The physical layer unit of data transmission is the "bit", the intermediate mass is an electrical signal, i.e., these bits ... 010101
Foreword
The physical layer is the first layer of computer networks, although it is at the bottom, but it is the basis of an open system. The physical layer provides transmission media interconnects and data communication between devices, to provide a reliable environment for data transmission. If you want as few words to remember this first layer, that is, signals and media .
Interpreted literally: the physical transfer of hardware, the physical properties. In Guangzhou, Beijing friends to chat with you, your computer must be able to access, what is the physical embodiment of that? Is not connected to a network cable, plug a router, Beijing friends over there is not also have network cable, they have to insert a router. That is the communication between the computer and the computer must be in communication with the underlying physical layer aspects, you would like to call, the intermediate is not necessary that even a telephone line.
Physical layer functions
The physical layer is considered how to connect the various computers on the transmission medium to transmit data bit stream, rather than to specific transmission medium. Means we use the Internet a lot, of course, a wide variety of hardware. The physical layer to do is block out as much as possible this difference, so you can make the data link layer need only consider how to complete this agreement and the level of service, regardless of what network specific transmission media is (twisted pair or fiber optic cable ) . Protocols for the physical layer is also often referred to the same procedure.
Physical media reality
Transmission medium: transmission media are also referred to as a transmission medium or a transmission medium, a physical path between the transmitter and the receiver, the transmission medium is an essential part. Divided into two categories: non-guided transmission media and transmission media guide type
1. guide type transmission medium (wired)
In the non-guided transmission medium, the electromagnetic wave is guided along the solid media (copper, fiber optic ,, cable, twisted pair) guided propagation
Advantages: high traffic volume, transmission distance, signal crosstalk is small, confidential performance, radiation-free, long life, unlike the outer light wave transmission wifi can be cracked.
Disadvantages: higher than over the physical facilities and ongoing maintenance costs of wireless transmission, not easy to move
2. Non-guided transmission medium (wireless)
Non-guide type transmission medium refers to the free space, showing unlimited transfers, non-guided transmission media transmission of electromagnetic waves often become wireless transmission, such transmission medium without the aid of solid media can achieve transmission purposes, simply to say that we usually used in the WIFI. They for transmission of signals through different frequency bands.
Advantages: does not require a restricted line convenient to move around, scalability, and low cost.
Cons: transmission rate is relatively slow, transmission susceptible to external electromagnetic interference, radiation, transport from the past, such as: you get a phone connection WIFI through a few walls open the web page will be excruciatingly slow.
There are many physical layer information such as:
channel: the link layer interfacing with
multiplexed channel used: frequency division multiplexing, time division multiplexing, wavelength division multiplexing, code division multiplexed
information channel transmission: analog information into and digital information. Generally refers to the medium in one direction to transfer information.
Channel bandwidth: bandwidth is divided into analog and digital bandwidth
symbol
Transmission channel and
Generally represents a channel communicating media information to one direction, usually it said communication line for transmitting information often contains a channel and a channel to receive information.
Simplex communication: that is one-way communication, the communication only in one direction and the opposite direction without interaction
duplex communication: that is, two-way communication can be, but can not communicate, one transmission data, the other party must accept the data, waiting for the other finished, and then send themselves can, for example, a television inside the military walkie-talkie, are only waiting for the party finished speaking, the other party to speak.
Full-duplex communication: with respect to the half-duplex communication is concerned, is able to communicate simultaneously and two-way, for example, telephone.