The Physical Layer of the OSI model

Physical media is thought of as some sort of wireThe Physical Layer consists of the basic hardware transmission technologies of a network sometime referred to as the physical media. Physical media provides the electro-mechanical interface through which data moves among devices on the network.

Initially physical media is thought of as some sort of wire. As technology progresses the types of media grows.

Bounded media transmits signals by sending electricity or light over a cable. Unbounded media transmits data without the benefit of a conduit-it might transmit data through open air, water, or even a vacuum. Simply put, media is the wire, or anything that takes the place of the wire, such as fiber optic, infrared, or radio spectrum technology.

Data communications definitions:

Public Switched Telephone Network (PSTN), also referred to as Plain Old Telephone Service (POTS), connections run over the standard copper phone lines found in most homes

Integrated Services Digital Network (ISDN) uses a single wire or fiber optic line to carry voice, data, and video signals.

In the early days of connecting your computer to the internet most folks had Public Switched Telephone Network (PSTN), also referred to as Plain Old Telephone Service (POTS), and all connections were run over the standard copper phone lines. In order for the digital world of computers to talk over analog phone lines you needed to use a MODEM.

The term MODEM comes from the words modulator and demodulator, it is a device that modulates a carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data.

Modem standards, or V dot modem standards, are defined by the ITU (International Telecommunications Union). The FCC has limited the speed of analog transmissions to 53 Kbps

Basic Rate Interface (BRI) is most commonly used in residential ISDN connections. It's composed of two bearer (B) channels at 64 Kbps each for a total of 128 Kbps (used for voice and data) and one delta (D) channel at 16 Kbps (used for controlling the B channels and signal transmission). The total bandwidth is up to 144 Kbps.

Primary Rate Interface (PRI) is most commonly used between a PBX (Private Branch Exchange) at the customer's site and the central office of the phone company. It is composed of 23 B channels at 64 Kbps and one D channel at 64 Kbps. The total bandwidth is up to 1,536 Kbps.

Digital Subscriber Line (DSL) technologies use existing, regular copper phone lines to transmit data. DSL hardware can transmit data using three channels over the same wire. In a typical set up, a user connected through a DSL hookup can send data at 640 Kbps, receive data at 1.5 Mbps, and still carry on a standard phone conversion over one line.

T-Carrier Technology is a digital transmission service used to create point-to-point private networks and to establish direct connections to Internet Service Providers. It uses four wires, one pair to transmit and another to receive.

T-1 lines support data transfer at rates of 1.544 megabits per second. Each T-1 line contains 24 channels. The E1 line is the European counterpart that transmits data at 2.048 Mbps.

T-3 has 672 (64 Kbps) channels, for a total data rate of 44.736 Mbps. The E3 line is the European counterpart that transmits data at 34.368 Mbps.

Cable connections provide access to the Internet through the same coaxial cable that brings cable TV into your home. A signal splitter installed by the cable company isolates the Internet signals from the TV signals. The two-way cable connection is always available and can be very fast. Speeds up to 30 Mbps are claimed to be possible, although speeds in the 1 to 2 Mbps range are more typical.

The Physical Layer Ethernet Specifications

Ethernet is a family of computer networking technologies for local area (LAN) and larger networks originally developed at Xerox PARC in the 1970s. Robert Metcalfe, one of the inventors of Ethernet, left Xerox PARC in 1979 to create 3Com Corporation to focus on deploying Ethernet technology.

In 1980, the Institute of Electrical and Electronics Engineers (IEEE) started project 802 to standardize local area networks (LAN). The IEEE 802 standards map to the lower two layers (Data Link and Physical) of the seven-layer OSI networking reference model. IEEE 802.3 is a working group and a collection of IEEE standards focusing on wired Ethernet.

Twisted-pair Ethernet cable has the following specifications: a maximum of 1,024 attached workstations, a maximum of 4 repeaters between communicating workstations, a maximum segment length of 328 feet (100 meters).

100BASE-TX specification uses two pairs of Category 5 UTP or Category 1 STP cabling at a 100 Mbps data transmission speed. Each segment can be up to 100 meters long.

100BASE-T4 specification uses four pairs of Category 3, 4, or 5 UTP cabling at a 100 Mbps data transmission speed with standard RJ-45 connectors. Each segment can be up to 100 meters long.

Fiber optic cable (IEEE 802.8) in which the center core, a glass cladding composed of varying layers of reflective glass, refracts light back into the core. Max length is 25 kilometers, speed is up to 2Gbps but very expensive. Best used for a backbone due to cost.

100BASE-FX specification uses two-strand 62.5/125 micron multi- or single-mode fiber media. Half-duplex, multi-mode fiber media has a maximum segment length of 412 meters. Full-duplex, single-mode fiber media has a maximum segment length of 10,000 meters.

Other wired LAN technologies

Ethernet has largely replaced competing wired LAN technologies such as token ring, token bus, and ARCNET.

IEEE standard 802.4 defined Token bus. It was mainly used for industrial applications. Token bus was used by General Motors for their Manufacturing Automation Protocol (MAP) standardization effort. The IEEE 802.4 Working Group is disbanded and the standard has been withdrawn

Token ring was IBM’s protocol of choice, standardized as IEEE 802.5. Introduced by IBM in 1984,Token ring was fairly successful in corporate environments, but gradually lost out to Ethernet.

ARCNET was a very early LAN system, a token-passing bus with a 2.5 Mbit/sec speed, popular in the 1980s.

Wireless standards

The standards defining the physical layer of wired Ethernet are known as IEEE 802.3, which is part of a larger set of project 802 standards by the Institute of Electrical and Electronics Engineers Standards Association.

IEEE 802.11 WLAN 802.11 and 802.11x refers to a family of specifications developed by the IEEE for wireless LAN (WLAN) technology. 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients.

IEE 802.15 defines Bluetooth Bluetooth,  a wireless technology standard for exchanging data over short distances (using short-wavelength UHF radio waves in the ISM band from 2.4 to 2.485 GHz[3]) from fixed and mobile devices, and building personal area networks (PANs).

IEEE 802.16 defines WIMAX standards for broadband for wireless metropolitan area networks. officially called WirelessMAN in IEEE, it has been commercialized under the name "WiMAX"

While in your world many of the older technologies of data communications may be replaced with modern one, there are many reasons why you may need to know about them. You may get a better understanding of how things are done on your current network if you understand the evolution of the network.

If your ever work in consulting you may be surprised to find out how much of what you call obsolete is still in use. You will also find questions on older technologies on various certification tests.

Related pages:

As an introduction to Ethernet and computer network cabling we have created the following pages: Ethernet computer network cable frequently asked questions answered and Computer network modular connectors and telephone registered jacks

Tags: