In the model of the real time communication, end users of the message application systems as source and destination residing in different host. The network interface of each host contains input queue and output queue.
Two buffer area called as input / output buffer are allocated to input and output queue to store queuing information. The queue are jointly maintained by two local servers as Transport Protocol Handler (TPH) and Network Access Control Handler (NACH).
The former interfaces with local application and provides them with message transport service. The next interface with the network below and provides network access and message transmission services to the TPH. The client- server architecture produce more delay such that it is not suitable communication network architecture for real time communication.
The fig. above shows the data path follow to transfer the message in between two hosts. Two marked circle TPH and NACH are transport protocol handler and network access control handler. When requested to send message by a local application task, the source TPH places the massage in output queue. Each outgoing message is delivered to network under the control of source NACH.
After the placement of message on network, the NACH of destination host place it on input queue of destination and notify the destination IPH. The destination IPH then moves the message to address space of the destination application task and notify the destination task of arrival of message.
Here the end to end model by chain of jobs is used to represent the message sending activity. The application task available on source and destination are modeled as the predecessor as well as successor of the chain. At the beginning and end of the chain are source and destination chain are the transport protocol processing job. In between them, each job that access the network or transmits the message becomes ready for execution after its predecessor completes. Ideally the network delivers messages to receiver with no delay. In reality there is some of the delay are associated with queuing delay at sender, network transmit time and queuing delay at receiver as well as network.
- Network is not always ready to accept a packet when it becomes available and data may be queued if produced faster than the network can deliver it such that Queuing delay arise at sender.
- Application task are not always ready to accept packets arriving from network and Network may deliver data in bursts form such that Queuing delay occurs at receiver.
- Due to cross-traffic or bottleneck links such that Queuing delay occurs in the network
- Network transit time also generates the delay.
Hence a synchronization protocol is needed to model the real time communication model.
1. Real Time Traffic Model
The real time traffic means isochronous or synchronous traffic, consisting stream of message that are generated by their sources and delivered to their respective destination on continuous basis. The traffic includes the periodic, aperiodic and sporadic messages. The periodic and sporadic message are synchronous in nature and there requirement of guarantee of on time delivery whereas aperiodic message are asynchronous in nature and shows the soft timing constraint.
In real time traffic model, each message (Mi) be characterized by tuples of inter-packet spacing (Pi), message length (ei), reception deadline (Di) as below.
Mi = (pi , ei , Di )
This traffic model is called peak rate model in real time communication.
2. Throughput, Delay and Jitter
- The throughput is the measure of the numbers of packets or message stream that the network can deliver per unit time.
- The delay (latency) is time taken to deliver the packet or message It is fixed due to minimum propagation delay due to speed of light and varying due to queuing on path.
- The term jitter indicates variance on
- Many real time communication protocol and algorithms are designed to keep not only the worst case delay and jitter as small.
Real Time Connections and Service Disciplines
The connection oriented approach is used for real time traffic. According to this approach, a simplex logical connection is set up for transmission of each message stream between source and destination. The fixed routing is used to route the packets and chosen route is fixed until torn down of system or invocation of adaptation mechanism. The new connection is established if the existing network meets quality of service parameter like end to end delay, jitter etc. Generally packet switching network is preferred for transmission of message streams.
- Fig below shows the packet switching network along with multi hop switch for multi hop network and circle of the network represents a switch.
- Switches are buffered e. there is a buffer pool for each output links, holding the packets that are queued for transmission on the link. Once the switch route the packets to the queue , the packets waits in the queue until schedulaer schedules it for transmission and then it is transmitted to next hop at the other end of the output link.
- The amount of time the switch takes to route the packets is small but the time a packets takes passing trough a swith is equal to its output time at output queue plus packet transmission time and this is called as hop delay o the packet.
- The end to end delay of each packet trough a switched network is equal to the sum of the per hope delays it suffers passing through all switches in the route plus total time it takes to propagate along the all links between the switches.
- The combination of acceptance test and admission control protocol , a synchronization protocol and a scheduling algorithms used for the purpose of rate control (jitter) and scheduling of packets transmission is called a service discipline.
- Service disciplines are divided into two categories as rate allocating and rate.
- Rate allocating disciplines allows the packets on each connection to be transmitted at higher rates than guaranteed rate.
- A service disciplines is said to rate controlled if it each connection guaranteed rate but never allows packet to send above guaranteed rate.