Data Link and Physical Layer Functions in Networking
Data Link Layer Fundamentals
The Data Link layer provides the means to exchange data over common local media. It performs two basic services:
- Allows higher layers to access the media using techniques such as framing.
- Controls how data is placed onto and received from the media using techniques such as media access control.
Layer 2 Terminology
- Frame: The Protocol Data Unit (PDU) of the Data Link layer.
- Node: A Layer 2 notation for network devices connected to a common medium.
- Media/Medium (physical): The physical means used to transfer information between nodes.
- Network (physical): Two or more nodes connected to a common medium.
Note: A physical network differs from a logical network. Logical networks are defined at the Network layer by establishing a hierarchical addressing scheme. Physical networks represent the interconnection of devices on a common medium. Sometimes, a physical network is also called a network segment.
Data Link Sublayers
To support a variety of network functions, the Data Link layer is often divided into two sublayers:
Logical Link Control (LLC)
The Logical Link Control (LLC) sublayer 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 IP and IPX, to utilize the same network interface and medium.
Media Access Control (MAC)
The Media Access Control (MAC) sublayer provides Data Link layer addressing and the delimitation of data according to the signaling requirements of the medium and the type of Data Link layer protocol in use.
Typical Frame Header Fields
- Start Frame Field: Indicates the beginning of the frame.
- Source and Destination Address Fields: Indicate the source and destination nodes on the media.
- Priority/Quality of Service Field: Indicates a particular type of communication service for processing.
- Type Field: Indicates the upper-layer service contained in the frame.
- Logical Connection Control Field: Used to establish the logical connection between nodes.
- Physical Link Control Field: Used to establish the link on the media.
- Flow Control Field: Used to start and stop traffic flow.
- Congestion Control Field: Indicates congestion on the media.
Physical Layer Essentials
The three essential functions of the Physical layer are:
- Physical Components
- Signaling
- Data Encoding
Encoding
Encoding is a method used to convert a stream of data bits into a predefined code. Codes are groups of bits used to provide a predictable pattern that both the sender and the receiver can recognize.
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. Physical layer standards must define what type of signal represents a “1” and what type represents a “0”.
Manchester Encoding
Rather than representing bit values as simple voltage pulses, in the Manchester encoding scheme, the bit values are represented as voltage transitions. A transition from low to high voltage might represent a ‘1’, while a high to low transition represents a ‘0’, or vice versa, occurring at the midpoint of each bit period.
Signal Patterns for Framing
One way of detecting a frame is to start each frame with a specific pattern of signals representing bits that the Physical layer recognizes as a start-of-frame indicator. Another distinct bit pattern signals the end of the frame. Bits received that do not conform to these patterns are typically ignored by the Physical layer standard being used.