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The physical layer is level one in the seven level OSI model of computer networking. It performs services requested by the data link layer.

This level is referring to network hardware, physical cabling or a wireless electromagnetic connection. It also deals with electrical specifications, collision control and other low-level functions.

The physical layer is the very simplest, defining only exactly what a bit is: in other words how to transmit a one or a zero. For example, you would specify at this layer things like what shapes the electrical connectors are, what frequencies to broadcast at, and what frequencies are allowed and will not blow up the network cards. In a snail-mail network, that is a network made up of people posting letters to one another, the physical layer is all about how you write and read individual letters of the alphabet.

The major functions and services performed by the physical layer are:

• establishment and termination of a connection to a communications medium;
• participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control;
• conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel.

1 Encodings

1.1 Two-binary, one-quaternary

Two-binary, one-quaternary (2B1Q) is a physical layer encoding used for Integrated Services Digital Network basic rate interface. 2B1Q uses four signal levels (1Q), equivalent to two bits (2B). Other encoding techniques are B8ZS, AMIIn computing and telecommunications, abbreviation for Alternate Mark Inversion a method of encoding data to be transmitted. Abbreviation of American Megatrends Inc , a computer hardware and software supplier. Abbreviation for acute myocardial infarction . or HDB3HDB3 H igh d ensity b ipolar 3 zeros) is a telecommunications line code mainly used in Japan and Europe (for example, in E-1 lines) and is based on AMI. It is also very similar to the B8ZS encoding used in T-1 lines. Four Zero bits in a row are coded in t.

2 Physical signaling sublayer

In a local area networkA local area network (LAN) is a computer network covering a local area, like a home, office or small group of buildings such as a college. The topology of a network dictates its physical structure. The generally accepted maximum size for a LAN is 1000m2. (LAN) or a metropolitan area networkMetropolitan area networks or MANs are large computer networks usually spanning a campus or a city. They typically use optical fiber connections to link their sites. For instance a university or college may have a MAN that joins together many of their loc (MAN) using open systems interconnection (OSI) architecture, the Physical signaling sublayer is the portion of the physical layer that:

• interfaces with the medium access control sublayer
• performs characterFor alternate meanings, see character. In computer terminology, a character is a unit of information that roughly corresponds to a grapheme, or written symbol, of a natural language, such as a letter, numeral, or punctuation mark. The concept also include encoding, transmissionIn communications, transmission is the act of transmitting electrical messages (and the associated phenonomena of radiant energy that pass through media). In telecommunication, the term transmission has the following meanings: 1. The dispatching, for rece, reception, and decoding
• performs optional isolation functions.

Source: from Federal Standard 1037C

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Topics: Physical Layer Network Functions Services Level Encoding...

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