The Physical Layer (Layer 1) is the lowest layer in the OSI reference Model. Its primary function is to move bits across a communications channel from one system to another. The basic unit of exchange at this layer is the binary digit, also known as a bit.
There are a variety of Physical Layer standards in existence, each of which fills a particular niche in the overall telecommunications marketplace. But all of the Physical Layer standards have to address common points, such as:
- Medium: Options include optical fiber, twisted pair, coaxial cable, and various forms of wireless, etc.
- Connectors: Type of connector, dimensions, number of pins, pin size, pin location, etc.
- Bit encoding: Is the communications system an analog or digital transmission environment? How are ones and zeroes represented on the medium using light, electricity, or electromagnetism?
- Transmission rate: How fast can signals be placed on the medium? How many bits can each signal represent? What is the overall bit rate of the transmission system? Is it fixed or variable?
- Transmission distances: How far can signals travel before they lose too much power through attenuation? This will provide information about spacing of repeaters or amplifiers on the medium.
- Transmission scheme: Does the transmission environment operate as full-duplex, half-duplex, or simplex?
- Topology: Is the circuit point-to-point (P2P), point-to-multipoint (P2MP), or multipoint-to-multipoint (MP2MP)?
These are many Physical Layer standards and technologies, including EIA-232-E, EIA-449, V.21bis, SONET, T-1, T-3, 10BASE-T, 100BASE-TX, and ISDN, to name a few. Any system that is going to be part of a data communications network needs to implement at least one Physical Layer standard to connect it to the network. Which standard it implements depends on where and how it is connected into the network, and what role it plays. Equipment designed to provide any kind of interconnection function will have at least two Physical Layer interfaces.
There are instances where a Physical Layer is transformed from one standard to another between two devices that are exchanging bits. This is most commonly found when a private organization purchases a service from a telecommunications carrier. Then this occurs, the end equipment that is the source and destination of the bit flow is called the data terminal equipment (DTE), and the equipment that transforms one Physical Layer to another is called data circuit-terminating equipment (DCE). This relationship is depicted in the graphic to the right. Computers and routers commonly play the role of a DTE. Examples of DCEs include modems, DSUs, and NT1s.
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