DATA LINK CONTROL

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Outline

LINK THOUGHPUT FLOW CONTROL LINK MANAGEMENT HIGH–LEVEL DATA LINK CONTROL (HDLC)

PROTOCOL POINT-TO POINT PROTOCOL

The major factors that cause throughput to be less than the transmission rate are listed below as follows:Frame overheadsPropagation delayAcknowledgementsRetransmissions Processing time Link utilization and efficiencyEffect of errors on throughputEffect of ARQ on throughput Optimum block length

Frame overheads

Frame overheadsLINK THOUGHPUT

: Not all of the contents of a frame are information bits. Typically, in addition to information bits, a frame also contains a header and a trailer. The header contains control information such as an address and sequence numbers. The trailer normally contains error-checking bits which are often called a frame check sequence. A frame might typically contain 256 bytes of which 251 are information bits, thus leading to a 2% reduction in potential throughput even before the frame is transmitted.

Frame overheads

Frame overheadsLINK THOUGHPUT

: This is the time that it takes for a frame to propagate from one end of a link to the other; that is, the difference in time between the first bit of a frame leaving the send node and arriving at the receive node. Propagation delay must not be confused with the frame transmission time, which is the difference in time between the first bit and the last bit of a frame leaving the send node. Propagation delay often has only a small effect on throughput but in some situations, such as long-distance wireless links and especially satellite links, it can be a major factor in reducing the throughput if acknowledgements are used.

Propagation delay

Propagation delayLINK THOUGHPUT

: Normally, some form of ARQ is used and time may be spent waiting for acknowledgements to reach the send node, particularly if there is a half duplex link. Since the acknowledgements will normally be much shorter than the information frames, the transmission time of the acknowledgements can often be ignored. However, the propagation delay of an acknowledgement will be the same as that of an information frame providing they take the same transmission path.

Acknowledgements

AcknowledgementsLINK THOUGHPUT

: Frames may need to be retransmitted as a result of errors or frames being discarded for whatever reason. The retransmission is accompanied by acknowledgements if ARQ is being used. If the error rate or discard rate is high then this is the most serious cause of reduction in throughput.

Retransmissions Retransmissions LINK THOUGHPUT

: Time is spent at the send and receive nodes in processing the data. This includes detecting (and possibly correcting) errors and also the implementation of flow control. If wireless links are used there will be further processing delays associated with the modulation and demodulation process.

Processing time Processing time LINK THOUGHPUT

Define two further terms1. link utilization

Link utilization is simply the average traffic over a particular link expressed as a percentage of the total link capacity

2. link efficiency. Link efficiency is a less commonly used term that is defined as the ratio of the time taken to transmit a frame (or frames) of data to the total time it takes to transmit and acknowledge the frame or frames.

Link utilization and efficiency

Link utilization and efficiencyLINK THOUGHPUT

Link efficiency will depend on the type of ARQ used. The efficiency of a link with stop-and-wait ARQ can be determined as follows. If the time taken to transmit a frame or block of data is tf, the propagation delay for both frame and acknowledgement is td, the time taken to transmit an acknowledgement is ta and the total processing time is tp, then:

In many situations the acknowledgement transmission time and processing times can be ignored, giving:

Link utilization and efficiency

Link utilization and efficiencyLINK THOUGHPUT

The effect of transmission impairments on a data communications link is to introduce errors . The number of errors present in a link is expressed as a BER. If a link has a BER of 0.000 001 (10−6), this means that there is a probability of 0.000 001 that any bit is in error. Alternatively, we can say that, on average, one in every 1 000 000 bits will be in error. This may seem a very low error rate but if bits are transmitted as a block in a frame then the probability of the frame being in error will be much greater. The frame error rate, P, can be obtained from the bit error rate, E, as follows. The probability of a bit being error free is 1 − E and the probability of a block of length n being error free is (1 − E)n. The frame error probability is therefore:

Effect of errors on throughput

Effect of errors on throughputLINK THOUGHPUT

: If a frame is transmitted m times then the probability of this occurring is the probability of transmitting m − 1 consecutive erroneous frames followed by a single correctly received frame. This is given by:

A problem which arises is that the number of times, m, that a frame is transmitted will vary according to some form of probability distribution. Since the determination of the value of m is not particularly simple, the value is just presented here without any analysis. For a full analysis see Bertsekas and Gallager (1992). If stop-and-wait ARQ is used then the average number of times that a frame is transmitted is given by:

Effect of ARQ on throughput

Effect of ARQ on throughputLINK THOUGHPUT

If go-back-n ARQ is used then an error detected in a frame causes that frame, along with all other unacknowledged frames, to be retransmitted.

1. Frames are retransmitted only when a frame is rejected at the receiver for being erroneous. In practice, there may be other reasons for frames being retransmitted.

2. The rejection of frame i by the receiver is followed by the transmitter sending frames i + 1, i + 2, . . . , i + n − 1 and then retransmitting the original frame i. This may not always be the case since there may be fewer than n − 1 frames waiting to be transmitted after frame I .

3. The resulting analysis, which is also carried out in Bertsekas and Gallager (1992), gives the number of times that a frame is likely to be transmitted as:

Effect of ARQ on throughput

Effect of ARQ on throughputLINK THOUGHPUT

An increased block size still produces a larger number of information bits transmitted in each block but a point will be reached at which throughput falls as a result of having to retransmit a large block of data each time an error is detected. This leads us to consider an optimum length of block. Figure 5.1

Optimum block length

Optimum block length

LINK THOUGHPUT

A maximum limit is set on the number of copies that are being held at the send node which is known as the send window. If the send node reaches its maximum window size it stops transmitting and, in the absence of any acknowledgements, it does not transmit any more frames. When the

Figure 5.2 Operation of send window: (a) window full; (b) continuous flow possible.

Window mechanisms

Window mechanismsFLOW CONTROL

The throughput of a frame-oriented link using stop-andwait ARQ depends upon the number of information bits, k, the frame transmission time, tf, and the propagation delay, td, according to the expression:

Effect of windows on throughput

Effect of windows on throughputFLOW CONTROL

This process seems almost trivial at first sight but the situation becomes more complex if a failure occurs on a link or at a node. A problem arises when frames have been accepted for transmission over a link but have not reached a receive node before a failure occurs. Link management procedures need to be able to cope with such failures.

Figure 5.4 Link set-up and disconnection.

LINK MANAGEMENT

The protocol allows for a variety of different types of link, the two nodes at either end of the link being referred to as stations. To satisfy the requirements of different types of link the protocol distinguishes between three modes of operation (although only two of them are normally used) and two types of link configuration:

1.Unbalanced configuration: 2.Balanced configuration:

HIGH–LEVEL DATA LINK CONTROL (HDLC) PROTOCOL

This is the situation in which a single primary station has control over the operation of one or more secondary stations. Frames transmitted by the primary are called commands and those by the secondary responses. A typical example of this type of configuration is a multidrop link in which a single computer is connected to a number of DTEs which are under its control. This mode of working is called normal response mode ( NRM).

HIGH–LEVEL DATA LINK CONTROL (HDLC) PROTOCOL

Unbalanced configuration Unbalanced

configuration

This refers to a point-to-point link in which the nodes at each end of the link have equal status, each capable of issuing a command. HDLC calls these combined stations and they can transmit both commands and responses. This mode of working is called asynchronous balanced mode ( ABM ).

HIGH–LEVEL DATA LINK CONTROL (HDLC) PROTOCOL

Balanced configuration

Balanced configuration

HDLC uses synchronous transmission with data being transmitted in frames. All frames have the common format shown in Figure 5.5. The address and control fields are known collectively as a header and the error-checking bits are called the frame check sequence (FCS) or trailer.

HIGH–LEVEL DATA LINK CONTROL (HDLC) PROTOCOL

HDLC frame structure HDLC frame structure

The frame header contains 1-byte address and control fields along with an additional 2-byte protocol field. PPP does not assign individual station addresses and the address field contains the binary sequence 11111111. The control field contains the binary sequence 00000011. The protocol field contains 2 bytes that identify the protocol encapsulated in the information field of the frame. In addition to IP, PPP supports other protocols, including Novell’s Internetwork Packet Exchange ( IPX) and IBM’s Synchronous Network Architecture (SNA). The FCS performs an identical function to that in an HDLC frame.

POINT-TO POINT PROTOCOL

Figure 5.10 PPP frame structure.

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PPP frame structure.

PPP frame structure.

PPP uses a Link Control Protocol ( LCP) to establish, configure and test the data link connection that goes through four distinct phases: Three classes of LCP frames exist. Link-establishment frames are used to establish and configure a link; link-termination frames are used to terminate a link; and link maintenance frames are used to manage and debug a link.

POINT-TO POINT PROTOCOL

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