Module 3

Digital data transmission – Serial, Parallel, Synchronous, Asynchronous and Isochronous transmission. Transmission mode-    Simplex - Half duplex – Full duplex, Noise- different types of noise – Basic Principles of Switching (circuit, packet, message switching)

 

 

Timing

Timing refers to how the receiving system knows that it received the start of a group of bits and the end of a group of bits. Two major timing schemes are used: Asynchronous and Synchronous Transmission.

1. Asynchronous Transmission sends only 1 character at a time. A character being a letter of the alphabet or number or control character. Preceding each character is a Start bit and ending each character is 1 or more Stop bits.

2. Synchronous Transmission sends packets of characters at a time. Each packet is preceded by a Start Frame which is used to tell the receiving station that a new packet of characters is arriving and to synchronize the receiving station's internal clock. The packets also have End Frames to indicate the end of the packet. The packet can contain up to 64,000 bits. Both Start and End Frames have a special bit sequence that the receiving station recognizes to indicate the start and end of a packet. The Start and End frames may be only 2 bytes each.

Conventional representation has asynchronous data flowing left to right and synchronous data flowing right to left.

 

 Asynchronous vs. Synchronous Transmission

Asynchronous transmission is simple and inexpensive to implement. It is used mainly with Serial Ports and dialup connections. Requires start and stop bits for each character - this adds a high overhead to transmission. For example: for every byte of data, add 1 Start Bit and 2 Stop Bits. 11 bits are required to send 8 bits! Asynchronous is used in slow transfer rates typically up to 56 kbps.

Synchronous transmission is more efficient as little as only 4 bytes (3 Start Framing bytes and 1 Stop Framing byte) are required to transmit up to 64 kbits. Synchronous transmission is more difficult and expensive to implement. It is used with all higher comunication transfer rates: Ethernet, Token Ring etc... Synchronous is used in fast transfer rates typically 56 kbps to 100 Mbps.

Historically, synchronous communications were operating over 2400/4800 baud modems on point-to-point communications, for example: IBM2770/IBM2780/IBM3780 (historical information courtesy of Jacques Sincennes, University of Ottawa)

 Asynchronous Communications

Asynchronous communications or transmission sends individual characters one at a time framed by a start bit and 1 or 2 stop bits.

Start/Stop bits

The purpose of the Start bit is to notify the receiving station of a new character arriving. Typically data is shown moving left to right. This is how it would appear on a Storage Oscilloscope or Network Analyser. The MSB ( Most Significant Bit) is sent first and the LSB (Least Significant Bit) is sent last.

The purpose of the Stop bits is to indicate the end of data. There could be 1 or 2 stop bits with 1 being the typical number of stop bits used today. In Asynchronous transmission, the characters are sent individually with a quiet period in between (quiet meaning 0 bit level). Asynchronous communications requires the transmitting station and the receiving station to have individual internal free-running clocks operating at the same frequency. Free-running means that the clocks are not locked together.

Both clocks operating at same frequency:

The receive station starts checking for data after the Start bit is received (Start bit is a wake up call!).

The receive station samples the transmitted data in the middle of each data bit. The samples are evenly spaced and match the transmitted data because both transmit and receive clocks are operating at the same frequency.

Receive clock frequency higher than transmitted frequency:

If the receive station's clock is higher in frequency, the samples will be spaced closer together (higher frequency - shorter period). In the above example, we transmitted the following data: 0100 1010 but we received the data: 0100 0101. The samples are out of synchronization with the transmitting data. We would have an error in receiving data.

Clocks are controlled by crystals (abbreviated: Xtal). Crystals are metal cans that hold a piezo-electric element that resonates at a certain frequency when a voltage is applied to it. If you drop a crystal or a printed circuit board (PCB) that has a crystal on it, the crystal can fracture inside the metal can. Either it will stop working or change its frequency, both result in a malfunctioning circuit! Crystals are also temperature sensitive and change frequency with temperature!

Receive clock frequency lower than transmitted frequency:

If the receiving station's clock is lower in frequency than the transmitted frequency, then the samples become farther apart (lower frequency - wider period). Again the samples become out of sync with the transmitted data!

The transmitted data is 0100 1010 but the receive data is 0101 0101! Again we would have receive data errors.

This is a basic problem with asynchronous communications, both transmitter and receiver require a very stable clock to work properly. At high frequencies (which result in high transfer rates), clock stability is critical and asynchronous transmission is very difficult to accomplish. Because of this inherent problem with asynchronous transmission, it is used at low frequency/slow transfer rates.

 

Data Transmission Modes

The term transmission mode defines the direction of data flow between two linked devices. The manner or way in which data is transmitted from one place to another is called Data Transmission Mode. There are three ways for transmitting data from one location to another. These are:

1. Simplex mode
2. Half-Duplex mode
3. Full-Duplex mode

1. Simplex Mode

In Simplex mode, the communication can take place in only one direction. In this mode, a terminal can only send data and cannot receive it or it can only receive data but cannot send it. It means that in this mode communication is uni-directional. Today, this mode of data communication is not popular, because most of the modem communications require two-way exchange of data. However, this mode of communication is used in business field at certain point-of-sale terminals in which sales data is entered without a corresponding reply. The other examples of simplex communication modes are Radio and T.V transmissions.

In computer system, the keyboard, monitor and printer are examples of simplex devices. The keyboard can only be used to enter data into computer, while monitor and printer can only accept (display/print) output.

2. Half-Duplex Mode

In Half-duplex mode, the communication can take place in both directions, but only in one direction at a time. In this mode, data is sent and received alternatively. It is like a one-lane bridge where two-way traffic must give way in order to cross the other.

In half-duplex mode, at a time only one end transmits data while other end receives. In addition, it is possible to perform error detection and request the sender to re-transmit information. The Internet browsing is an example of half duplex. When we issue a request to download a web document, then that document is downloaded and displayed before we issue another request.

3. Full-Duplex Mode

In Full-duplex mode, the communication can take place in both directions simultaneously, i.e. at the same time on the same channel. It is the fastest directional mode of communication. Example of this mode is conversation of the persons through telephone. This type of communication is similar to automobile traffic on a two-lane road. The telephone communication system is an example of MI-duplex communication mode.

Types of Data Transmission Modes

There are two types of data transmission modes. These are:

1. Parallel Transmission
2. Serial Transmission

1. Parallel Transmission

In parallel transmission, bits of data flow concurrently through separate communication lines. Parallel transmission is shown in figure below. The automobile traffic on a multi-lane highway is an example of parallel transmission. Inside the computer binary data flows from one unit to another using parallel mode. If the computer uses 32-bk internal structure, all the 32-bits of data are transferred simultaneously on 32-lane connections. Similarly, parallel transmission is commonly used to transfer data from computer to printer. The printer is connected to the parallel port of computer and parallel cable that has many wires is used to connect the printer to computer. It is very fast data transmission mode.

2. Serial Transmission

In serial data transmission, bits of data flow in sequential order through single communication line. Serial dat& transmission is shown in figure below. The flow of traffic on one-lane residential street is an example of serial data transmission mode. Serial transmission is typically slower than parallel transmission, because data is sent sequentially in a bit-by-bit fashion. Serial mouse uses serial transmission mode in computer.

Synchronous & Asynchronous Transmissions

Synchronous Transmission

In synchronous transmission, large volumes of information can be transmitted at a time. In this type of transmission, data is transmitted block-by-block or word-byword simultaneously. Each block may contain several bytes of data. In synchronous transmission, a special communication device known as synchronized clock' is required to schedule the transmission of information. This special communication device or equipment is expensive.

Asynchronous Transmission

In asynchronous transmission, data is transmitted one byte at a 'time'. This type of transmission is most commonly used by microcomputers. The data is transmitted character-by-character as the user types it on a keyboard.

An asynchronous line that is idle (not being used) is identified with a value 1, also known as 'Mark' state. This value is used by the communication devices to find whether the line is idle or disconnected. When a character (or byte) is about to be transmitted, a start bit is sent. A start bit has a value of 0, also called a space state. Thus, when the line switches from a value of 1 to a value of 0, the receiver is alerted that a character is coming.

 

Circuit switching

In circuit switching, transmission between a source and the destination is achieved through a dedicated physical link for the entire duration of transmission. The entire link remains dedicated and no other potential and/or emergent users can use it even when the path happens to be idle. Circuit switching is used in telephones. A circuit switching telephone circuit is only 30-40 per cent efficient as most of the time is spent on the listening mode. Synchronous transfer mode (STM) uses circuit switching. The common-T carrier for digitised voice uses STM.

In order to avoid the irregular gaps/delays in between words and/or bunches of words, synchronised time-division multiplexing is used in voice communication. Each slot in the multiplexing system is assigned to a voice call and thereby the access is guaranteed as long as the call lasts. All the multiplexed time slots form a frame. Each slot in the frame is synchronised through the frame bit and position.

Advantages of Circuit Switching:

    * Once the circuit has been set up, communication is fast and without error.

    * It is highly reliable

Disadvantages:

    * Involves a lot of overhead, during channel set up.

    * Waists a lot of bandwidth, especial in speech whereby a user is sometimes listening, and not talking.

    * Channel set up may take longer.

 

To overcome the disadvantages of circuit switching, packet switching was introduced, and instead of dedicating a channel to only two parties for the duration of the call it routes packets individually as they are available. This mechanism is referred to as being connectionless.

 

Packet Switching

Since its introduction in the early 1970s, packet switching has received widespread acceptance. Public networks have been constructed in most developed countries and many developing countries. The internetwork ITU-T X.75 protocol provides for interlinking of national networks at an international level. The ITU-T X.25 Recommendation is the original standard for packet-switching architecture.

Packet switching has several advantages over conventional circuit-switched networks. The circuit-switched network maintains a fixed bandwidth between the transmitter and receiver for the duration of a call. Also, the circuit-switched network is bit stream transparent, meaning it is not concerned with the data content or error-checking process. This is not the case for packet switching, where bandwidth is allocated dynamically on an "as required" basis. Data is transmitted in packets, each containing a header that contains the destination of the packet and a tail, or footer, for error-checking information. Packets from different sources can coexist on the same customer-to-network physical link without interference. The simultaneous call and variable bandwidth facilities improve the efficiency of the overall network. The buffering in the system which allows terminals operating at different bit rates to interwork with each other is a significant advantage of packet switching. The obvious disadvantage is the extra dimension of complexity with respect to the switches and network-to-customer protocol.

Furthermore, in certain circumstances, packet switching has several advantages over other methods of data communication:

1. Packet switching might be more economical than using private lines if the amount of traffic between terminals does not warrant a dedicated circuit.

2. Packet switching might be more economical than dialed data when the data communication sessions are shorter than a telephone call minimum chargeable time unit.

3. Destination information is contained in each packet, so numerous messages can be sent very quickly to many different destinations. The rate depends on how fast the data terminal equipment (DTE) can transmit the packets,

4. Computers at each node allow dynamic data routing. This inherent intelligence in the network picks the best possible route for a packet to take through the network at any particular time. Throughput and efficiency are therefore maximized.

5. The packet network inherent intelligence also allows graceful degradation of the network in the event of a node or path (link) failure. Automatic rerouting of the packets around the failed area causes more congestion in those areas, but the overall system is still operable.

6. Other features of this intelligence are error detection and correction, fault diagnosis, verification of message delivery, message sequence checking, reverse billing (charging), etc.

 

Message switching

A form of store-and-forward switching known as message switching has existed for many decades in telegraphy. e-mail is a latest example of message switching.

The message, a meaningful information or a set of data, as a whole is sent from a source to its nearest node from where, depending on the availability of free link, it is either passed or stored for next change.

A major problem in message switching is that if a message is long, an urgent message may have to be sacrificed for an ordinary one, because once a message is on transmission, it cannot be stopped. In order to avoid this problem and to make message switching more practical, the concept of packet switching was introduced.

Comparison

A packet switching network is expected to deliver its packets in a fraction of a second, whereas a message switching network is expected to deliver a message typically in a fraction of an hour.

A packet switching mode deletes a message from the memory as soon as an acknowledgement of correct receipt is received from the next node. On the contrary, message switching system files the message for any possible retrieval in future. Due to this, a message switching network has a mesh topology where no particular node dominates the structure. Thus, message switching incorporates the advantage of both circuit switching and message switching.

In packet switching, there are two approaches—virtual circuit and datagram—for transmitting data from a source to the destination.In virtual circuit approach, a logical connection between the source and the destination is established prior to transmission of packets. The approach is similar to circuit switching, but the established path is not physically dedicated and may be shared by other sets of users. In the datagram service, each packet uses a path throughout the network in between a source and a destination based on current information available to the node.

The basic difference between the virtual circuit and datagram approach is that the node does not take a routing decision for each packet in virtual circuit approach as is done in datagram approach.

Circuit switching is best suited for time-sensitive communications, like voice, because this service does not tolerate delay and jittering. Low delay and avoidance of jittering are possible with circuit switching. On the other hand, packet switching is suitable for time-insensitive communications, like data, because it does not tolerate error but can tolerate delay. Circuit switching can be considered equivalent to packet switching where one packet in the information is transmitted over entire duration of the call.

 

Labels: computer networks, data communication

Labels: computer networks, data communication