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OSI Networking Model Layer 4

 

4 - TRANSPORT - End-to-end connections and reliability (flow control & error-handling) (TCP)

 

Function: Responsible for end-to-end data transmission integrity. Makes sure that the data actually gets there, with no errors, in the right order, regardless of transmission problems.

 

Data unit: Segments & Packets.

 

Protocols (examples): TCP, UDP, SPX, ARP, RARP, NWLink, NetBIOS / NetBEUI, ATP.

 

Components: Gateway, Advanced Cable Tester, Brouter.

 

Topics & Methods Explained


Address/name resolution

Service-requestor-initiated:
Each requestor sends out a special packet. This packet requests that entities that correspond to a given name, address, or service type respond by providing their name or address.

 

Service-provider-initiated:
Every service provider routinely sends out a broadcast packet announcing its availability. These packets include name and address information that can be collected by other entities or by special directory servers.

Addressing

Connection identifier:
(Also called Connection ID, port, or socket) identifies each conversation. Using the connection ID, a service provider, such as a print server can concurrently communicate with more than one client.

 

Transaction identifier:
Transaction identifiers are similar to connection identifiers, but they refer to a unit (transaction, which is composed of a request and response) that is smaller than a conversation.

Segment development

Division and combination:
Messages that are too large for the network and lower layers to handle are divided, and messages that are smaller than the acceptable size are combined into one combination segment to reduce network overhead.

Connection services

Segment sequencing:
A message that is divided into segments, which can arrive at the destination out of order, is rebuilt by shuffling the segments into their correct order.

 

Error control:
Protects against errors that occur when a data segment is lost or arrives with the same segment sequence number as a totally different segment, and against errors occurring from corrupt segments.

 

End-to-end flow control:
Just the sender and receiver are involved in recovering from lost or delayed packets on the internetwork. End-to-end flow control is performed using acknowledgments.

 

 

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