OSI (Open Systems Interconnection):

*We are going to spend the next several days learning about various communication network equipment. Before we do that it will be helpful to understand the OSI Reference Model.

OSI (Open Systems Interconnection): A Reference Model developed by the ISO (International Organization for Standardization, as translated into English). The OSI Reference Model is the only internationally accepted framework of standards for communication between different systems made by different vendors. ISO's goal is to create an open systems networking environment where any vendor's computer system, connected to any network, can freely share data with any other computer system on that network or a linked network. Most of the dominant communication protocols used today have a structure based on the OSI model.

Although OSI is a model and not an actively used protocol, and there are still very few pure OSI-based products on the market today, it is still important to understand its structure. The OSI model organizes the communications process into seven different categories and places these categories in a layered sequence based on their relation to the user. Layers 7 through 4 deal with end to end communications between the message source and the message destination, while layers 3 through 1 deal with network access.

Layer 1 - The Physical Layer deals with the physical means of sending data over lines (i.e. the electrical, mechanical and functional control of data circuits). Examples include EIA-232 , T-Carrier and SONET.

Layer 2 - The Data Link Layer is concerned with procedures and protocols for operating the communications lines. It also has a way of detecting and correcting message errors. Examples include Frame Relay, PPP, and SLIP (Serial Line Internet Protocol). ATM runs at Layers 1 & 2, as do LANs.

Layer 3 - The Network Layer determines how data is transferred between computers. It also addresses routing within and between individual networks. The most visible example is IP (Internet Protocol).

Layer 4 - The Transport Layer defines the rules for information exchanges and manages end-to-end delivery of information within and between networks, including error recovery and flow control. TCP (Transmission Control Protocol) is an example, as is the OSI Transport Protocol (TP), which comprises five layers of its own. Layer 4 protocols ensure end-to-end integrity of the data in a session. The X.25 packet-switching protocol operates at Layers 1, 2, 3 and 4.

Layer 5 - The Session Layer is concerned with dialog management. It controls the use of the basic communication facility provided by the Transport layer. If you’ve ever lost your connection while Web surfing, you've likely experienced a session time-out, so you have some sense of the Session Layer.

Layer 6 - The Presentation layer provides transparent communications services by making the differences of varying data formats (character codes, for example) between dissimilar systems. Conversion of coding schemes (e.g., ASCII to EBDCIC to Unicode) and text compression and decompression exemplify Presentation Layer functions.

Layer 7 - The Applications layer contains functions for particular applications services, such as file transfer, remote file access and virtual terminal. TCP/IP application protocols such as FTP (File Transfer Protocol), Simple Mail Transfer Protocol (SMTP), SNMP (Simply Network Management Protocol) and TELNET (TELecommunications Network) take place at Layer 7.

The OSI Model is an important concept to understand, at least at a high level. I have attached one of Novell's Network Tutorials which goes into the OSI Model in more detail and also provide some very useful diagrams, including one which illustrates where different protocols operate in the OSI model. The tutorial can be found at the following link:

http://www.novell.com/info/primer/prim05.html.

Switch Access Line Service, etc.

Switched Access Line Service (Switched Access): All residential and most businesses use this type of telephone access. It refers to the connection between your phone and the long distance companies' switch (POP or Point-of-Presence) when you make a regular local or LD (long distance) telephone call over standard phone lines.

Switched Access: A method of obtaining test access to telecommunications circuits by using electromechanical circuitry to switch test apparatus to the circuit.

Switched Local Service: You pick up the phone. You dial a local number. Bingo, you have switched local phone service. The reason this trivial definition is even in Newton's dictionary is because many states in the US now, finally, allow companies to offer local switched telephone service in competition with the established company, e.g. United Telecom (Embarq). Previously, they had only allowed competition in leased lines. And then previous to that they had not allowed any competition in any area of local phone service.

Reciprocal Compensation

(Recip Comp) - A form of financial compensation that occurs when a local or LD (Long Distance) service provider terminates a call on another provider's facilities. Imagine a phone call from New York to Los Angeles. It may start with the customer of a new phone company, then proceed to a local phone company (e.g., New York Telephone, part of Verizon). Then it may proceed to a LD company before ending in Los Angeles and going through another one or two local phone companies before reaching the person dialed. Under the existing rules, all the companies carrying these phone calls have to be paid in some way for their transmission and switching services. There are programs in place such that the company doing the billing and collecting the money pays over some of those monies to the other phone companies in the chain. One such program is called "reciprocal compensation." The opposite of recip comp is called "Bill and Keep." Under this program, the company billing the call gets to keep all the money. The others in the chain (or most of the others in the chain) get nothing.

IXC (IntereXchange Carrier)

Also less commonly known as IEC (InterExchange Carrier) and IC. Long-haul long distance carriers, IXCs include all facilities-based inter-LATA carriers. The largest IXCs are AT&T (acquiredby SBC in 2005), MCI (merged with Verizon in 2006) and Sprint; a huge number of smaller, regional companies also fit this definition. The term generally applies to voice and data carriers, but not to Internet carriers. IXC is in contrast to LEC (Local Exchange Carrier), a term applied to traditional telephone companies which provide local service and intraLATA toll service. IXCs also provide intraLATA toll service and operate as CLECs (Competitive Local Exchange Carriers) in many states. Once upon a time the non-AT&T IXCs were called OCCs (Other Common Carriers), a status which they resented for understandable reasons.

Below is a link to an annual FCC publication (pdf format), "Statistics of Common Carriers." This document provides a wealth of data on common carriers. The FCC publishes a wide range of useful documents which are available at http://www.fcc.gov. The most up-to-date version is the 2005-2006 annual report released 6/08.

Document: http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-282813A1.pdf

InterLATA Service

As defined by the Telecom Act of 1996, the term interLATA service means telecommunications between a point located in a local access and transport area (LATA) and a point located outside such area. InterLATA services, traffic or facilities originate in one LATA and cross over and terminate in another LATA. This can be either Interstate or Intrastate service, traffic or facilities. Under provisions of Divestiture, the BOCs (Bell Operating Companies) were restricted from provided interLATA services, but could provide intraLATA services (since changed, right?). Reminder, the term LATA means a contiguous geographic area: (a) established before the date of enactment of the Telecommunications Act of 1996 by a BOC such that no exchange area includes points within more than 1 metropolitan statistical area, consolidated metropolitan statistical area, or State, except as expressly permitted under the AT&T Consent Decree; or (b) established or modified by a BOC after such date of enactment and approved by the Commission. IntraLATA services originate and terminate in the same LATA.

Long Distance

Now that we've learned all about LATA's and numbering plans, we'll shift our focus to long distance. Although today's word is rather succinct, there is plenty to learn in this area (which we will dive into later). Stay tuned in future weeks for more information on the various companies spawned from the original AT&T and terminology pertaining to the architecture of the local loop!

Long Distance: Any telephone call to a location outside the local service area. Also called a toll call or trunk call.

NANP (North American Numbering Plan)

Invented in 1947 by AT&T and Bell Telephone Laboratories (now Lucent). The NANP assigns area codes and sets rules for calls to be routed across North America (i.e. the US and Canada). The new one, put into effect in January, 1995 has one major change: The middle number in a North American area code no longer is required to be a 1 or a 0; rather, it can range between 0 and 9. NANP numbers are 10 digits in length, in the format NXX-NXX-XXXX. The first three digits are the NPA code (i.e., area code). The second three are the central office code or central office prefix, and the last four are the line number. NANP numbers conform to E.164, which is the ITU-T (International Telecommunication Union) international standard for numbering plans. NANP administration was shifted from Bell Labs to Bellcore, when it was formed in 1986. Due to Bellcore's obvious conflict of interest, responsibility was shifted to NANC (North American Numbering Council) in 1995; it was shifted again in 1997 to Lockheed Martin. In November, 1999, it was shifted to NeuStar Inc., when it was discovered that Lockheed Martin had a conflict of interest. NeuStar originally was an independent business unit of Lockheed Martin, but was spun off in order to resolve the conflict.