Chapter 4 : Network Access

      This chapter begins with the general functions of the physical layer and the standards and protocols that manage the transmission of data across local media. It also introduces the functions of the data link layer and the protocols associated with it.

The Physical Layer

The process that data undergoes from a source node to a destination node is:

• The user data is segmented by the transport layer, placed into packets by the network layer, and further encapsulated into frames by the data link layer.

• The physical layer encodes the frames and creates the electrical, optical, or radio wave signals that represent the bits in each frame.

• These signals are then sent on the media, one at a time.

• The destination node physical layer retrieves these individual signals from the media, restores them to their bit representations, and passes the bits up to the data link layer as a complete frame.

Physical Layer Media

      There are three basic forms of network media. The physical layer produces the representation and groupings of bits for each type of media as:

• Copper cable: The signals are patterns of electrical pulses.

• Fiber-optic cable: The signals are patterns of light.

• Wireless: The signals are patterns of microwave transmissions.

Physical Layer Standards

       The physical layer consists of electronic circuitry, media, and connectors developed by engineers. Therefore, it is appropriate that the standards governing this hardware are defined by the relevant electrical and communications engineering organizations.

Function

The physical layer standards address three functional areas:

Physical Components – The physical components are the electronic hardware devices, media, and other connectors that transmit and carry the signals to represent the bits. Hardware components such as NICs, interfaces and connectors, cable materials, and cable designs are all specified in standards associated with the physical layer. The various ports and interfaces on a Cisco 1941 router are also examples of physical components with specific connectors and pinouts resulting from standards.

Encoding – Encoding or line encoding is a method of converting a stream of data bits into a predefined “code”. Codes are groupings of bits used to provide a predictable pattern that can be recognized by both the sender and the receiver. In the case of networking, encoding is a pattern of voltage or current used to represent bits; the 0s and 1s.

Signaling – The physical layer must generate the electrical, optical, or wireless signals that represent the “1” and “0” on the media. The method of representing the bits is called the signaling method. The physical layer standards must define what type of signal represents a “1” and what type of signal represents a “0”. This can be as simple as a change in the level of an electrical signal or optical pulse. For example, a long pulse might represent a 1 whereas a short pulse represents a 0.

Bandwidth

      Bandwidth is the capacity of a medium to carry data. Digital bandwidth measures the amount of data that can flow from one place to another in a given amount of time.

Physical media properties, current technologies, and the laws of physics all play a role in determining the available bandwidth.

The table shows the commonly used units of measure for bandwidth.

Throughput

Throughput is the measure of the transfer of bits across the media over a given period of time.

Due to a number of factors, throughput usually does not match the specified bandwidth in physical layer implementations. Many factors influence throughput, including:

• The amount of traffic

• The type of traffic

• The latency created by the number of network devices encountered between source and destination

Latency refers to the amount of time, to include delays, for data to travel from one given point to another.

Types of Physical Media

       The physical layer produces the representation and groupings of bits as voltages, radio frequencies, or light pulses. Various standards organizations have contributed to the definition of the physical, electrical, and mechanical properties of the media available for different data communications. These specifications guarantee that cables and connectors will function as anticipated with different data link layer implementations.

The figure shows different types of interfaces and ports available on a 1941 router.

Copper Media

There are three main types of copper media used in networking:

• Unshielded Twisted-Pair (UTP)

• Shielded Twisted-Pair (STP)

• Coaxial

These cables are used to interconnect nodes on a LAN and infrastructure devices such as switches, routers, and wireless access points. Each type of connection and the accompanying devices has cabling requirements stipulated by physical layer standards.

The Data Link Layer

The data link layer of the OSI model (Layer 2), as shown in Figure, is responsible for:

• Allowing the upper layers to access the media

• Accepting Layer 3 packets and packaging them into frames

• Preparing network data for the physical network

• Controlling how data is placed and received on the media

• Exchanging frames between nodes over a physical network media, such as UTP or fiber-optic

• Receiving and directing packets to an upper layer protocol

• Performing error detection

Data Link Sublayers

The data link layer is divided into two sublayers:

• Logical Link Control (LLC) – This upper sublayer communicates with the network layer. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to utilize the same network interface and media.

• Media Access Control (MAC) – This lower sublayer defines the media access processes performed by the hardware. It provides data link layer addressing and access to various network technologies.