(Due on 1/30/2018 at 8:00 PM)
Does the link layer encapsulate the
datagram received from the network layer into a frame?
Yes, link layer encapsulate the diagram received
from the network layer into a frame. As we know that frame consists of a data field,
in which the network layer datagram is introduced, and a number of header
fields. I other word I can say that a frame may also comprise trailer fields
and we will mention to both header and trailer fields as header fields. A data link
protocol requires the structure of the frame, as well as a channel access
protocol that specifies the rules by which a frame is transmitted onto the
link. For point to point relations that have a single sender on one end of the
link and a single receiver at the other end of the link, the link entrance
protocol is simple the sender can send a frame whenever the link is shiftless.
The more stimulating case is when multiple nodes share a single broadcast link
the so known as multiple access problem.
Is the link layer incapable of
detecting an error in a received frame?
No, link layer is
capable of detecting an error in a received frame. Here we can see that how
Data link layer find out the error. Data link layer uses some error control
mechanism to ensure that frames are conveyed with certain level of accurateness.
But to understand how errors is controlled, it is vital to know what types of
errors may occur.
There may be three types
Single bit error
Multiple bits error
There are many reasons
for error such as noise, cross talk and many more which may help data to get
corrupted during transmission. The higher layers work on some sweeping view of system
architecture and are not aware of actual hardware data processing. So, higher
layers expect error-free communication between systems. Applications such as
voice and video may not be that affected and with some errors they may still
Moreover, Error control
device may involve two possible ways which is mention following:
Errors in the received
frames are detected by two ways means of Parity Check and Cyclic Redundancy
Check. We have to check in both situations, few extra bits are sent along with
actual data to confirm that bits received at other end are same as they were
sent. If the checks at receivers end fail, the bits are corrupted.
One extra bit is directed
along with the original bits to make number of 1s either even, in case of even
parity or odd, in case of odd parity.
The dispatcher while
creating a frame counts the number of 1s in it, for example, if even equality
is used and number of 1s is even then one bit with value 0 is added. This way number
of 1s remainders even. Or if the number of 1s is odd, to make it even a bit
with value 1 is added.
The receiver simply
counts the number of 1s in a frame. If the count of 1s is even and even parity
is used, the frame is considered to be not degraded and is accepted. If the
count of 1s is odd and odd parity is used, the frame is still not corrupted.
If a single bit flips
in transport, the receiver can detect it by counting the number of 1s. But when
more than one bit is in error it is very hard for the receiver to detect the
error. So this way we can check Error detection in Parity check.
Cyclic Redundancy Check
CRC is a different
approach to detect if the frame received contains legal data. This technique
involves binary division of the data bits being sent. The divisor is generated
using polynomials. The sender performs a division operation on the bits being
sent and calculates the remainder. Before transfer the actual bits, the sender
adds the remainder at the end of the actual bits. Actual data bits plus the
remainder is called a code word. The sender transmits data bits as code words.
In the digital world,
error correction can be done in two ways:
Backward Error Alteration: When the headset
identifies an error in the data received, it requirements back the sender to
retransmit the data unit.
Forward Error Alteration: When the
receiver identifies some error in the data received, it executes error modifying
code, which helps it to auto recover and to correct some kinds of errors.
So, this is way link
layer find detecting an error in a received frame.
3. Is flow control one of the services provided
by the link layer?
Yes, Flow control is provided by the link layer. The three major types
of facilities offered by data link layer are:
Ø Unacknowledged connectionless service.
Ø Acknowledged connectionless service.
Ø Acknowledged connection oriented
Ø Unacknowledged Connectionless Service:
type of service source machine sends frames to endpoint machine but the
destination machine does not send any credit of these frames back to the
source. Hence it is called unacknowledged service.
is no connection formation between source and destination machine before data
transfer or release after data transfer. Therefore it is known as
is no error control. For example, if any frame is lost due to noise on the
line, no effort is made to recover it.
type of service is used when error rate is low.
It is appropriate
for real time traffic such as speech.
Ø Acknowledged Connectionless Service:
service, neither the connection is established before the data transfer is it
released after the data transfer between source and destination.
the sender sends the data frames to destination, endpoint machine sends back
the acknowledgement of these frames.
type of service provides additional dependability because source machine
retransmit the frames if it does not receive the heading of these frames within
the specified time.
service is useful over defective channels, such as wireless systems.
Ø Acknowledged Connection Oriented Service:
service is the most refined service provided by data link layer to network
connection oriented. It means that connection is founding between source &
destination before any data is transferred.
service, data transfer has three distinct phases:-
(i) Connection establishment
(ii) Actual data transfer
(iii) Connection release
frame being spread from source to destination is given a exact number and is recognized
by the destination machine.
frames are received by destination in the same order in which they are sending
by the source.
What operations does the link layer
The Data Link Layer is accountable
for spreading data across a physical network link. Each physical medium has
link layer specifications for network and link layer protocol features such as
physical addressing, network topology, error notification, frame sequencing,
and flow control.
Physical addressing is
different from network addressing. Network addresses separate between nodes or
devices in a network, permitting traffic to be routed or switched through the
network. In contrast, physical addressing identifies devices at the link layer
level, differentiating between individual devices on the same physical medium.
The main procedure of physical addressing is the media access control address.
specifications identify how devices are linked in a network. Some media allow
devices to be connected by a bus topology, while others require a ring
The Data Link Layer
provides error notifications that alert higher layer protocols that an error
has occurred on the physical link. Instances of link level errors include the damage
of a signal, the loss of a clocking signal across serial connections, or the
loss of the remote endpoint on a T1 or T3 link.
The frame sequencing
capabilities of the Data Link Layer allow frames that are transmitted out of
sequence to be reordered on the receiving end of a transmission. The reliability
of the packet can then be verified by means of the bits in the Layer 2
header, which is transmitted along with the data payload.
Flow control within the
Data Link Layer allows receiving devices on a link to detect congestion and
notify their upstream and downstream neighbors. The neighbor devices communicate
the congestion information to their higher layer protocols so that the flow of
traffic can be altered or rerouted.
Data Link Sub layers:
The Data Link Layer is
divided into two sub layers: logical link control and media access control. The
LLC sub layer attains communications between devices over a lone link of a
network. This sub layer supports fields in link layer frames that enable
multiple higher layer protocols to share a single physical link.
On other hand, the MAC sub
layer administers protocol access to the physical network medium. Through the
MAC addresses that are typically assigned to all ports on a device, multiple
devices on the same physical link can uniquely identify one another at the Data
Link Layer. MAC addresses are used in addition to the network addresses that
are typically configured manually on ports within a network.
A MAC address is the
serial number forever stored in a device adapter to uniquely identify the
device. MAC addresses operate at the Data Link Layer, while IP addresses
operate at the Network Layer.
A brief description on the IEEE
(Institute of Electrical and Electronics Engineers) 802 standard development.
is an international association and organization of specialists working to the growth,
request and preservation of technology absorbed
products and services. IEEE is a public
organization founded in 1963. It works exclusively toward innovating, educating
and standardizing the electrical and electronic development industry.
mainly originates new automated products and services, designs the values that rule
them and communicates, publishes and promotes industry knowledge done
publications, meetings and partnering with academic institutes. The major areas
of focus for IEEE are electrical, electronics, computer engineering, processor
science, information technology and most of their related disciplines.
IEEE in calculating is widely
general for the development of standards for computer networking and its suite
of services. IEEE develops many different standards, such as IEEE 802 and IEEE 802.11
such as commonly known as Wi-Fi, and provides enduring innovation, alterations
and maintenance services for these standards. IEEE also preserves thousands of
student and expert chapters globally, has many focus societies and promoters
regular conferences and seminars. While the organization is US based, its
standards often become internationally accepted.
A brief description on Wi-Fi.
There are several standards of Wi-Fi with different speeds
and power requirements. The earliest standard was 802.11b, though it is the
slowest but least expensive in terms of cost. This was soon upgraded to 802.11a
and 802.11g. Both of these standards included technology for splitting the
radio signal and thus reducing intervention. The newest typical is 802.11n,
which lets even extended ranges and data throughput.
A Wi-Fi network may be public called a hotspot, for private
use by an organization or for home use. A network may even be citywide. For
example, the South African city of Cape Town has a Wi-Fi network in large parts
of the city. Paris, France is currently rolling out an ambitious high speed
citywide Wi-Fi network.
Wi-Fi was originally only used by organizations because of
its innovativeness and cost. But as with the lot electronic, the channel of
time means it became more reasonable to more users. Several tech practicality
home users now hook up their home Internet connections and device
communications using Wi-Fi. Most current computers are Wi-Fi allowed and comprise
built in hardware for connection to a Wi-Fi network.
Despite its many advantages, Wi-Fi is not without drawbacks.
First and foremost is the issue of security, since the network is widely
available, it is an open request for hackers. Hotspot users are at risk as
their computers are easily accessible to all types of hackers and identity
thieves. Second is the issue of network congestion. All Wi-Fi devices in one
area are communicating with one router, if the numbers of users are too large
or the users are communicating and receiving too much data, the router gets overcome
and cannot transmit data fast enough. This can be resolved by adding more
routers, which adds to the expenses.
How Wi-Fi Networks Works:
Wi-Fi networks have no
physical wired connection between sender and receiver by using radio frequency
technology a frequency within the electromagnetic spectrum associated with
radio wave spread. When an RF present is comprehensive to a projection, an
electromagnetic field is shaped that then is able to mix through space.
of any wireless network is an access point. The primary job of an access point
is to broadcast a wireless signal that computers can detect and tune into.
In order to connect to an access point and join a wireless network, computers
and devices must be equipped with wireless network adapters.
A brief description on the link layer
The data link layer is separated
into two sub layers: Logical Link Control and Media Access Control. The LLC sub
layer manages communications between devices over a single link of a network.
This sub layer provisions fields in link-layer frames that allow multiple
higher layer procedures to share a single physical link.
The MAC sub layer oversees
protocol access to the physical network medium. Over the MAC statements that
are classically assigned to all ports on a router, numerous devices on the same
physical link can uniquely identify one another at the data link layer.
LLC and MAC:
The data link layer
functionality is usually fragmented it into logical sub-layers, the upper
sub-layer, termed as LLC, that interacts with the network layer above and the
lower sub-layer, called as MAC, that interrelates with the physical layer
below, as exposed in the diagram given below:
While LLC is accountable
for handling multiple Layer3 protocols and link services like consistency and
flow control, the MAC is responsible for framing and media access control for
broadcast media. The hands-on indication of LLC and MAC sub-layers are given in
the diagram below:
The MAC sub-layer
interacts with the physical layer and is primarily accountable for framing or de-framing
and accident resolution.
On the distribution
side, the MAC sub-layer is accountable for creation of frames from network
layer packages, by totaling the frame header and the frame trailer. While the
frame header contains of layer2 reports and a few other fields for control drives,
the frame trailer consists of the checksum of the whole frame. After generating
a frame, the MAC layer is accountable for cooperating with the physical layer
processor to transmit the frame.
On the receiving side,
the MAC sub-layer receives frames from the PHY and is accountable for accepting
each frame, by examining the frame header. It is also responsible for verifying
the checksum to accomplish whether the frame has come unspoiled through the
link without bit errors.
8. A brief description on Network Interface Cards.
is short for network
interface card. Most computers have them made in but you can also improve
your own NIC to expand the functionality of the system. The NIC is what
provides the hardware interface
between a computer and a network.
cards that join over USB are
not actually cards but in its place regular USB devices that enable network
connections through the USB port.
These are called network
Does a NIC Do?
network interface card enables a device to network with other strategies. This
is accurate whether the plans are connected to a central network or even if
they’re paired together, directly from one device to the other.
a NIC isn’t always the only component needed to interface with other devices. For
instance, if the device is part of a larger network and you want it to have
access to the internet, like at home or in a business, a router is
required too. The device, then, uses the network interface card to join to the
router, which is connected to the internet.
cards come in many different systems but the two main ones are wired and
NICs need to use wireless technologies to access the network, so they have one
or more antennas stabbing out of the card. Wired NICs fair use an RJ45 port
since they have an Ethernet cable attached to the end. This makes them much
flatter than wireless network cards.
matter which is used, the NIC projects from the back of the computer next to
the other plugs, like for the monitor.
Types of data link protocols.
SDLC: This protocol was originally
developed by IBM as part of IBM’s SNA. It was used to connect remote devices to
processer computers at central locations in either point to point connection.
HDLC: This protocol is based on SDLC
and provides both a best effort untrustworthy service and a reliable service.
It is used with many serial interface protocols distinct in the physical layer.
SLIP is a data link control facility for communicating IP
packets, usually between an ISP and a home user over a dial up link. Slip has
some borders, including a lack of any error finding and correction mechanisms.
It is up to higher layer protocols to perform these checks. Used over much of
the same serial borders as HDLC.
PPP delivers the same functionality as slipup but it is a
more strong protocol that can transference not only IP, but also other types of
packets. Frames hold a field that identifies the type of protocol being approved.
LAP has constancy service structures and comes in three variations.
LAP Balanced is a protocol that provides point to point networks on packet switched
networks. LAPD provides the data link control over the D channel of an Integrated
Services Digital Network connection.
LAN Data Link Controls:
The actual data link layer is divided into
two sub layers, called the Medium Access Control sub layer and the Logical Link
Control sub layer. The lower MAC layer defines the media access method, which
can be collision detection, token ring, or other IEEE physical interface. The
LLC sub layer provides a way for the network layer to communicate with one of
Data link protocols may provide any of the following
Framing: Data is broken up into frames that
are communicated as sovereign units. If errors are detected in a frame, it is
only necessary to retransmit that frame.
Session setup and finish: For consistent services,
session control messages are used by end systems to exchange status information
about the session.
Error uncovering: Controls whether a frame has
been transported exactly. A checksum is calculated on a frame by the sender,
and the receiver must perform the same calculation and originate up with the
same checksum. If not, the frame is considered corrupted. For no reliable
services, the frame is released and upper layer protocols are relied on to
handle the problem.
Flow control: A technique that averts the
sender from sending more, overflowing the receiver with more data than it can
Media Access Control design and Cyclic Redundancy
Check error detection scheme.
access control is a sub layer of the data link layer in the seven layer OSI
network position model. MAC is accountable for the transmission of data packets
to and from the network-interface card, and to and from another remotely shared
The architectural model
described in Division 1 is used in this section to provide a functional
description of the LAN sub layer. The MAC sub layer defines a medium autonomous
facility, built on the medium needy physical facility provided by the Physical
Layer. It is relevant to a general class of local area broadcast media suitable
for use with the media access correction known as Carrier Sense Multiple Access
with Collision Detection. The LLC sub layer and the MAC sub layer composed are
intended to have the same function as that described in the OSI model for the
Data Link Layer alone.
In a broadcast network,
the idea of a data link between two network entities does not correspond
directly to a separate physical connection. They are as follows:
a) Data encapsulation
b) Media Access Management
Cyclic Redundancy Check:
An error detection mechanism in which a special number is attached
to a block of data in order to detect any changes introduced during storage. For
sample, a single dishonored bit in the data results in a one bit change in the designed
CRC, but multiple corrupt bits may cancel each other out.
Necessities of CRC:
A CRC will be valid if
and only if it contents the following supplies:
should have just one less bit than divisor.
the CRC to the end of the data unit should result in the bit sequence which is
exactly divisible by the divisor.
The many stages
followed in the CRC method are
A thread of n as is joined to the data
unit. The length of predetermined divisor is n+ 1.
The again formed data unit for instance creative data string of
n as are separated by the divisor using binary division and remainder is
obtained. This remainder is called CRC.
Now, string of n Os attached to data
unit is replaced by the CRC remainder.
The data unit + CRC are then communicated
The receiver on in receipt of it divides
data unit + CRC by the same divisor & checks the remainder.
If the remainder of division is zero,
receiver adopts that there is no error in data and it accepts it.
If residue is non-zero then there is an
error in data and handset rejects it.
Margolies, “Resource allocation for the Internet of Everything: from energy
harvesting tags to cellular networks,” Ph.D. Thesis, Columbia University, 2015
G. D. Celik, G. Zussman, W. F. Khan, and E.
Modiano, “MAC for networks with multipack reception capability and spatially
distributed nodes,” in Proc. IEEE