Data link layer tutorial

Introduction
Data Link Layer is second layer of OSI Layered Model. This layer is one of the most complicated layers
and has complex functionalities and liabilities. Data link layers hides the details of underlying hardware
and represents itself to upper layer as the medium to communicate.
Data link layer works between two hosts which are directly connected in some sense. This direct
connection could be point to point or broadcast. Systems on broadcast network are said to be on same
link. The work of data link layer tends to get more complex when it is dealing with multiple hosts on
single collision domain.
Data link layer is responsible for converting data stream to signals bit by bit and to send that over the
underlying hardware. At the receiving end, Data link layer picks up data from hardware which are in the
form of electrical signals, assembles them in a recognizable frame format, and hands over to upper
layer.
Data link layer has two sub-layers:
· Logical Link Control: Deals with protocols, flow-control and error control
· Media Access Control: Deals with actual control of media
Functionality of Data-link Layer
Data link layer does many tasks on behalf of upper layer. These are:
· Framing:
Data-link layer takes packets from Network Layer and encapsulates them into Frames. Then, sends
each Frame bit-by-bit on the hardware. At receiver’s end Data link layer picks up signals from hardware
and assembles them into frames.
· Addressing:
Data-link layer provides layer-2 hardware addressing mechanism. Hardware address is assumed to be
unique on the link. It is encoded into hardware at the time of manufacturing.
· Synchronization:
When data frames are sent on the link, both machines must be synchronized in order to transfer to take
place.
· Error Control:
Sometimes signals may have encountered problem in transition and bits are flipped. These error are
detected and attempted to recover actual data bits. It also provides error reporting mechanism to the
sender.
· Flow Control:
Stations on same link may have different speed or capacity. Data-link layer ensures flow control that
enables both machine to exchange data on same speed.
· Multi-Access:
Hosts on shared link when tries to transfer data, has great probability of collision. Data-link layer
provides mechanism like CSMA/CD to equip capability of accessing a shared media among multiple
Systems

Introduction
There are many reasons such as noise, cross-talk etc., which may help data to get corrupted during
transmission. The upper layers works on some generalized view of network architecture and are not
aware of actual hardware data processing. So, upper layers expect error-free transmission between
systems. Most of the applications would not function expectedly if they receives erroneous data.
Applications such as voice and video may not be that affected and with some errors they may still
function well.
Data-link layer uses some error control mechanism to ensure that frames (data bit streams) are
transmitted with certain level of accuracy. But to understand how errors is controlled, it is essential to
know what types of errors may occur.
There may be three types of errors:
· Single bit error:
[Image: Single bit
error]
In a frame, there is only one bit, anywhere though, which is corrupt.
· Multiple bits error:
[Image: Multiple
bits error]
Frame is received with more than one bits in corrupted state.
· Burst error:
[Image: Burst
error]
Frame contains more than1 consecutive bits corrupted.
Error control mechanism may involve two possible ways:
· Error detection
· Error correction
Error Detection
Errors in the received frames are detected by means of Parity Check and CRC (Cyclic Redundancy
Check). In both scenario, 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 receiver’s end fails, the bits are corrupted.
PARITY CHECK
One extra bit is sent 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 sender while creating a frame counts the number of 1s in it, for example, if even parity is used and
number of 1s is even then one bit with value 0 is added. This way number of 1s remain even. Or if the
number of 1s is odd, to make it even a bit with value 1 is added.
[Image: Even
Parity]
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-corrupted 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 transit, the receiver can detect it by counting the number of 1s. But when more than
one bits are in error it is very hard for the receiver to detect the error
CYCLIC REDUNDANCY CHECK
CRC is a different approach to detect if the frame received contains valid 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 sending the
actual bits, the sender adds the remainder at the end of the actual bits. Actual data bits plus the
remainder is called a codeword. The sender transmits data bits as codewords.
[I
mage: CRC in action]
At the other end, the receiver performs division operation on codewords using the same CRC divisor. If
the remainder contains all zeros the data bits are accepted, otherwise there has been some data
corruption occurred in transit.
Error Correction
In digital world, error correction can be done in two ways:
· Backward Error Correction: When the receiver detects an error in the data received, it
requests back the sender to retransmit the data unit.

· Forward Error Correction: When the receiver detects some error in the data received, it uses
an error-correcting code, which helps it to auto-recover and correct some kinds of errors.
The first one, Backward Error Correction, is simple and can only be efficiently used where retransmitting
is not expensive, for example fiber optics. But in case of wireless transmission retransmitting may cost
too much. In the latter case, Forward Error Correction is used.
To correct the error in data frame, the receiver must know which bit (location of the bit in the frame) is
corrupted. To locate the bit in error, redundant bits are used as parity bits for error detection. If for
example, we take ASCII words (7 bits data), then there could be 8 kind of information we need. Up to
seven information to tell us which bit is in error and one more to tell that there is no error.
For m data bits, r redundant bits are used. r bits can provide 2r combinations of information. In m+r bit
codeword, there is possibility that the r bits themselves may get corrupted. So the number of r bits used
must inform about m+r bit locations plus no-error information, i.e. m+r+1.
Introductioin
Data-link layer is responsible for implementation of point-to-point flow and error control mechanism.
Flow Control
When data frames (Layer-2 data) is sent from one host to another over a single medium, it is required
that the sender and receiver should work on same speed. That is, sender sends at a speed on which the
receiver can process and accept the data. What if the speed (hardware/software) of the sender or
receiver differs? If sender is sending too fast the receiver may be overloaded (swamped) and data may
loss.
Two types of mechanism can be deployed in the scenario to control the flow:
· Stop and Wait
This flow control mechanism forces the sender after transmitting a data frame to stop and wait until the
acknowledgement of the data-frame sent is received.
[Image: Stop and Wait Protocol]
· Sliding Window

In this flow control mechanism both sender and receiver agrees on the number of data-frames after
which the acknowledgement should be sent. As we have seen, stop and wait flow control mechanism
wastes resources, this protocol tries to make use of underlying resources as much as possible.
Error Control
When data-frame is transmitted there are probabilities that data-frame may be lost in the transit or it is
received corrupted. In both scenarios, the receiver does not receive the correct data-frame and sender
does not know anything about any loss. In these types of cases, both sender and receiver are equipped
with some protocols which helps them to detect transit errors like data-frame lost. So, either the sender
retransmits the data-frame or the receiver may request to repeat the previous data-frame.
Requirements for error control mechanism:
· Error detection: The sender and receiver, either both or any, must ascertain that there’s been
some error on transit.
· Positive ACK: When the receiver receives a correct frame, it should acknowledge it.
· Negative ACK: When the receiver receives a damaged frame or a duplicate frame, it sends a
NACK back to the sender and the sender must retransmit the correct frame.
· Retransmission: The sender maintains a clock and sets a timeout period. If an
acknowledgement of a data-frame previously transmitted does not arrive in the timeout period, the
sender retransmit the frame, thinking that the frame or it’s acknowledge is lost in transit.
There are three types of techniques available which Data-link layer may deploy to control the errors by
Automatic Repeat Requests (ARQ):
· STOP-AND-WAIT ARQ

[Image: Stop and Wait ARQ]
The following transition may occur in Stop-and-Wait ARQ:
o The sender maintains a timeout counter.
o When a frame is sent the sender starts the timeout counter.
o If acknowledgement of frame comes in time, the sender transmits the next frame in
queue.
o If acknowledgement does not come in time, the sender assumes that either the frame
or its acknowledgement is lost in transit. Sender retransmits the frame and starts the timeout counter.
o If a negative acknowledgement is received, the sender retransmits the frame.
· GO-BACK-N ARQ
Stop and wait ARQ mechanism does not utilize the resources at their best. For the acknowledgement is
received, the sender sits idle and does nothing. In Go-Back-N ARQ method, both sender and receiver
maintains a window.

[Image: Go-Back-N ARQ]
The sending-window size enables the sender to send multiple frames without receiving the
acknowledgement of the previous ones. The receiving-window enables the receiver to receive multiple
frames and acknowledge them. The receiver keeps track of incoming frame’s sequence number.
When the sender sends all the frames in window, it checks up to what sequence number it has received
positive ACK. If all frames are positively acknowledged, the sender sends next set of frames. If sender
finds that it has received NACK or has not receive any ACK for a particular frame, it retransmits all the
frames after which it does not receive any positive ACK.
· SELECTIVE REPEAT ARQ
In Go-back-N ARQ, it is assumed that the receiver does not have any buffer space for its window size
and has to process each frame as it comes. This enforces the sender to retransmit all the frames which
are not acknowledged.

[Image: Selective Repeat
ARQ]
In Selective-Repeat ARQ, the receiver while keeping track of sequence numbers, buffers the frames in
memory and sends NACK for only frame which is missing or damaged.
The sender in this case, sends only packet for which NACK is received.

Data link layer tutorial

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