The transport layer provides a mechanism for the exchange of data between end systems. It optimizes the use of network services with providing a requested quality of service to session entities. The size and complexity of a transport protocol depend on how reliable or unreliable the undelying network and network layer services are. Essentially, this layer is responsible for the reliable data transfer between two end nodes and is sometimes referred to as host-to-host layer.
The transport layer decomposes messages into packets and combines packets into messages, possibly after resequencing them. This is required, because the size of the message may be larger than the size of the packets accepted by the network layer. To perform this packetization function, the transport layer numbers the packets belonging to the same message. This process is called segmentation and reassembly.
The transport layer controls the flow of packets to prevent the source from sending packets faster than the destination can handle them. Same function takes place in data link layer also. But here, the flow control is performed end to end rather than across a single link.
The transport layer delivery of messages is either connection oriented or connectionless. A connection oriented transport layer deliveres error free messages in the correct order. Such a transport layer provides the following services. CONNECT, DATA and DISCONNECT. A connectionless transport layer deliveres messages one by one possibly with errors with no guaranteed order of the messages. The services in UNIT-DATA, the connectionless delivery of single message.
Other processes at transport layer are :
1. Error detection and recovery to minimize data loss and timeloss due to retransmission of frames.
2. Sets quality of service (QOS) for network layer packets to assure end-to-end message integrity.
3. A process called blocking is employed when requested by sending station. Blocking is used to prevent a specific frames to reach a particular node.
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