Data Link Layer and Transport Layer: Functions and Importance in Networking and Cloud Computing

Data link layer
This is the second layer in the OSI model and governs the transfer of data
between two nodes on same network. The data is transferred on basis of
Physical ID of the devices. As per OSI standard, the layer itself is sandwiched
between Physical layer and Network Layer. Physical layer deals with the
underlying medium of the network. Network layer manages the routing of data
across networks. Data link layer manages data packet between these two
layers.
The data link layer is used for the encoding, unraveling and predictable
relationship of data bits. Data bundles are handled by this layer. The layer itself
is divided into two sublayers. The data link layer's first sublayer is the Media
Access control (MAC) layer. It holds a 48 bit ID which is used for unique
identification of a node with in a network. The MAC layer allows the data link
layer to identify itself on the same network. All data packets routed on
network are directed based on this MAC address.
The data link layer's second sublayer is the logical link control. It manages two
way communication of data stream from higher layers.
Some important functions of Data link layer are given below:

 Maintain bit rate - bits should be passed on at rates not to overwhelm the
receiver. This is done by making use of suitable buffers and use of
acknowledgments between communicating parties.
 Data link layer is to pass on the data in and out from network layer. The bit
by bit data received at the input must be framed as a data packet and
transferred to the upper layers.
 Data link layer transforms data packet in to data bits. These data bits are
then encoded in physical layer for transmission of the data. Reverse is also
done when getting the data from physical layer. The bit stream is converted
into data packet and forwarded.
 Keep track of any errors in bit values by making using of CRC bits in data
packets and perform CRC checks. Due to noise in physical channels, some
bits received might be in error. The various parity and redundancy bits are
used to identify single bit or burst errors. The measures are then taken
accordingly to correct the error or request retransmission of packet.
 Link Configuration Control: The layer must also handle the mode of
transmission. Some network media may support full duplex mode, others
may only support half duplex or simplex modes of transmission. Not all
channels full two way communication. This must be taken care of by the
layer and data must only be transmitted when media is available for
transmission.

 Framing of packet must also be done by the layer adding the MAC address in
the header for other devices to recognize and accept the packet at the
matching node. This might need use of ARP from higher layers to resolve
MAC addresses of receiving stations. This is done by sending broadcast
messages on network and processing an ARP reply received by the node
responsible for providing the MAC id.
 Packet Synchronization: The packets must be framed with synchronization
bits on each side to allow the receiving terminals detect start and end of the
packet on media. This is important for physical media to be able to recognize
the frame boundaries.
 Packet Identification: Data Link layer also needs to decode the packets
received on its input and match the destination MAC address in the packet
with its own MAC address. The packet should be buffered and processed if
the address matches and forwarded to upper layers for further processing.
 Carrier Sense and Media Access: Physical layer only converts the signals into
streams of bits. It makes no decision based on values of those bits. It’s up to
the data link layer to make sense out of it. The layer must decide if the
carrier is occupied or free at any given moment before it starts transmission.
There are two methodologies used for carrier access, one is based on
transmission and waiting for collision to occur called as CSMA/CD (Carrier
sense Multiple Access/ Collision Detection) and other one senses carrier

before transmission and tries to avoid collision, called as CSMA/CA (Carrier
Sense Multiple Access/Collision Avoidance).
CSMA/CD is usually preferred for Wired Networks, where each node
transmits the data and senses the media at the same time. If there is
transmission by more than one node at the same time, both the nodes cease
to transmit data, wait for random amount of time and transmit again.
CSMA/CA is usually preferred for wireless networks, since two nodes might
not be in range of each other but in range of third device. In such a case
collision will be hard to detect by transmitting stations. So transmitting
stations first send a request to reserve a channel before transmission. Only if
the channel is free, the node starts transmission. And relinquishes the
channel after it is done transmitting.
All these functions are carried out by the data link layer for every single
packet that passes through the layer either upstream or downstream. This
way the layer keeps the node connected in a network.