of Ethernet due to the inherent complexity of the wireless
environment compared to that of a wired system. With Ethernet, a station just wait until the
ether goes silent and starts transmitting. If it does not receive a noise burst back within the first
64 bytes, the frame has almost assuredly been delivered correctly. With wireless, this situation
does not hold.
To start with, there is the hidden station problem mentioned earlier and illustrated again in Fig.
(a) & (b).
Since not all stations are within radio range of each other, transmissions going on in one part of
a cell may not be received elsewhere in the same cell. In this example, station C is transmitting
to station B. If A senses the channel, it will not hear anything and falsely conclude that it may
now start transmitting to B.

In addition, there is a inverse problem, the exposed station problem in a (b) diagram Here B
wants to send to C so it listens to the channel. Want it hears a transmission; it falsely concludes
that it may not send to C, even though A may be transmitting to D. In addition, most radios are
half duplex, meaning that they cannot transmit and listen for noise bursts at the same time on a
single frequency. As a result of these problems, 802.11 do not use CSMA/CD, as Ethernet
does.
To deal with this problem, 802.11 support two modes of operation.
 Distributed Coordination Function (DCF)
 Point Coordination Function (PCF)
The protocol starts when A decides it wants to send data to B. It begins by sending an RTS
frame to B to request permission to send it a frame. When B receives this request, it may
decide to grant permission, in which case it sends a CTS frame back. Upon receipt of the CTS,
A now sends its frame and starts an ACK timer. Upon correct receipt of the data frame, B
responds with an ACK frame, terminating the exchange. If A’s ACK timer expires before the
ACK gets back to it, the whole protocol is run again.
Now Let us consider this exchange from the viewpoints of C and D. C is within range of A, so
it may receive the RTS frame. If it does, it realizes that someone is going to send data soon, so
for the good of all it desists from transmitting anything until the exchange is completed. From
the information provided in the RTS request, it can estimate how long the sequence will take,
including the final ACK, so it asserts a kind of virtual channel busy for itself, indicated by
NAV (Network Allocation Vector) in diagram D does not hear the RTS, but it does hear the
CTS, so it also asserts the NAV signal for itself. Note that the NAV signals are not transmitted;
they are just internal reminders to keep quiet for a certain period of time.
To deal with problem of noisy channels, 802.11allows frames to be fragmented into smaller
pieces, each with its own checksum. The fragments are individually numbered and ACK using
stop-and-wait protocol.

Broadband Wireless
Wireless broadband is high-speed Internet and data service delivered through a wireless local
area network (WLAN) or wide area network (WWAN).
As with other wireless service, wireless broadband may be either fixed or mobile. A fixed
wireless service provides wireless Internet for devices in relatively permanent locations, such
as homes and offices. Fixed wireless broadband technologies include LMDS (Local Multipoint
Distribution System) and MMDS (Multichannel Multipoint Distribution Service) systems for
broadband microwave wireless transmission direct from a local antenna to homes and
businesses within a line-of-sight radius. The service is similar to that provided through digital
subscriber line (DSL) or cable modem but the method of transmission is wireless.
The 802.16 Protocol Stack
The general structure is similar to that of the other 802 networks, but with more sub layers.
The bottom sub layer deals with transmission. Traditional narrow-band radio is used with
conventional modulation schemes. Above the physical transmission layer comes a

convergence sublayer to hide the different technologies from the Data Link Layer.
The DLL consists of three sublayers. The bottom one deals with privacy and security, which is
far more crucial for public outdoor networks than for private indoor networks. It manages
encryption, decryption and key management.
Next comes the MAC sub layer common part. This is where the main protocols, such as
channel management, are located. The model is that the base station controls the system. It can
schedule the downstream channels very efficiently and plays a major a role in management the
upstream channel as well.
The service-specific convergence sublayer takes the place of the logical link sublayer in the
other 802 protocols. Its function is to interface to the network layer. A complication here is that
802.16 was designed to integrate seamlessly with both datagram protocols and ATM. The
problem is that packet protocols are connection-less and ATM is connection oriented.
The 802.16 Frame Structure
All MAC frames begin with a generic header. The header is followed by an optional payload
and an optional checksum (CRC). The Payload is not needed in control frames, for ex:- those

requesting channel slots. The checksum is also optional due to the error correction in the
physical layer and the fact that no attempt is ever made to retransmit real-time frames.
Bluetooth
In 1994, the L.M. Ericson company interested in connecting its mobile phones to other devices
without cables. Together with four other companies ( IBM, Intel, Nokia and Toshiba), it
formed a SIG to develop a wireless standard for interconnecting computing and
communication devices and accessories using short-range, low-power, inexpensive wireless
radios. The project was named Bluetooth, after Harald Blatand II, a Viking king who unified
Denmark and Norway, also without cables.
Undaunted by all this, in July 1999 the Bluetooth SIG issued a 1500-page specification of
V1.0. Shortly, thereafter, the IEEE standards group looking at wireless personal area networks,
802.15, adopted the Bluetooth document as a basis and began hacking on it.
Bluetooth Architecture
The basic unit of a Bluetooth system is a piconet, which consists of a master node and upto
seven active slave nodes within a distance of 10 meters. Multiple piconets can exist in the
same room and can even be connected via a bridge node, interconnected collection of piconets
is called a Scatternet.
In addition to the seven active slave nodes in a piconet, there can be up to 255 parked nodes in
the net. These are devices that the master has switched to a low-power state to reduce the drain
on their batteries. In parked state, a device cannot do anything except respond to an activation
or beacon signal from the master. There are also two intermediate power states, hold and sniff,
but these will not concern us here.

The reason for the master/slave design is that the designers intended to facilitate the
implementations of complete Bluetooth chips for under $5. The consequence of this decision is
that the slaves are fairly dum, basically just doing whatever the master tells them to do. At its
heart, a piconet is a centralized TDM system, with the master controlling the clock and
determining which devices get to communicate in which time slot. All communication is
between the master and a slave; direct slave-slave communications is not possible.
Bluetooth Application
The Bluetooth Protocol Stack
The basic Bluetooth protocol architecture as modified by the 802 committee is shown as
above.
The bottom layer is the physical radio layer, which corresponds fairly well to the physical layer
in the OSI and 802 models. It deals with radio transmission and modulation. Many of the
concerns here have to do with the goal of making the system inexpensive so that it can become

a mass market item.
The baseband layer is somewhat analogous to the MAC sublayer but also includes elements of
the physical layer. It deals with how the master controls time slots and how these slots are
grouped into frames.
Next comes a layer with a group of somewhat related protocols. The Link manager handles the
establishment of logical channels between devices, including power management,
authentication and quality of service. The L2CAP shields the upper layer from the details of
transmission. The audio and control protocols deal with audio and control respectively.
The next layer, which contains a mix of different protocols. The RFcomm, telephony and
service discovery protocols are native.
The top layer is where the application and profiles are located. They make use of the protocols
in lower layers to get their work done. Each application hs its own dedicated subset of the
protocols. Specific devices, such as headset, usually contain only those protocols needed by
that application and no them.

Computer Network MAC Layer Notes as per RGPV syllabus

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Computer Network MAC Layer Notes as per RGPV syllabus