Chapter Seven

Data Link Layer

This chapter is about various protocols being used in data link layer which is the second lowest layer in ISO/OSI reference model. The protocols are generally classified into byte oriented and bit oriented protocols. The former protocol is the oldest technology and has been widely proved to be a reliable protocol for low speed low volume data transfer while the latter protocol is more efficient in data processing. Error detection and correction is done in this layer by re-transmitting the error frame. Upon completion of this chapter, you should:

Understand the functions of data link layer

Identify the structure of Bisync frame or block

Understand high level Data Link Control Procedures

Data Link Layer

This is the second layer principally responsible for sending and receiving the data in an error-free environment. In addition, it also maintains the data flow gracefully and is able to correct the sequence of transmission in case a distorted frame is detected. The functions provided are summarized as follows:

One party may be temporarily incapable of handling the current traffic load. For instance, due to temporary hardware problem, buffer space depletion, sudden processor load demanded by other functions.

The data link peer parties may reside in a host-node configuration, or a node-node configuration as shown in Figure . The host-node refers to the interface between the Data Terminating Equipment and the network boundary(DCE in this diagram). The frame size between host-node and node-to-node may be varied, as a result, the frames may be fragmented (break into a few smaller frames) and reassembled (combine a few smaller frames into a larger frame) while traversing along a pre-determined transmission path.

Explain why data link layer is concerned about frames between two adjacent nodes.

It is error-free data transmission between two machines.

The block diagram indicating the relationship between network layer and physical layer is shown in Figure .

Additional frame header embedding the physical address of recipient, one control byte, and frame delimiter are added to the Level two frame. The transmission is carried out by the physical layer using synchronous/asynchronous data transmission. A transmission flag is added to the frame by physical layer as well.

Hundreds of different link protocols are used by data communications in industry. A few examples are given below:

X.25 uses LAP or LAPB as the data link layer.

Layer 2 frame

Protocol Data Unit (PDU) for data link layer is often called a frame with frame structure as given below:

The data field is used to carry layer 3 PDU, while the header (also called) PCI in OSI model) contains control information. E.g. Sequence No.,frame type, acknowledgment, etc.). Layer 2 passes the entire frame to layer 1 for transmission.

The internal structure of the frame should be mechanical to the peer layer-2’s only.

Layer 1 accepts a layer 2 frame as a block of data to be embedded in layer 1 frame for transmission. Moreover, its internal structure is of no concern.

Can you Figure out how many bytes are required for the header and trailer? Also find out the improvement in utilization if they are not added.

Three bytes including transmission flag, physical address of recipient and control byte for header.

Error Detection and Control

The function of data link layer is to provide an error free transmission environment. The cause of noise and remedial methods are firstly discussed. The noisy signals which causes the data lost or corruption are classified into three types:

Sources of error

This section discusses where the error comes from. Error is mainly caused by random signal that is unpredictable. Here, random signal refers to the noise.

White Noise

It is present in all electronic devices and cannot be eliminated by any circuits. It increases with temperature, but is independent of frequency. That means the white noise covers the whole frequency spectrum and will be picked up by both low or high frequency devices.

Interference

It is caused by picking up the unwanted electromagnetic signals nearby such as crosstalk due to adjacent cables transmitting electronic signals or lightning causing power surge.

Crosstalk can be reduced by twisting the telephone wires. This will cancel the effect of interference.

Human error

Noise sometimes is caused by human being such as plugging or unplugging the signal cables, or power on/off the related communications equipment.

Other Errors

Other errors that are less relevant are:

It will produce random electromagnetic wave which will be picked up by computer. Proper earthing is therefore vital to the reliability of computer systems.

Figure shows the effect of corrupting the data by the presence of noise. The binary data being transmitted will be incorrectly altered by the noise. That is why the protocols available in data link layer must be capable to detect the correctness of received data. The factors to measure how effective the protocols to detect the transmission error include the percentage of detecting an error (single bit, double bit or triple bit errors), and undetected error. BER (bit error rate = Incorrect Data/Total transmitted Data ) is a measurement of how well the medium transmission quality. The typical values are 10^-4 for telephone line and 10^-10 for optical fiber.  

The prevention methods are: Shielding, Moving cables, Changing multiplexing techniques, Improving connection quality with better equipment, Amplifies and repeaters, Equalization takes of attenuation and delay distortion, Condition by using expensive error free circuits.

Error Detection

A few codes or bits in association with the data are appended to the frame so that the receiver can detect the presence of error by using a simple algorithm.

Vertical Redundancy Check

A single bit either ‘1’ or ‘0’ is appended to the end of each character to produce an odd or even number of ‘1’ bits. This method is commonly used in start/stop transmission (Asynchronous) as shown in Figure .

An ASC character transmitting at asynchronous mode will have:

 1 start bit + 7 data bits + 1 parity bit + 1 or 2 stop bits = 10 bits, not just 8 bits

The advantages and disadvantages are:

Can you explain why overhead is 10%?

Parity Bit/Total Bits

Longitudinal Redundancy Check

This method is better than a simple Vertical Redundancy Check. The data is grouped into blocks of characters, say for example 12 bytes from the Hong Kong Stock Exchange, each of the character has a parity bit added. A block check character (BCC) is then appended to the end of each transmission block. The size of BCC can be 8, 12 or 16 bits depending on the algorithm.

The parity bits at the end of each character are known as Vertical Redundancy Check (VRC) while the BCC is called the Block Check Character (BCC).

The advantage and disadvantage is:

Both parity and two-coordinate parity checking are commonly used in low speed communication.

Cyclic Redundancy Check

This is a faster method and is commonly used in protocol operating at synchronous mode such as SDLC. In this method, the data to be sent is divided by a set of binary constant called the generator polynomial. The remainder after division called Frame Check Sequence (FCS) will be appended to the original data stream. The original data plus the frame check sequence will be transmitted over the transmission media. At the receiving end, the received data will be verified by using the same generator polynomial to determine whether the message received is error-free. CRC can be implemented by means of hardware such as Synchronous Communication Chip (SCC) chip which provides the necessary circuit in physical layer to detect the transmission error.

The generator polynomial defined by CCITT for use on the switched telephone is:

X¹⁶ + X¹² + X⁵ + X⁰

The equivalent binary expression is 10001000000100001

Another commonly used code is the CRC-16:

X¹⁶ + X¹² + X² + X⁰

Write down the binary expression.

These 16 bit codes can detect the following errors:

Error is measured in terms of Bit Error Rate. Typical value for switched line is 10^-5.

What is the FCS if the frame transmitted in decimal is 728 and the generator polynomial is 13? Assume two digits are used for error checking.

Byte and Bit protocols

The data link layer protocols based on the frame structure (whether frames are constructed out of bits or characters) can be classified into:

The control information in a frame does not need to be continuous. Typically, the delimiter field and the error checking field are separated from other control fields.

Can you list the dummy character used by SNA/SDLC, when there is no data being transmitted?

It will send out a string of flags

The frame format for Byte oriented frame is:

And the Bit oriented frame:

What is the advantage of bit oriented frame compared to byte oriented frame in terms of data position identification?

The protocol can detect the exact location of control and data filed without processing the individual characters.

Byte-oriented Protocols

This type of protocols was developed a long time ago. A well known example is the IBM BISYNC protocol used in Remote Job Entry (RJE). The computer has to stripe down the character by character within the frame in order to determine the meaning and action of received data.

The Byte oriented protocols such as BISYNC have the following characteristics:

BISYNC is widely used in:

A typical BSC format delineated by a pair of PADs is:

Byte stuffing

Since BISYNC uses a lot of binary characters (STX, SYNC) for control purpose, a pair of special characters DLE is used to quote the binary data so that the receiver will transparently treat the bracket data. Use of DLE to quote data is called byte stuffing as given below:

BSC Line States

There are FOUR line states that BSC will be used to transfer data. Initially, BSC will be in disconnected state until it receives an event, it will then change to other state and take appropriate action.

Data Flow

The principal means of flow control is to lay down rules to control if a sender can transmit a frame. For simplicity, both send and receive party can be considered to consist of a sender and a receiver as shown in Figure

BSC uses simple stop-and-wait protocol. The sender, after sending a frame, will stop and wait for an acknowledgment frame (from the receiver of the peer party) before sending another frame. The data link layer frames of such a protocol may consist of an information and acknowledge frame as shown in Figure .

The general frame flow diagram is given in Figure

	Primary Stations
	It is responsible for controlling data link by issuing commands such as Receive Ready.
	Secondary
	It receives commands from the primary and return responses.

	Link Initialization & Disconnection
	Sliding Window (The normal value is 7 for 3-bit control field). It consists of
	Sending Window
	Receiving Window
	Error Handling (about the receipt of corrupted message(s))
	Go Back N, ask the sender to repeat the whole sequence starting from the first rejected message. (Some frames had been receivd by the receiver. As a result, it is less efficient.)
	Selective repeat, just to ask the sender to send the message(S) that was/were rejected by use of SREJ message. (It is more efficient. However, it needs buffer to store the previously received frames.)

	Bad Address field
	Fewer than 32 bits between flags
	Information field is not a multiple of 8 bits

	List the FOUR types of supervisory frames.
	List the THREE modes defined by HDLC.
	What is the function of the P-bit?
	When is a FRMR sent instead of a REJ?
	What is a problem of having a large window size?
	What is the function of a sending window?

	How are frames delineated in BSC? (What character, PAD, STX, ETX...)
	How to fix the problem of duplicated frames?
	What is the function of the EOT character?

	UA is sent in response to SNRM.
	RNR is used to respond to RR.
	Information frame must be responded by Information frame .
	REJ is sent against all the invalid frames for physical layer operating at half duplex mode.