Group07_Rothmans : Transmission Media

A transmission medium is a material substance (solid, liquid or gas) which can propagate energy waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound.

The absence of a material medium (the vacuum of empty space) can also be thought of as a transmission medium for electromagnetic waves such as light and radio waves. While material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media through which they pass, for instance by absorption or by reflection or refraction at the interfaces between media.

The term transmission medium can also refer to the technical device which employs the material substance to transmit or guide the waves. Thus an optical fiber or a copper cable can be referred to as a transmission medium.

A transmission medium can be classified as a:

1. Linear medium, if different waves at any particular point in the medium can be superposed;
2. Bounded medium, if it is finite in extent, otherwise unbounded medium;
3. Uniform medium or homogeneous medium, if its physical properties are unchanged at different points;
4. Isotropic medium, if its physical properties are the same in different directions.

Electromagnetic radiation can be transmitted through an optical media, such as optical fiber, or through twisted pair wires, coaxial cable, or dielectric-slab waveguides. It may also pass through any physical material which is transparent to the specific wavelength, such as water, air, glass, or concrete. Sound is, by definition, the vibration of matter, so it requires a physical medium for transmission, as does other kinds of mechanical waves and heat energy. Historically, various aether theories were used in science and thought to be necessary to explain the transmission medium. However, it is now known that electromagnetic waves do not require a physical transmission medium, and so can travel through the “vacuum” of free space. Regions of the insulative vacuum can become conductive for electrical conduction through the presence of free electrons, holes, or ions.

Group07_Rothmans : GO BACK N ARQ

Go-Back-N ARQ is a specific instance of the Automatic Repeat-reQuest (ARQ) Protocol, in which the sending process continues to send a number of frames specified by a window size even without receiving an ACK packet from the receiver.
The receiver process keeps track of sequence number of the next frame it expects to receive, and sends that number with every ACK it sends. The receiver will ignore any frame that does not have the exact sequence number it expects — whether that frame is a “past” duplicate of a frame it has already ACK’ed [1] or whether that frame is a “future” frame past the lost packet it is waiting for. Once the sender has sent all of the frames in its window, it will detect that all of the frames since the first lost frame are outstanding, and will go back to sequence number of the last ACK it received from the receiver process and fill its window starting with that frame and continue the process over again.

Go-Back-N ARQ is a more efficient use of a connection than Stop-and-wait ARQ, since unlike waiting for an acknowledgement for each packet, the connection is still being utilized as packets are being sent. In other words, during the time that would otherwise be spent waiting, more packets are being sent. However, this method also results in sending frames multiple times — if any frame was lost or damaged, or the ACK acknowledging them was lost or damaged, then that frame and all following frames in the window (even if they were received without error) will be re-sent. To avoid this, Selective Repeat ARQ can be used.

Group07_Rothmans : GO BACK N ARQ

Stop-and-wait ARQ is the simplest kind of automatic repeat-request (ARQ) method. A stop-and-wait ARQ sender sends one frame at a time. After sending each frame, the sender doesn’t send any further frames until it receives an ACK (acknowledgement) signal. After receiving a good frame, the receiver sends an ACK. If the ACK does not reach the sender before a certain time, known as the timeout, the sender sends the same frame again.

These are problems

1. Typically the transmitter adds a redundancy check number to the end of each frame. The receiver uses the redundancy check number to check for possible damage. If the receiver sees that the frame is good, it sends an ACK. If the receiver sees that the frame is damaged, the receiver discards it and does not send an ACK — pretending that the frame was completely lost, not merely damaged.

2. One problem is where the ACK sent by the receiver is damaged or lost. In this case, the sender doesn’t receive the ACK, times out, and sends the frame again. Now the receiver has two copies of the same frame, and doesn’t know if the second one is a duplicate frame or the next frame of the sequence carrying identical data.

3.Another problem is when the transmission medium has such a long latency that the sender’s timeout runs out before the frame reaches the receiver. In this case the sender resends the same packet. Eventually the receiver gets two copies of the same frame, and sends an ACK for each one. The sender, waiting for a single ACK, receives two ACKs, which may cause problems if it assumes that the second ACK is for the next frame in the sequence.

To avoid these problems

The most common solution is to define a 1 bit sequence number in the header of the frame. This sequence number alternates (from 0 to 1) in subsequent frames. When the receiver sends an ACK, it includes the sequence number of the next packet it expects. This way, the receiver can detect duplicated frames by checking if the frame sequence numbers alternate. If two subsequent frames have the same sequence number, they are duplicates, and the second frame is discarded. Similarly, if two subsequent ACKs reference the same sequence number, they are acknowledging the same frame.

Stop-and-wait ARQ is inefficient compared to other ARQs, because the time between packets, if the ACK and the data are received successfully, is twice the transit time (assuming the turnaround time can be zero). The throughput on the channel is a fraction of what it could be. To solve this problem, one can send more than one packet at a time with a larger sequence number and use one ACK for a set. This is what is done in Go-Back-N ARQ and the Selective Repeat ARQ.

Group07_Rothmans : GO BACK N ARQ

Go-Back-N ARQ is a specific instance of the Automatic Repeat-reQuest (ARQ) Protocol, in which the sending process continues to send a number of frames specified by a window size even without receiving an ACK packet from the receiver.

The receiver process keeps track of the sequence number of the next frame it expects to receive, and sends that number with every ACK it sends. The receiver will ignore any frame that does not have the exact sequence number it expects — whether that frame is a “past” duplicate of a frame it has already ACK’ed, or whether that frame is a “future” frame past the lost packet it is waiting for. Once the sender has sent all of the frames in its window, it will detect that all of the frames since the first lost frame are outstanding, and will go back to sequence number of the last ACK it received from the receiver process and fill its window starting with that frame and continue the process over again.

The sending window size must be no more than the number of sequence numbers (if they are numbered from zero to n-1) to verify transmission in cases of any packet (any data or ACK packet) being dropped.

Go-Back-N ARQ is a more efficient use of a connection than Stop-and-wait ARQ, since unlike waiting for an acknowledgement for each packet, the connection is still being utilized as packets are being sent. In other words, during the time that would otherwise be spent waiting, more packets are being sent. However, this method also results in sending frames multiple times — if any frame was lost or damaged, or the ACK acknowledging them was lost or damaged, then that frame and all following frames in the window (even if they were received without error) will be re-sent. To avoid this, Selective Repeat ARQ can be used

Group07_Rothmans : GO BACK N ARQ

Go-Back-N error recovery is a procedure which is implemented in some communications protocols to provide reliability. Go-Back-N ARQ (is one of a number of error recovery procedures to detect and retransmit I-frames which have been corrupted due to errors in the physical link.

Features required for Go-Back-N ARQ

1.To support Go-Back-N ARQ, a protocol must number each PDU which is sent. (PDUs are normally numbered using modulo arithmetic, which allows the same number to be re-used after a suitably long period of time. The time period is selected to ensure the same PDU number is never used again for a different PDU, until the first PDU has “left the network” (e.g. it may have been acknowledged)).

2.The local node must also keep a buffer of all PDUs which have been sent, but have not yet been acknowledged.

3.The receiver at the remote node keeps a record of the highest numbered PDU which has been correctly received. This number corresponds to the last acknowledgement PDU which it may have sent.

Recovery of lost PDUs using Go-Back-N.

The recovery of a corrupted PDU proceeds in three stages:

1.First, the corrupted PDU is discarded at the remote node’s receiver.

2.Second, the remote node requests retransmission of the missing PDU using a control PDU (sometimes called a NACK or REJECT). The receiver discards all PDUs which do not have the number of the requested PDU.

3.The final stage consists of retransmission of the lost PDU(s).

A remote node may request retransmission of corrupted PDUs by initiating Go-Back-N error recovery by sending a control PDU indicating the last successfully received PDU. This allows the remote node to instruct the sending node where to begin retransmission of PDUs. The remote node does not store any out-of-sequence PDUs and therefore must discard all received PDUs until one is received with the expected sequence number.

Upon receipt of a Go-Back-N control PDU (by the local node), the transmitter winds-back the sequence of PDUs pending transmission to the indicated PDU in its buffer of unacknowledged PDUs. The transmitter then retransmits (Goes Back-to-N) the requested PDU followed by all successive PDUs. This is sometimes known as “wind back” of the transmitter.

Example of Go-Back-N. The sender in this example transmits four PDUs (1-4) and the first one (1) of these is not successfully received. The receiver notes that it was expecting a PDU numbered 1 and actually receives a PDU numbered 2. It therefore deduces that (1) was lost. It requests retransmission of the missing PDU by sending a Go-Back-N request (in this case N=1), and discards all received PDUs with a number greater than 1.The sender receives the Go-Back-N request and retransmits the missing PDU (1), followed by all subsequently sent PDUs (2-4) which the receiver the correctly receives and acknowledges.

Group07_Rothmans : ARQ

Stop and Wait transmission is the simplest reliability technique and is adequate for a very simple communications protocol. A stop and wait protocol transmits a Protocol Data Unit (PDU) of information and then waits for a response. The receiver receives each PDU and sends an Acknowledgement (ACK) PDU if a data PDU is received correctly, and a Negative Acknowledgement (NACK) PDU if the data was not received. In practice, the receiver may not be able to reliably identify whether a PDU has been received, and the transmitter will usually also need to implement a timer to recover from the condition where the receiver does not respond.

Under normal transmission the sender will receive an ACK for the data and then commence transmission of the next data block. For a long delay link, the sender may have to wait an appreciable time for this response. While it is waiting the sender is said to be in the “idle” state and is unable to send further data.

Stop and Wait ARQ – Waiting for Acknowledgment (ACK) from the remote node.

The blue arrows show the sequence of data PDUs being sent across the link from the sender (top to the receiver (bottom). A Stop and Wait protocol relies on two way transmission (full duplex or half duplex) to allow the receiver at the remote node to return PDUs acknowledging the successful transmission. The acknowledgements are shown in green in the diagram, and flow back to the original sender. A small processing delay may be introduced between reception of the last byte of a Data PDU and generation of the corresponding ACK.

When PDUs are lost, the receiver will not normally be able to identify the loss (most receivers will not receive anything, not even an indication that something has been corrupted). The transmitter must then rely upon a timer to detect the lack of a response.

Stop and Wait ARQ – Retransmission due to timer expiry

In the diagram, the second PDU of Data is corrupted during transmission. The receiver discards the corrupted data (by noting that it is followed by an invalid data checksum). The sender is unaware of this loss, but starts a timer after sending each PDU. Normally an ACK PDU is received before this the timer expires. In this case no ACK is received, and the timer counts down to zero and triggers retransmission of the same PDU by the sender. The sender always starts a timer following transmission, but in the second transmission receives an ACK PDU before the timer expires, finally indicating that the data has now been received by the remote node.

State Diagram for a simple stop and wait protocol

Group07_Rothmans : GO BACK N ARQ

Go-Back-N is one of the automatic repeat-request (ARQ) techniques. With go-back-n the sender sends a number of frames specified by a window size even without the need to wait for individual ACK from the receiver as in stop-and-wait. The receiver includes with each ACK the sequence number of the next expected frame. The receiver ignores any out-of-order frames. The window size at the sender shrinks with every sent frame and increases with every ACK it receives. If the sender does not receive an ACK for a frame after a timeout, the sender will go back and retransmit all frames starting from that frame.

Group07_Rothmans : Wireless Transmission Media

In wireless transmission media, communication devices communicate with each other and data is communicated through the air or space using broadcast radio signals, microwave signals and infrared signals. This transmission medium is used when it is impossible to install the cables. The data can be transmitted all over the world through this medium. The commonly used wireless transmission media are below.

i. Broadcast Radio

It is a wireless transmission medium that is used to communicate information through radio signals in air, over long distance such as between cities and countries. In this medium, a transmitter is required to send messages (signals) and receiver is required to receive them. To receive the broadcast radio signal, the receiver has an antenna that is located in the range of signal. Some networks use a special device called transceiver used to send and to receive messages in the form of radio signals. The data transmission speed of broadcast radio is up to 54Mbps.

ii. Cellular Radio

Cellular radio is a form of broadcast radio that is used for mobile communications such as cellular telephones and wireless modems. A cellular telephone is a telephone device that uses high frequency radio waves to transmit voice and digital messages. Some mobile users connect their laptop computer or other mobile device to a cellular telephone to access the Web, send and receive e-mail etc. while away from a standard telephone line.

iii. Microwaves

Microwaves are radio waves that provide a high speed transmission. In Microwaves transmission, data is transmitted from one station to another. Microwave station contains an antenna, transceiver (transmitter & receiver) and other equipments that are required for microwave communication. Microwave uses the line-of-sight transmission, which means that in microwave transmission system the data signals travel in a straight path and cannot bend. Microwave stations or antennas are usually installed on the high towers or buildings. Thus microwave stations need to be placed within 20 to 30 miles of each other. Each microwave station receives signals from the previous stations. In this way, data is transmitted from one place to another.

The data transmission speed of microwave transmission is up to 150 Mbps. Microwave transmission is used in environments where installing physical transmission media is impossible and where line-of-sight transmission is available. It is used in wide-open areas. Today, it is used by telephone companies, cable television providers, universities etc.

Group07_Rothmans :Error Detection And Correction

Data processing and transmission systems use a variety of techniques to detect and correct errors that occur, usually for any of the following reasons:

1.Electrostatic interference from nearby machines or circuits.

2.Attenuation of the signal caused by a resistance to current in a cable.

3.Distortion due to inductance and capacitance.

4.Loss in transmission due to leakages.

5.Impulses from static in the atmosphere.
It has been estimated that an error occurs for every 1 in 200,000 bits. While most LAN technologies and optical cable networks reduce errors considerably, wireless networks and WAN links can have high error rates.

Bit errors are errors that corrupt single bits of a transmission, turning a 1 into a 0, and vice versa. These errors are caused by power surges and other interference. Packet errors occur when packets are lost or corrupted. Packet loss can occur during times of network congestion when buffers become full and network devices start discarding packets. Errors and packet loss also occur during network link failures.

There are two solutions to this problem:

1.Error correction strategy :-Send enough additional information to correct problems at the destination. This is called FEC (forward error correction). The receiver is responsible for fixing errors, if possible.

2.Error detection strategy :-Send only enough extra information to detect an error; then request a retransmission from the source. This is called ARQ (automatic repeat request). In this case, both the sender and receiver may take part in retransmitting lost information.