Wide Area Networks: Characteristics, Technologies, and Protocols
Wide Area Networks
5: Wide Area Networks. Characteristics:
- Circuit Switching (CC): Involves the existence of a path between two stations, which is a sequence of links connected between the network nodes (switches) to allow information transfer.
- Packet Switching (CP): The information is chopped into small blocks of information.
Different Technologies: CC and CP. They differ in that switch nodes link the information between communication points.
Circuit Switching (CC)
Involves the existence of a path between two stations, which is a sequence of links connected between the network nodes. It has 3 phases: establishment of the circuit, data transfer, and disconnection of the circuit. The use of this technique is exclusive because it is inefficient. There is a delay because of the call transfer. It is used mainly for voice traffic and has no risk of data loss or congestion.
Packet Switching (CP)
The information is chopped into small blocks of information. Here, resources are not reserved. Each packet contains a data part of the information and a control part, which is what the network needs for routing. The delay is variable. There is a risk of congestion. Advantages of CP over CC: higher efficiency, possibility of conversion in data speed, no calls when traffic increases, and the ability to use priorities.
How to manage approaches on the network sequence of these packages:
Datagram
Each packet is independent. Each one forms its destination. There is no call establishment phase. If sending few packets quickly, the datagram will be more prone to failure on the switch. There is more flexibility in intermediate control failure. Control of congestion is difficult, and the receipt is not guaranteed in order or without duplicates. Service without connection, no errors, and no warranty control of delivery. Good for short transmissions.
Virtual Circuits (VC)
A route of packets is established previously. The service is connection-oriented; there is an exchange to perform a call. If data is sent for a long period, the VC offers a sequential service and secures control of ordered reception without errors or duplicates. The control of congestion is simpler, and if there is a failure, there is an intermediate link that switches the connection. The addressing is restarted through the CV identifier.
Routing
For a packet to reach its destination, tables are created that relate input information with outputs. This can be created in two places:
- Distributed: The routing decision is taken by each node locally, using information such as the cost associated with the different output links and the information of adjacent nodes.
- Centralized: A central node uses information from all nodes.
When creating: static (road establishment initializing the network), dynamic (with information updates over time).
Congestion
Jams are created in the excess of switches due to traffic. Due to congestion, delays increase, and information is lost.
Frame Relay (FR): Retransmission by frames. It operates efficiently with an error rate of up to 2Mbps. It detects but does not correct. Variable plot size 8-250B. It is a connection-oriented service. It is reliable, and the probability of lost frames is small. Protocol of the 2nd level: line control protocol (IMP). Standards admitted: switched virtual circuits (SVC) and permanent virtual circuits (PVC).
ATM: It is an evolution of FR. Long packets are used, physically called standard cells. It is for fast packet switching (ISDN). Ideal for voice and data transmission. Characteristics: connection-oriented service. Its cells are small. There are almost negligible delays. It is hardware switching, which is faster than software.
ATM Adaptation Layer (AAL)
Adapts the user information and formats it for ATM. It segments the data to send it and reconstructs it at a higher level. Adjusting the speed of generation and absorption of the user speed allows buffering. Recovery of cells, detection of erroneous and missed cells from end to end.
ATM Level
The cell is in charge of reaching its destination in order and without errors. It constructs or functions headers to send the information. To expedite the switching, it uses virtual channels (between two end-users) or virtual paths (grouping the VC). Within each plot, there is an ATM virtual circuit identifier.
Form Level
This is divided into two levels: PMD (depends on the MDIO level, just below the physical level. The function is to pick up and put the bits in the MDIO) and CT (convergence transmission, level accepts the ATM cells and prepares their shipment, also bits comprising cells to be received pass the ATM level).