Transport and Data Link Layers of the OSI Model

This is the fourth layer of the Open Systems Interconnection (OSI) Model, which is necessary and responsible for the overall end-to-end, logical communication between network applications. This layer encompasses the following functions:

• Service-point addressing: it provides a mechanism for multiplexing of the upper-layer applications; enabling multiple applications in a single device to communicate over the network on a single physical link at the same time.
• Connection control: it is responsible for establishing, maintaining, and terminating virtual circuits between two communicating hosts in connection-oriented communication, ensuring that all the data is received in correct order by the correct application. This is by sending an acknowledgement for received data.
• Segmentation and re-assembly: this involves breaking data units into smaller segments for transmission; each datagram contains a sequence number that enables re-assembly of the data to the original format.
• Error Control: it ensures the delivery of error-free data; and in delivery is in proper sequence. Error checking involves detecting errors that may have occurred during transmission, whereas error recovery is concerned with the retransmission request of the same data to resolve any errors previously detected.
• Flow control: this manages data transmission between communicating hosts to ensure that any transmitting device never overwhelms the receiving device by sending more data than the recipient can process.

The Transmission Control Protocol and the User Datagram Protocol of the Internet Protocol Suite; and the Sequenced Packet Exchange of the Internetwork Packet Exchange protocol suite (IPX) are the common categories of the transport layer protocols.

Devices at the transport layer include personal computers, workstations and servers.

Data Link Layer

This is the second layer of the Open Systems Interconnection model; and is divided into two layers: the Logical Link Control, which is responsible for managing communication between two devices in a network over a single link; and the Media Access Control which is responsible for protocol access management at the physical network medium. MAC addresses are used in a network to uniquely identify devices at the data link layer. This layer is responsible for the following functions:

• Physical addressing: this defines how network devices are addressed for unique identification with each other at the data link layer. This is usually by adding a header to a frame defining a sender and receiver.
• Framing: this is by dividing the data packets received from the network layer into frames.
• Network topology definition: this consists of the data link layer specifications that determine how devices are physically connected; for instance, a bus or ring topology.
• Error control: this layer has a mechanism for error detection, retransmission of lost packets, and notification. Error notification involves sending alerts to the upper-layer protocols notifying them that an error has occurred during data transmission.
• Flow control and sequencing: this encompasses moderating data transmission to ensure that the receiving device only receives the amount of traffic it can accommodate, and is not overwhelmed with traffic it can accommodate at once. Sequencing involves the reordering of frames that have not been transmitted in proper sequence.

The protocol suite for the data link layer includes: Ethernet, Logical Link Control (LLC), Token Ring, High-Level Data Link Control, Fiber Distributed Data Interface, Point-to-Point Protocol, Synchronous Data Link Control, Sub-Network Access Protocol (SNAP), Frame Relay, and Serial Line Interface Protocol (SLIP).

Switches and bridges are the network components of this layer.