Networking Software - Services, Protocols and Standards

In this section I tried to give answers to the following questions:

What are services?

In the networking environment the term service is used with two different meanings:
Networking services
This is any kind of infrastructure that telecommunication companies offer to their customers. So the service provider is a company, and the service user is a customer or another company.
The normal set of offers for using the telephone network to make calls to other people is a service in this meaning, but as the customer's needs can differ when data is to be transmitted (faster line, optical instead of electromagnetical transmission, etc.), there is now a lot of services other than the normal phone communication.
Prominent networking services are nowadays:
  • Switched Multimegabit Data Services (SMDS)
  • X.25 Networks
  • Frame Relay
  • Narrowband ISDN
  • Broadband ISDN/ Asynchronous Transfer Mode (ATM)
  • Distributed Queue Dual Bus (DQDB)
Layer services
When speaking about a layered network architecture, a service is the set of tasks that are performed by the layer for the layer above. So, a service in this meaning acts as an interface between two layers of a network reference model, with the lower layer being the service provider and the upper layer being the service user.
Formally, a layer service is specified by a set of primitives (operations) available to the layer above. These primitives tell the layer to perform or report on some action. Primitives can for instance be classified as follows:

Request An entity wants the service to do some work
Indication An entity is to be informed about an event
Response An entity wants to respond to an event
Confirm The response to an earlier request has come back

Example:
To see how primitives are used, consider the steps required to call your aunt and invite her to your house for tea:

Class of primitives    Primitive Description
CONNECT Request You dial your aunt's phone number.
CONNECT Indication Her phone rings.
CONNECT Response She picks up the phone.
CONNECT Confirm You hear her ringing stop.
DATA Request You invite her to tea.
DATA Indication    She hears your invitation.
DATA Request She sais she would be delighted to come.
DATA Indication You hear her acceptance.
DISCONNECT Request You hang up the phone.
DISCONNECT Indication She hears it and hangs up, too.

(from: Andrew S Tanenbaum: Computer Networks, pp.26/27)
The layer services, however, only define what the layer must provide to the layer above, and not how. The "how" is defined in the protocols.

What are protocols?
Protocols are sets of rules how frames, packages and messages are formatted and exchanged. They are implementations of layer services. Hence the protocol is an agreement of how layers perform their work, and not what work they do. The "what" is defined in the services.
Sender and receiver must agree upon a protocol to use, otherwise communication will fail. As there are different layers in each network architecture, there are also different protocols, each defining how the work of this very layer is to be done. And as there are always different ways of doing a work, there are multiple protocols for either network layer.
Prominent data link protocols are:
Prominent network protocols are:
  • Internet Protocol (IP)
  • Address Resolution Protocol (ARP)
Prominent transport protocols are:
Prominent session protocols are:
Prominent presentation protocols are:
  • Pretty Good Privacy (PGP)
  • Privacy Enhanced Mail (PEM)
Prominent application protocols are:
  • Hyper Text Transfer Protocol (HTTP)
  • Network News Transfer Protocol (NNTP)
  • File Transfer Protocol (FTP)

What are standards?
Standards rule the outlook of how computers are physically linked in a network, how a single station may get access to the network, and how the data frame must look like. Hence they are governing the lowest levels of any network architecture:
  1. the transmission line,
  2. the physical layer, and
  3. the lower part of the data link layer.
Seen in this light, a standard would be a conglomerate of transmission line-, level 1- and level 2-protocols. This is not quite true, though: A protocol is always something sender and receiver have to agree upon if communication is to be successful. But sender and receiver may use different standards and can still be able to communicate, as there are network components that "translate" from one standard to another (the brigdes).
Prominent LAN standards are nowadays:
© 2000 Peter Gallert, last updated on 24 January 2001