Network Architecture: Protocol

What are protocol and network architecture?
Protocol: In a computer network, entities in different hosts need to communicate. For two entities for successfully communicate, they must “speak the same language”. What is communicated must conform to same manually accepted set of conventions is referred to as protocol, which may be defined as a set of rules governing the exchange of data between two entities. Network architecture: A protocol may be monolithic or structured. The task of communication between entities on different hosts is too complex to be handled as a unit. An alternative is to use structured design and implemented techniques. Instead of a single protocol, there is a set of protocols that exhibit a hierarchical or layered structure. More primitives functions are implemented in lower-level entities that provide service to higher-level entities when structured protocol design is used, we refer to the hardware and software used to implement the communications functions as network architecture.


what are the key elements of a protocol?

The key elements of a protocol are: i) Syntax: Includes such things as data format, coding, and signal levels. ii) Semitics: Includes control information for coordination and error handling. iii) Timing: Includes speed matching and sequencing.
a) Describe the important characteristics of a protocol.

Some important characteristics of a protocol are:

i) Direct/Indirect
ii) Monolithic/Structured
iii) Symmetric/Asymmetric
iv) Standard/Nonstandard

i) Direct/Indirect: Communication between two entities may be direct/indirect. If two hosts share a point-to-point link, the entities in these hosts may communicate directly; that is, data and control information pass directly between entities with no intervening active agent. If hosts connect through a switched communication networks, the two entities must depend on the functioning of other entities to exchange data.

ii) A protocol may be monolithic or structured. The task of communication between entities on different hosts is too complex to be handled as a unit. An alternative is to use structured design and implementation techniques. Instead of a single protocol, there is a set of protocols that exhibit a hierarchical or layered structured. More primitive functions are implemented in lower-level entities that provide functions service to higher-level entities. When structured protocol design is used, we refer to the hardware and software used to implement the communications functions as network architecture.

iii) Symmetric/Asymmetric: Protocol may be symmetric/asymmetric. The symmetric protocols involve communication between peer entities. Asymmetric may be detected by the logic of an exchange or by the desire to keep one of the entities as simple as possible.

iv) A protocol may either standard or nonstandard. A nonstandard protocol is one built for a specific communication situation. The increasing use of distributed processing dictate that all vendors implement protocols that conform to an agreed upon standard.

Describe the various categories of functions of a protocol.

We can group protocol functions into the following categories:
i) Segmentation and reassembly
ii) Encapsulation
iii) Connection control
iv) Ordered delivery v) Flow control
vi) synchronization vii) Addressing viii) Multiplexing
ix) Transmission services.

i) Segmentation and reassembly: A protocol is concerned with exchanging streams of data between entities. Usually, the transfer can be categorized as consisting of a sequence of blocks of data of some bounded size. Lower level protocols may need to break the data up into blocks of some smaller bounded size. This process is called segmentation or fragmentation. A block of data exchanged between two entities via a protocol is referred to as a protocol data unit (PDU). The counterpart of segmentation is reassembled.

ii) Encapsulation: Each PDU contains not only data but also control information. Indeed, some PDU’s consists solely of control information ans no data. The control information falls into three genereal categories:
a) Address: The address of the sender and/or receiver may be indicated.
b) Error-detecting code: Some sort of frame check sequence often included for error detection.
c) Protocol control: Additional information is included to implement the protocol function. The addition of control information to data is referred to as encapsulation.

iii) Connection control: An entity may transmit data another data to another entity in an unplanned fashion and without prior coordination. This is known as connectionless data transfer. If stations anticipate a lengthy exchange of data and/or certain details of their protocol must be worked out dynamically, a logical connection is established between the entities. This is known as connection-oriented data transfer. In connection-oriented data transfer three phases occur: a) Connection establishment
b) Data transfer
c) Connection termination


iv) Ordered delivery: If two communication entities are indifferent hosts connected by a network, there is a risk that PDUs will not arrive in the order in which they are sent, because traverse different paths through the network. In connection-oriented protocols, it is generally required that the PDU order be maintained.

v) Flow control: Flow control is a function performed by a receiving entity to limit the amount or rate of data that is sent by a transmitting entity. Flow control is a function that must be implemented in several protocols.

vi) Error control: Techniques are needed to guard against loss or damage of data and control information. Most techniques involve error detection and PDU retransmission. Error control is function that must be performed at various levels of protocols.

vii) Synchronization: It is occasionally important that two communicating protocol entities simultaneously in a will defined state, for example at initialization, check pointing and termination. This is termed synchronization.

viii) Addressing: For entities to communicate, other than over a point-to-point link, they must somehow be able to identify each other.
ix) Multiplexing: Multiplexing permits multiple simultaneous connections.

x) Transmission services: A protocol may provide a variety of additional services to the entities that use it. Three common examples of services are:
a) Priority: Certain message, such as control messages, may need to get through to the destination entity with minimum delay. Thus priority may be assigned on a message basis.
b) Grade of service: Certain classes of data may require a minimum throughput threshold.
c) Security: Security mechanism may be involved.