Connection oriented service

 CONNECTION ORIENTED SERVICE :-

The connection - oriented service establishes a logical connection between source destination machines before any data are transferred that provides error control and flow control. 

Each frame sent over connection is numbered, and the data link layer guarantee that each frame sent indeed received. 

Furthermore, it guarantees that each frame is recorded exactly once and that all frames are received in the right order. 

When connection - oriented service is used, go through three distinct phases. 

In the first phase, the connection is established by having both sides initialize variables and counters needed to keep track of which frames have been received and which ones have not. 

In the second phase, one or more frames are actually transmitted. 

In the third and final phase, the connection is released, freeing up the variables, buffers and other resources used to maintain the connection. 

The used error control mechanism is the ARQ (Automatic Repeat Request), ARQ treats with two different error :Lost PDU and Damage PDU. The general performance of ARQ is :

The transmitter sends the frame. 

When the frame is in the receiver, the station checks whether there are any errors in the frame using a cyclic redundancy check (CRC). 

The transmitter will retransmit the frame if receive a negative ACK or does 't receive any ACK. 

* Consider a typical example :- 

A WAN subnet consisting of routers connected by point - to - point leased telephone lines. When a frame arrives at a router, the hardware checks it for errors, then passe the frame to the data link layer software. The data link layer software checks to see if this is the frame expected, and if so gives the packet contained in the payload field to routing software. 

Software, which then transmits it. 

The routing code frequently wants the job done right, that is with reliable, sequenced connections on each of the point - to - point lines. 

It does not want to be bothered too often with packets that got lost on the way. 

It is up to the data link Protocol, shown in the dotted rectangle, to make unreliable communication lines look perfect or, at least, fairly good. 

* Connection - oriented network also support two types of connections :

1. Point - to Point (private) connections refer to a pathway from one location to another (Node A to Node B). 

An example is a mainframe terminal connected to a mainframe front - end - processor. Point - to point networks have many connections between individual pairs machines. 

2. Point - to - Multipoint (conference) connections refer to a pathway from one location to many locations (Node A to Note B, C, and D). 

Years age, Multipoint links were used to connect multiple terminals to a mainframe front - end processor. 

In today 's LAN environment, Multipoint links are used to connect multiple devices in bus, tree or star topologies. 

Point - to - point links are different than Multipoint because it implies dedicated bandwidth. 

Because the bandwidth is dedicated in a point to point environment, no addressing is needed. 

In a Multipoint link the channel (the communication path) is shared which complicates communication and the efficiency of using the channel.