Advantages and disadvantages

Transcription

1 Advantages and disadvantages Advantages Disadvantages Asynchronous transmission Simple, doesn't require synchronization of both communication sides Cheap, timing is not as critical as for synchronous transmission, therefore hardware can be made cheaper Set-up is very fast, so well suited for applications where messages are generated at irregular intervals, for example data entry from the keyboard Large relative overhead, a high proportion of the transmitted bits are uniquely for control purposes and thus carry no useful information Synchronous transmission Lower overhead and thus, greater throughput Slightly more complex Hardware is more expensive

2 3.3 Data Transmission Modes The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous. Figure 3.3: Data transmission and modes Serial and Parallel Serial Transmission In serial transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices. The advantage of serial over parallel transmission is that with only one communication channel, serial transmission reduces cost of transmission over parallel by roughly a factor of n. Since communication within devices is parallel, conversion devices are required at the interface between the sender and the line (parallel-to-serial) and between the line and the receiver (serial-

3 to-parallel). Serial transmission occurs in one of three ways: asynchronous, synchronous, and isochronous. Parallel Transmission Binary data, consisting of 1 s and 0 s, may be organized into groups of n bits each. Computers produce and consume data in groups of bits much as we conceive of and use spoken language in the form of words rather than letters. By grouping, we can send data n bits at a time instead of 1. This is called parallel transmission. The mechanism for parallel transmission is a simple one: Use n wires to send n bits at one time. That way each bit has its own wire, and all n bits of one group can be transmitted with each clock tick from one device to another. The advantage of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor on n over serial transmission. But there is a significant disadvantage: cost. Parallel transmission requires n communication lines just to transmit the data stream. Because this is expensive, parallel transmission is usually limited to short distances Simplex, Half duplex and Full duplex There are three modes of data transmission that correspond to the three types of circuits available. These are: a) Simplex b) Half-duplex c) Full-duplex Figure 3.4: Different Modes of Data Transmission Simplex

4 Simplex communications imply a simple method of communicating, which they are. In simplex communication mode, there is a one-way communication transmission. Television transmission is a good example of simplex communications. The main transmitter sends out a signal (broadcast), but it does not expect a reply as the receiving units cannot issue a reply back to the transmitter. A data collection terminal on a factory floor or a line printer (receive only). Another example of simplex communication is a keyboard attached to a computer because the keyboard can only send data to the computer. At first thought it might appear adequate for many types of application in which flow of information is unidirectional. However, in almost all data processing applications, communication in both directions is required. Even for a one-way flow of information from a terminal to computer, the system will be designed to allow the computer to signal the terminal that data has been received. Without this capability, the remote used might enter data and never know that it was not received by the other terminal. Hence, simplex circuits are seldom used because a return path is generally needed to send acknowledgement, control or error signals. Half-duplex In half-duplex mode, both units communicate over the same medium, but only one unit can send at a time. While one is in send mode, the other unit is in receiving mode. It is like two polite people talking to each other one talks, the other listens, but neither one talks at the same time. Thus, a half duplex line can alternately send and receive data. It requires two wires. This is the most common type of transmission for voice communications because only one person is supposed to speak at a time. It is also used to connect a terminal with a computer. The terminal might transmit data and then the computer responds with an acknowledgement. The transmission of data to and from a hard disk is also done in half duplex mode. Full duplex In a half-duplex system, the line must be turned around each time the direction is reversed. This involves a special switching circuit and requires a small amount of time (approximately 150 milliseconds). With high speed capabilities of the computer, this turn-around time is unacceptable in many instances. Also, some applications require simultaneous transmission in both directions. In such cases, a full-duplex system is used that allows information to flow simultaneously in both directions on the transmission path. Use of a full-duplex line improves efficiency as the line turnaround time required in a half-duplex arrangement is eliminated. It requires four wires Synchronous and Asynchronous transmission Synchronous Transmission In synchronous transmission, the bit stream is combined into longer frames, which may contain multiple bytes. Each byte, however, is introduced onto the transmission link without a gap between it and the next one. It is left to the receiver to separate the bit stream into bytes for decoding purpose. In other words, data are transmitted as an unbroken sting of 1s and 0s, and the receiver separates that string into the bytes, or characters, it needs to reconstruct the information. Without gaps and start and stop bits, there is no built-in mechanism to help the receiving device adjust its bits synchronization midstream. Timing becomes very important, therefore, because the accuracy of the received information is completely dependent on the ability of the receiving device to keep an accurate count of the bits as they come in.

5 The advantage of synchronous transmission is speed. With no extra bits or gaps to introduce at the sending end and remove at the receiving end, and, by extension, with fewer bits to move across the link, synchronous transmission is faster than asynchronous transmission of data from one computer to another. Byte synchronization is accomplished in the data link layer. Figure 3.5: Synchronous transmission Asynchronous Transmission Asynchronous transmission is so named because the timing of a signal is unimportant. Instead, information is received and translated by agreed upon patterns. As long as those patterns are followed, the receiving device can retrieve the information without regard to the rhythm in which it is sent. Patterns are based on grouping the bit stream into bytes. Each group, usually 8 bits, is sent along the link as a unit. The sending system handles each group independently, relaying it to the link whenever ready, without regard to t timer. Without synchronization, the receiver cannot use timing to predict when the next group will arrive. To alert the receiver to the arrival of an new group, therefore, an extra bit is added to the beginning of each byte. This bit, usually a 0, is called the start bit. To let the receiver know that the byte is finished, 1 or more additional bits are appended to the end of the byte. These bits, usually 1s, are called stop bits. By this method, each byte is increased in size to at least 10 bits, of which 8 bits is information and 2 bits or more are signals to the receiver. In addition, the transmission of each byte may then be followed by a gap of varying duration. This gap can be represented either by an idle channel or by a stream of additional stop bits. The start and stop bits and the gap alert the receiver to the beginning and end of the each byte and also it to synchronize with the data stream. This mechanism is called asynchronous because, at the byte level, the sender and receiver do not have to be synchronized. But within each byte, the receiver must still by synchronized with the incoming bit stream. That is, some synchronization is required, but only for the duration of a single byte. The receiving device resynchronizes at the onset of each new byte. When the receiver detects a start bit, it sets

6 a timer and begins counting bits as they come in. after n bits, the receiver looks for a stop bit. As soon as it detects the stop bit, it waits until it detects the next start bit. Figure 3.6: Asynchronous transmission Isochronous Transmission In real-time audio and video, in which uneven delays between frames are not acceptable, synchronous transmission fails. For example, TV images are broadcast at the rate of 30 images per second; they must be viewed at the same rate. If each image is send by using one or more frames, there should be no delays between frames. For this type of application, synchronization between characters is not enough; the entire stream of bits must be synchronized. The isochronous transmission guarantees that the data arrive at a fixed rate. Self Assessment Questions: State whether the following statements are True or False: 4. Serial transmission is costlier than parallel transmission. 5. In half duplex, transmission is done at a time from the sender and receiver. 6. In full duplex turn-around time is eliminated to the sender and the receiver. 7. In asynchronous transmission the bits are received by patterns 3.4 Switching A network is a set of connected devices. Whenever we have multiple devices, we have the problem of how to connect them to make one-to-one communication possible. One of the better solutions is switching. A switch is network consists of a series of interlinked nodes, called switches. Switches are devices capable of crating temporary connections between two or more devices linked to the switch. In a switched network, some of these nodes are connected to the end systems (computers or telephones). Others are used only for routing. Switched networks are divided, as shown in the figure.

7 Figure 3.7: Different types of switching techniques Circuit switching A circuit switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. It is mainly used for telephones to call from one to one. Figure 3.8: Circuit switching in telephone In the figure, each office has three incoming lines and three outgoing lines. When call passes through a switching office, a physical connection is established between the line on which the call came in and one of the output lines, as shown by the dotted lines. An important property of circuit switching is the need to set up an end-to-end path before any data can be sent. The elapsed time between the end of dialing and the start of ringing can easily be 10 sec, more on long-distance or international calls. Before data transmissions begin, the destination telephone should give acknowledgement. Once call setup, the only delay for data is the propagation time for the electromagnetic signal, about 5 msec per 1000 km. There is no problem of congestion Message switching The message switching is used to transfer the messages form one end to other end. There is no physical path is establishes in advance between the sender and receiver. Initially the messages were converted to Morse code i.e. in the form of dots and dashes. Each dot or dash was communicated by transmitting short and long pulses of electrical current over a copper wire. In this method a human operator was needed to encode the text messages, routing decision, error checking. Here also we use the concept of store-and forward, where the entire messages were

8 received fully, inspected for errors, and then transmitted to the destinations. The same method with slight change we are following in our applications Packet switching In packet switching, we transfer the messages in terms of small block fixed sizes called packets. In packet switching, there is no path; packets are routed independently by sharing the network at time to time, by following the best path to the destination. Packets can be in order to the destination. Packet switching is more fault tolerant than circuit switching. The store-and-forward transmission is used to route to the destination, while storing the packet in the routers main memory. Congestion may occur when more packets are sending from the various hosts Comparison of switching techniques Self Assessment Questions: Parallel Transmission a. Binary data consisting of 1s and 0s may be organized into groups of n bits each b. By grouping we can send data n bits at a time instead of one bit

9 Serial Transmission a. One bit follows another, so we need only one channel rather than n to transmit data between two devices b. Conversion devices are required at the interface Advantage COST Types of Serial Transmission There are two types of Serial Transmission: Asychronous Transmission Synchronous Transmission Asynchronous Transmission a. It is so named because the timing of the signal is unimportant. Instead information is received and translated by agreed upon patterns b. Start and Stop Bits Asynchronous Transmission Advantages Cheap Effective Disadvantages Slow Synchronous Transmission

10 Data is transmitted as an unbroken string of 1 s and 0 s and the receiver separates that string into the bytes or characters it need to reconstruct the information Synchronous Transmission Advantage Speed Figure Different modes of transmission Parallel Transmission Parallel transmission involves grouping several bits, say n, together and sending all the n bits at a time. Figure shows how parallel transmission occurs for n = 8. This can be accomplishes with the help of eight wires bundled together in the form of a cable with a connector at each end. Additional wires, such as request (req) and acknowledgement (ack) are required for asynchronous transmission. Primary advantage of parallel transmission is higher speed, which is achieved at the expense of higher cost of cabling. As this is expensive for longer distances, parallel transmission is feasible only for short distances. Figure Parallel mode of communication with n = 8 Serial Transmission

11 Serial transmission involves sending one data bit at a time. Figure shows how serial transmission occurs. It uses a pair of wire for communication of data in bit-serial form. Version 2 CSE IIT, Kharagpur Since communication within devices is parallel, it needs parallel-to-serial and serial-to- parallel conversion at both ends. Serial mode of communication widely used because of the following advantages: Reduced cost of cabling: Lesser number of wires is required as compared to parallel connection Reduced cross talk: Lesser number of wires result in reduced cross talk Availability of suitable communication media Inherent device characteristics: Many devices are inherently serial in nature Portable devices like PDAs, etc use serial communication to reduce the size of the connector

12 However, it is slower than parallel mode of communication. There are two basic approaches for serial communication to achieve synchronization of data transfer between the source-destination pair. These are referred to as asynchronous and synchronous. In the first case, data are transmitted in small sizes, say character by character, to avoid timing problem and make data transfer selfsynchronizing, as discussed later. However, it is not very efficient because of large overhead. To overcome this problem, synchronous mode is used. In synchronous mode, a block with large number of bits can be sent at a time. However, this requires tight synchronization between the transmitter and receiver clocks. Figure Serial mode of communication Direction of data flow: There are three possible modes in serial communication: simplex, full duplex and half duplex. In simplex mode, the communication is unidirectional, such as from a computer to a printer, as shown in Fig (a). In full-duplex mode both the sides can communicate simultaneously, as shown in Fig (b). On the other hand, in halfduplex mode of communication, each station can both send and receive data, as shown in Fig (c). But, when one is sending, the other one can only receive and vice versa. Version 2 CSE IIT, Kharagpur (a)

13 (b) (c) Figure Direction of data flow Framing and Synchronization Why Framing and Synchronization? Normally, units of data transfer are larger than a single analog or digital encoding symbol. It is necessary to recover clock information for both the signal (so we can recover the right number of symbols and recover each symbol as accurately as possible), and obtain synchronization for larger units of data (such as data words and frames). It is necessary to recover the data in words or blocks because this is the only way the receiver process will be able to interpret the data received; for a given bit stream. Depending on the byte boundaries, there will be seven or eight ways to interpret the bit stream as ASCII characters, and these are likely to be very different. So, it is necessary to add other bits to the block that convey control information used in the data link control procedures. The data along with preamble, postamble, and control information forms a frame. This framing is necessary for the purpose of synchronization and other data control functions Synchronization Data sent by a sender in bit-serial form through a medium must be correctly interpreted

14 at the receiving end. This requires that the beginning, the end and logic level and duration of each bit as sent at the transmitting end must be recognized at the receiving end. There are three synchronization levels: Bit, Character and Frame. Moreover, to achieve synchronization, two approaches known as asynchronous and synchronous transmissions are used. Frame synchronization is the process by which incoming frame alignment signals (i.e., distinctive bit sequences) are identified, i.e. distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission. The usual Version 2 CSE IIT, Kharagpur practice is to insert, in a dedicated time slot within the frame, a non-information bit that is used for the actual synchronization of the incoming data with the receiver. In order to receive bits in the first place, the receiver must be able to determine how fast bits are being sent and when it has received a signal symbol. Further, the receiver needs to be able to determine what the relationship of the bits in the received stream have to one another, that is, what the logical units of transfer are, and where each received bit fits into the logical units. We call these logical units frames. This means that in addition to bit (or transmission symbol) synchronization, the receiver needs word and frame synchronization Synchronous communication (bit-oriented)

15 Timing is recovered from the signal itself (by the carrier if the signal is analog, or by regular transitions in the data signal or by a separate clock line if the signal is digital). Scrambling is often used to ensure frequent transitions needed. The data transmitted may be of any bit length, but is often constrained by the frame transfer protocol (data link or MAC protocol). Bit-oriented framing only assumes that bit synchronization has been achieved by the underlying hardware, and the incoming bit stream is scanned at all possible bit positions for special patterns generated by the sender. The sender uses a special pattern (a flag pattern) to delimit frames (one flag at each end), and has to provide for data transparency by use of bit stuffing (see below). A commonly used flag pattern is HDLC's flag as shown in Fig The bit sequence is used for both preamble and postamble for the purpose of synchronization. A frame format for bit-oriented synchronous frame is shown in Fig Apart from the flag bits there are control fields. This field contains the commands, responses and sequences numbers used to maintain the data flow accountability of the link, defines the functions of the frame and initiates the logic to control the movement of traffic between sending and receiving stations (a) Specific pattern

16 to represent start of frame (b) Specific pattern to represent end of frame Figure Bit oriented framing (a) Data to be sent to the peer, (b) Data after being character stuffed. Version 2 CSE IIT, Kharagpur Figure Frame format for synchronous communication Summary of the approach:

17 Initially 1 or 2 synchronization characters are sent Data characters are then continuously sent without any extra bits At the end, some error detection data is sent Advantages: Much less overhead No overhead is incurred except for synchronization characters Disadvantages: No tolerance in clock frequency is allowed The clock frequency should be same at both the sending and receiving ends Bit stuffing: If the flag pattern appears anywhere in the header or data of a frame, then the receiver may prematurely detect the start or end of the received frame. To overcome this problem, the sender makes sure that the frame body it sends has no flags in it at any position (note that since there is no character synchronization, the flag pattern can start at any bit location within the stream). It does this by bit stuffing, inserting an extra bit in any pattern that is beginning to look like a flag. In HDLC, whenever 5 consecutive 1's are encountered in the data, a 0 is inserted after the 5th 1, regardless of the next bit in the data as shown in Fig On the receiving end, the bit stream is piped through a shift register as the receiver looks for the flag pattern. If 5 consecutive 1's followed by a 0 is seen, then the 0 is dropped before sending the data on (the receiver destuffs the stream). If 6 1's and a 0 are seen, it is a flag and either the current frame are ended or a new frame is started, depending on the current state of the receiver. If more than 6 consecutive 1's

18 are seen, then the receiver has detected an invalid pattern, and usually the current frame, if any, is discarded. a). b) Specific pattern to represent start of frame s stuffed after every five 1 s Specific pattern to represent end of frame Figure Bit oriented (a) Data to be sent to the peer, (b) Data after being bit stuffed. Version 2 CSE IIT, Kharagpur

19 With bit stuffing, the boundary between two frames can be unambiguously recognized by the flag pattern. Thus, if receiver loses track of where it is, all it has to do is to scan the input for flag sequence, since they can only occur at frame boundaries and never within data. In addition to receiving the data in logical units called frames, the receiver should have some way of determining if the data has been corrupted or not. If it has been corrupted, it is desirable not only to realize that, but also to make an attempt to obtain the correct data. This process is called error detection and error correction, which will be discussed in the next lesson Asynchronous communication (word-oriented) In asynchronous communication, small, fixed-length words (usually 5 to 9 bits long) are transferred without any clock line or clock is recovered from the signal itself. Each word has a start bit (usually as a 0) before the first data bit of the word and a stop bit (usually as a 1) after the last data bit of the word, as shown in Fig The receiver's local clock is started when the receiver detects the 1-0 transition of the start bit, and the line is sampled in the middle of the fixed bit intervals (a bit interval is the inverse of the data rate). The sender outputs the bit at the agreed-upon rate, holding the line in the appropriate state for one bit interval for each bit, but using its own local clock to determine the length of these bit intervals. The receiver's clock and the sender's clock may not run at the same speed, so that there is a relative clock drift (this may be caused by variations in the crystals used, temperature, voltage, etc.). If the receiver's clock drifts too much relative to the sender's clock, then the bits may be sampled while the line is in transition from one state to another, causing the receiver to misinterpret the received data. There can be variable amount of gap between two frames as shown in Fig

20 Figure Character or word oriented format for asynchronous mode Version 2 CSE IIT, Kharagpur Figure Data units sent with variable gap sent in asynchronous mode Advantages of asynchronous character oriented mode of communication are summarized below: Simple to implement Self synchronization; Clock signal need not be sent Tolerance in clock frequency is possible The bits are sensed in the middle hence ± ½ bit tolerance is provided This mode of data communication, however, suffers from high overhead incurred in data transmission. Data must be sent in multiples of the data length of the word, and the two or more bits of synchronization overhead compared to the relatively short data length causes the effective data rate to be rather low. For example, 11 bits are required to transmit 8 bits of data. In other words, baud rate (number of signal elements) is higher