Network Architecture

The initial computer networks were designed with the hardware as the primary concern and the software as the secondary concern.

However, today, network software is highly structured. For reliable and error-free communication of information between two computers over a network, the enabling software running on both computers must follow a set of well defined rules and conventions (called Protocols).

In the earlier computer network implementations, protocols were non-standard. Each computer manufacturer or a closed group of network users developed their own private protocols for communication. Such implementations were closed system implementations, because they only allowed communication amongst hardware/ software developed by the manufacturer or the closed user community. They did not allow integration with hardware/ software developed by others.

So, the need for a standard set of protocols was felt so as to enable all kinds of computers with hardware and software from any manufacturer/ vendor to talk to each other over a network.

Hence, the International Standards Organization (ISO) has developed a standard model known as Open System Interconnection (OSI) reference model. The idea behind this standardized model is to provide for standardized procedure for exchange of information among terminal devices, computer, people, network, processes, etc.

Further, the design of such network communication software is rather complex as the software needs to take care of several issues that arise when two processes communicate across process boundaries.

Some of the important issues are:

• Data Signal types
• Data encoding techniques
• Multiplexing of signals
• Error detection and control
• Switching techniques
• Network Topology
• Medium Access control techniques
• Addressing
• Naming
• Routing
• Flow control
• Congestion control
• Security aspects

To reduce their design complexity, most networks are organized as a series of layers or levels, each one built upon the one below it.

The number of layers, the name of each layer, and the function of each layer differ from network to network. However, in all networks, the purpose of each layer is to offer certain services to the higher layers, thereby shielding the higher layers from the details of how the services are actually implemented by this layer and its lower layers.

Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as the layer n protocol.

Basically, a protocol is an agreement between the communicating parties on how the communication is to proceed. Violating the protocol will make communication more difficult, if not impossible.

A four-layer network is illustrated below:

Some terms to understand

1. Peers: The entities comprising the corresponding layers on different machines are called peers. It is the peers that communicate using the protocol. In reality, no data are directly transferred from layer n on one machine to layer n on another machine. Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached.

2. Physical medium: Below layer 1 is the physical medium through which actual communication occurs.

3. Interface: Between each pair of adjacent layers there is an interface. The interface defines which primitive operations and services the lower layer offers to the upper one.

   The layer and interface design is based on two objectives –

   • The amount of information passed through the interface should be minimum.
   • Each layer should perform a specific set of functions, and the interface design should be such that it should be simpler to replace the implementation of one layer with a completely different implementation.

   Eg: All the telephone lines are replaced by satellite channels. All that is required of the new implementation is that it offers exactly the same set of services to its upstairs neighbor as the old implementation did.

4. Network Architecture: A set of layers and protocols is called a network architecture. The specification of the architecture must contain enough information to allow an implementer to write the program or build the hardware for each layer so that it will correctly obey the appropriate protocol. The details of the implementation or the specification of the interfaces are not part of the architecture, as these are hidden away inside the machines and are not visible from the outside.

5. Protocol Stack: A list of protocols used by a particular system, one protocol per layer, is called a protocol stack.

6. Simplex, Half duplex and Full duplex communication: Defines the rules for data transfer.

   • Simplex - Data only travels in one direction.
   • Half Duplex - Data can travel in either direction but not simultaneously.
   • Full Duplex - Data travels in both directions at once.

Interfaces and Services

The function of each layer is to provide services to the layer above it.

The active elements in each layer are termed as entities.

An entity can be -

• a software entity (such as a process), or
• a hardware entity (such as an intelligent I/O chip).

Entities in the same layer on different machines are called peer entities. The entities in layer n implement a service used by layer n+1. Here, layer n is referred to as the service provider and layer n+1 as the service user. Layer n may use the services of layern-1 in order to provide its service.

Services are available at Service Access Points (SAPs). The layer n SAPs are the places where layer n+1 can access the services offered. Each SAP has an address that uniquely identifies it.

In order for 2 layers to exchange information, there has to be an agreed upon set of rules about the interface. At a typical interface, the layer n+1 entity passes an IDU (Interface Data Unit) to the layer n entity through the SAP. The IDU consists of - an SDU (Service Data Unit), and some Control Information (interface Control Information)

The SDU is the information passed across the network to the peer entity and then up to layer n+1.

The control information is needed to help the lower layer do its job but is not part of the data itself. The control information could be things like – the number of bytes in the SDU.

In order to transfer the SDU, the layer n entity may have to fragment it into pieces, each of which is given a header and sent as a separate PDU (Protocol Data Unit) such as a packet.

The PDU headers are used by the peer entities at the destination machine. These headers identify which PDUs contain data and which contain control information, sequence numbers, counts, etc.