We are in the process of negotiating with the US General Services Administration (GSA) to have Teracom listed on government procurement systems. One of the steps in this process is to have our products and services anonymously evaluated and rated by past customers.

The results are in, and we couldn't be happier: an overall score of 97, with quality and personnel ratings 100% positive!
We're not surprised, but are gratified to know our customers agree the quality of our training and customer service is at the very top of the scale.
See for yourself:

To communicate, we need a convention or agreement that specifies how we are going to do so. This is the definition of a protocol. The term protocol is used in the data communications business the same way it is used in the diplomacy business: it is a plan for how two different systems will interact. Mutual adherence to an agreed protocol or set of protocols make communication possible.

In an open system, the protocols are published standards: everyone agrees on the plan. There are two basic choices for the plan: monolithic or structured. A monolithic protocol would embody all of the required functions in a single standard. The problem with this approach is that it becomes unwieldy when all possible variations are included in the single document, and makes maintenance impossible.
A structured approach, where we divide the totality of functions into easy pieces, then write protocols covering each of the pieces, is more workable.

One approach to implementing structured communications protocols is the OSI Reference Model. In 1983, the International Organization for Standardization (ISO) adopted a “Basic Reference Model for Open Systems Interconnection”. The purpose of this model was to “provide a common basis for the coordination of standards development for the purpose of systems interconnection, while allowing existing standards to be placed into perspective within the overall reference model”. The key point is that this is a model for discussing protocols and standards. It does not specify how to actually perform a function, but more describes what functions must be performed, and organizes these functions into manageable groups or layers.

A layer is a subset of the totality of functions that must be implemented to interwork systems. We write software packages that implement one or more of the layers on each system. Each layer relies on services provided by the lower layer, takes those and adds value to them, and provides that as a value-added service to the higher layer above it.
 

The OSI Reference Model is referred to as a 7-layer model because the total set of functions required to interwork diverse systems was defined and then broken up into seven groups or layers, and arranged in a hierarchy.

Each layer has a name and a number. We start numbering at the bottom:

1: Physical Layer  The physical layer provides a raw bit stream service, moving 1s and 0s between the systems. This is all it does, but it has to do this completely. The physical layer includes the mechanical, electrical, functional and procedural specifications for moving binary digits over a physical medium. Category 5 LAN cables and Ethernet are an example of the implementation of the physical layer.

2: Data Link Layer  There is no such thing as an error-free, unlimited-capacity physical connection. The data link layer manages communications on a single circuit, a single link. There may be several stations connected to the circuit – a multidrop circuit – but it is one single physical circuit. The link layer typically controls access to the circuit (which station can transmit next), flow control and error control. This allows communications of blocks of data to another station on the same circuit. The 802.2 MAC addresses and frame formats on LANs are an example of the implementation of the link layer.

3: Network Layer  What happens if we don’t have a single link, but 86,000 of them, and we do not want our data broadcast to all of the stations on all 86,000 circuits, but rather want it routed and delivered to just one destination? This is the definition of a network. The network protocol allows the user to specify a particular destination, and the network to route the data internally to that destination, moving data from one circuit to the next, essentially a forwarding function. IP is a network layer protocol.

4. Transport Layer  If the receiver isn’t on our network, but on another one, and the networks are connected together with multiple intervening networks, how do we know that our data got delivered? If we are using IP, there are no guarantees our data will be delivered, when that might happen, nor how often that might happen. Nothing. The transport layer implements error checking between the source and destination end-to-end to verify that the data was successfully delivered, and in some cases, retransmit data that was not. TCP is a transport layer protocol.

5. Session Layer  The session layer manages sessions between applications, including initiation, maintenance and termination of information transfer sessions. Usually this is visible to the user by having to log on with a password. The POP email protocol implements a session layer.

6. Presentation Layer  The presentation layer is very important: this is the coding step. How are we going to represent our message in 1s and 0s? ASCII is an example of a presentation layer protocol. Compression and encryption are also presentation layer protocols: methods of coding messages into 1s and 0s.

7. Application Layer  Sitting on top of all of this is the application layer. The application layer defines the vocabulary, format and syntax of messages that will be exchanged, and is usually bundled with a Human-Machine Interface. Regular email messages are formatted following the Simple Mail Transfer Protocol (SMTP), an application-layer protocol.

Enjoy!