Layer 1: The Physical Layer

When you get down to the nitty-gritty details of spitting the ones and zeros onto a cable, you are dealing with the Physical layer.

Manchester encoding

A method of identifying the beginning and end of a bit signal (one or zero) when it is transmitted on the network.

asynchronous communication

Begins transmission of each character with a start bit and ends transmission of each character with a stop bit. This method is not as efficient as synchronous communication but is less expensive.

synchronous communication

Transmits data by synchronizing the data signal between the sender and receiver and sending data as a continuous stream. This is the most efficient way of sending large amounts of data but requires expensive equipment.

The Physical layer of the OSI model determines:

• The physical network structure you will use.

• The mechanical and electrical specifications of the transmission media that will be used.

• How data will be encoded and transmitted. There are all kinds of schemes used for encoding data, which specify how the ones and zeros will be transmitted. For example, Ethernet networks use a scheme called Manchester encoding.

The following graphic illustrates devices in the Physical layer (including the T-connector, which is used in some networks for connecting the computer to the cable).

Note 

Topologies, Ethernet, and Token Ring are covered in greater detail in Chapter 10, 'Data-Link and Network Layer Protocols.'

Physical Layer Technologies

baseband

A technology that permits a single transmission of data at a fixed frequency on a wire at any given moment.

broadband

A type of transmission that can use a single wire to transmit multiple streams of data simultaneously using different frequencies. This method is similar to the method used by radio stations all sharing the airwaves to send their signals.

hub

A Physical layer device that connects computers and other devices to make a network. A hub regenerates an incoming signal from one device and broadcasts the signal out all other ports.

repeater

A network device, similar to a hub but with only two or three ports, that can be used to extend the transmission distance of a network signal or to join two networks.

The category of technologies that the Physical layer defines include:

Connection type  Connections can be point-to-point or multipoint. Point-to-point assumes that only two devices are connected. Multipoint means that three or more devices are connected.

Physical topology  Topologies define the physical layout of the network. Common topologies include star, bus, and ring.

Type of signal  Signals can be analog or digital. Local computer networks use digital signals, and the public telephone system uses analog signals.

Type of bit synchronization  You can send data by asynchronous or synchronous means. Asynchronous communication requires less expensive equipment but is less efficient when transferring large blocks of data. Synchronous communication requires more expensive equipment but provides better efficiency.

Transmission signaling technology  You can use baseband technology, which is the signaling type used on Ethernet networks, or broadband signaling, which is used on ISDNs.

Physical Layer Hardware

Examples of Physical layer hardware are:

• Network interface cards

• Network cables

• Hubs (also referred to as concentrators)

• Repeaters

• Modems

• ISDN adapters

  Note 

  At the Physical layer, data is referred to as bits.

frame

Data that has been encapsulated by the Layer 2 protocol before being transmitted on the wire.

Token Ring

A Layer 2 protocol developed by IBM that uses a token-passing method for transmitting data. Each device on the ring takes turns using the token. The token can be used by only one device at a time.

hexadecimal

A numbering system that uses 16 instead of 10 as its base; it uses the digits 0-9 and the letters A-F to represent the decimal numbers 0-15.

Note 

At the Physical Layer, data is referred to as bits.