What is meant by multiplexing?

- Multiplexing or muxing is a very important process in computer networks and the telecommunications. 

Using this process, a number of digital data streams or analog message signals are combined as one signal and then transported over the common medium. 

- Multiplexing is used wherever it is required to share a resource that is very expensive. 

- The most common example of multiplexing is of using one wire for several telephone calls. 

- The origin of the multiplexing dates back to 1870s when telegraphy was started.

- Now it is used to a great extent in the field of communications. 

- The telephone carrier multiplexing was developed by George Owen Squire in the field of telephony. 

- The communication channel over which the multiplexed signal might be transmitted might be a physical transmission medium. 

- The high level communication channel’s capacity is divided by multiplexing process in to a number of low level logical channels where for each message or data stream one channel is used. 

- Demultiplexing is the reverse process of multiplexing. 

- This is used for the extraction of the original signals on the reception side.

- A multiplexer or MUX is a device that is used for carrying out the multiplexing process and the demultiplexer or DEMUX is the device that performs demultiplexing. 

- IMUX or inverse multiplexing is another process whose aim is just the opposite of the multiplexing.

- It breaks down a single data stream in to various streams while transferring them at the same time over various communication channels.

- Later, the original stream is recreated.

Types of Multiplexing

Many different types of multiplexing technologies are available today. Each has its own significance:

Ø SDM or space-division multiplexing: 

This technique implies on using different point – to – point wires for individual communication channels. For example, an audio cable of analogue stereo, multi – pair telephone cable, switched star network, mesh network. However typically the wired SDM is not usually considered as multiplexing. In SDM a phased array antenna is formed by multiple antennas. For example MIMO (multiple – input and multiple – output), SIMO (simple – input and multiple – output), MISO (multiple – input and single – output) etc.

Ø  FDM or frequency-division multiplexing: 

This is considered to be an analog process, here the signals are sent in to different frequency ranges over a shared medium. For example, TV and radio broadcasting from satellite stations through the earth’s atmosphere. One cable is given in each house but over this cable many signals can be sent to other subscribers also. For accessing the desired signal, the users require to tune to that particular frequency. WDM or wavelength division multiplexing is a variant of FDM.

Ø TDM or time-division multiplexing: 

Unlike FDM, TDM is a digital technology but very rarely it might be used as an analog technology also. The process involves putting bytes in a sequence for each input stream one by one. this sequencing is done in such a way that the receiver can appropriately  receive them. If this is done quickly, the fact that another logical communication path was served in that circuit time won’t be detected by the receiver.

Ø  CDM or code-division multiplexing: 

In this multiplexing technique, the same frequency spectrum is shared by the several channels at the same time. Also the bandwidth of the spectrum is quite high when compared to the symbol rate or the bit rate. It is implemented in either of the two forms namely direct sequence spread spectrum and frequency hopping.

Some other types of multiplexing techniques which are less prominent are:

Polarization-division multiplexing: Used in optical and radio communications.

Orbital angular momentum multiplexing

Explain the technique of admission control to control congestion in virtual circuit subnets?

- Virtual circuits can be thought of as a virtual channel in the telecommunication networks as well as computer networks. 

- Virtual circuit sub-nets represent a communication service that is connection oriented. 

- This service is used through the packet mode communication.

- A stream of data bytes can be exchanged between the two nodes only if a virtual circuit has been established between them. 

- Without the presence of the higher level protocols, data division has to be dealt with unnecessarily. 

- Therefore, the virtual circuits always allow the high level protocols. 

- There is a resemblance between the circuit switching mode and the virtual circuits because of the fact that both of them are based up on connection. 

- The packets that are transmitted through a virtual circuit sub-net consist of a circuit number and not the destination address. 

- This is why the memory requirement of the packets is less when using virtual circuits and more in the others. 

- This also makes the virtual circuit sub nets less expensive when compared to other sub nets. 

In this article we discuss about the technique for congestion control in virtual circuit sub nets. 

- One most popular technique is of the admission control. 

- Most of the congestion control methods are based up on an open loop i.e., the congestion is prevented rather than managing it after it has occurred. 

- Admission control is a dynamic method for controlling the congestion problems in the virtual circuit sub nets. 

- Admission control technique has been widely accepted for preventing the congestion problem from getting worse over the time. 

- The technique is based up on a very simple idea which is that no virtual circuit is set up until and unless the congestion problem that was detected has been resolved. 

- Therefore, any attempt that is made for establishing a new virtual connection with the transport layer is failed. 

- The things get even worse if the access is granted to more and more people. 

- Simplicity of the technique is one of its characteristic that makes it easy to be implemented. 

- The technique can be implemented in a straightforward manner. 

- The admission control technique is also used by the telephone systems for combating with the congestion problems. 

- The admission control technique is implemented whenever a switch in the network gets overloaded. 

- At this time no dial tone is heard. 

- Establishing new virtual connections represents another way of coping with this problem. 

- Here, the new connections have to be routed carefully such that there are no problems. 

Another method for eliminating the problem of congestion is to strike an agreement between the virtual circuit subnet and the host. 

- By this we mean establishing a new virtual circuit. 

- But this arrangement requires specifying how the traffic has to be shaped and what would be its volume, QoS (quality of service), other parameters and so on. 

- The virtual circuit sub net has to reserve the resources on its part of the agreement established. 

- These resources lie on the route where the virtual circuit has been established. 

- The resources might include the following:

Ø  Space in the router’s buffer.

Ø  Tables

Ø  Bandwidth of the lines and so on.

- The newly virtual circuits are less likely to experience the congestion problems. 
- This is because to them the availability of the resources has been guaranteed.
- Resources can be reserved in this way only if the existing sub nets are experiencing congestion problem or when the standard operating procedure is being followed. 
- One disadvantage of the admission control technique is that it leads to the wastage of the resources. 
- Also, sometime the bandwidth is left unused. 

What is the concept of flow control?

- Flow control is an important concept in the field of data communications. 

- This process involves management of the data transmission rate between two communicating nodes. 

- Flow control is important to avoid a slow receiver from being outrun by a fast sender. 

- Using flow control, a mechanism is designed for the receiver using which it can control its speed of transmission.

- This prevents the receiving node from getting overwhelmed with traffic from the node that is transmitting.

- Do not confuse yourself with congestion control and flow control. Both are different concepts. 

- Congestion control comes in to play when in actual there is a problem of network congestion for controlling the data flow. 

On the other hand the mechanism of flow control can be classified in the following two ways:

1. The feedback is sent to the sending node by the receiving node.
2. The feedback is not sent to the sending node by the receiving node.

- The sending computer might tend to send the data at a faster rate than what can be received and processed by the other computer. 

- This is why we require flow control. 

- This situation arises when the traffic load is too much up on the receiving computer when compared to the computer that is sending the data. 

- It can also arise when the processing power of the receiving computer is slower than the processing power of the one that is sending the data.

Stop and Wait Flow Control Technique 

- This is the simplest type of the flow control technique. 

- Here, when the receiver is ready to start receiving data from the sender, the message is broken down in to a number of frames. 

- The sending system then waits for a specific time to get an acknowledgement or ACK from the receiver after sending each frame. 

- The purpose of the acknowledgement signal is to make sure that the frame has been received properly. 

- If during the transmission a packet or frame gets lost, then it has to be re-transmitted. 

- We call this process as the automatic repeat request or ARQ. 

- This technique has a problem which is that it is capable of transmitting only one frame in one go. 

- This makes the transmission channel very inefficient. 

- Therefore, until and unless the sender gets an acknowledgement it will not proceed further for transmitting another packet. 

- Both the transmission channel and the sender are left un-utilized during this period. 

- Simplicity of this method is its biggest advantage. 

- Disadvantage is the inefficiency resulting because of this simplicity. 

- Waiting state of the sender creates inefficiency. 

- This happens usually when the transmission delay is shorter than the propagation delay. 

- Sending longer transmissions is another cause for inefficiencies. 

- Also, it increases the chance for the errors to creep in this protocol. 

- In short messages, it is quite easy to detect the errors early. 

- By breaking down one big message in to various separate smaller frames, the inefficiency increases. 

- This is so because these pieces altogether take a long to be transmitted.

Sliding window Flow Control Technique 

- This is another method of flow control where permission is given to the sender by the receiver for continuously transmitting data until a window is filled up. 

- Once the window is full, sender stops transmission until a larger window is advertised. 

- This method can be utilized in a better way if the size of the buffer is kept limited. 

- During the transmission, space for say n frames is allocated to the buffer. 

- This means n frames can be accepted by the receiver without having to wait for ACK. 

- After n frames an ACK is sent consisting of the sequence number of the next frame that has to be sent. 

What are the goals and properties of a routing algorithm?

Routing requires the use of routing algorithms for the construction of the routing tables.

A number of routing algorithms are today available with us such as:

1.   Distance vector algorithm (bellman ford algorithm)

2.   Link state algorithm

3.   Optimized link state routing algorithm (OLSR)

- In a number of web applications, there are a number of nodes which require communicating with each other via communication channels. 

- Few examples of such applications are telecommunication networks (such as POTS/ PSTN, internet, mobile phone networks, and local area networks), distributed applications, multiprocessor computers etc. 

- All nodes cannot be connected to each other since doing so will require many high powered transceivers, wires and cables. 

- Therefore, the implementation is such that the transmissions of nodes are forwarded by the other nodes till the data or info reaches its correct destination. 

- Thus, routing is the process of determining where the packets have to be forwarded and doing so.

Properties of Routing Algorithm
- The packets must reach their destination if there are no factors preventing this such as congestion.

- The transmission of data should be quick.

- There should be high efficiency in the data transfer.

- All the computations involved must not be long. They should be as easy and quick as possible.

- The routing algorithm must be capable of adapting to the two factors i.e., changing load and changes in topology (this includes the channels that are new and the deleted ones.)

- All the different users must be treated fairly by the routing algorithm.

The second and the third properties can be achieved using fastest or the shortest route algorithms. 

- Graphical representation of the network is a crucial part of the routing process.

- Each network node is represented by a vertex in the graph whereas an edge represents a connection or a link between the two nodes. 

- The cost of each link is represented as the weight of the edge in the graph. 

- There are 3 typical weight functions as mentioned below:

1.   Minimum hops: The weight of all the edges in the graph is same.

2.  Shortest path: The weight of all the edges is a constant non – negative value.

3.   Minimum delay: The weight of every edge depends up on the traffic on its link and is a non – negative value.

However in real networks, the weights are always positive.

Goals of Routing Algorithms

- The goal of these routing algorithms is to find the shortest path based up on some specified relationships that if used will result in the maximum routing efficiency. 

- Another point is to use as minimum information as possible.

- Goal of the routing algorithm is also to keep the routing tables update with all alternative paths so that if one fails, the other one can be used.

- The channel or the path that fails is removed from the table. 

- The routing algorithms need to be stable in order to provide meaningful results but at the same time is quite difficult to detect the stable state of an algorithm. 

- Choosing a routing algorithm is like choosing different horses for different courses. 

- The frequency of the changes in the network is one thing to be considered. 

- Other things to be considered include the cost function that is needed to be minimized and the calculation of the routing tables in a centralized fashion.

- For static networks the routing tables are fixed and therefore they require only simple routing algorithms for calculation. 

- On the other hand, the networks that are dynamic nature require distributed routing algorithms which are of course complex.

What properties are common between WDMA and GSM channel access protocols?

About Wavelength Division Multiplexing Access(WDMA)

- Wavelength division multiplexing access or WDMA is a concept of fiber optics communications that is all about multiplexing the number of optical carrier signals in to one optical fiber. 

- This is done by using a number of different wavelengths or colors of the LASER lights. 

- This technique has made it possible to make bidirectional communications through one fiber strand and multiplication of capacity.

- The frequency division multiplexing is applied to radio carrier whereas this WDMA is used in the optical fibers. 

- An inverse relationship ties the frequency and wavelength together since the same concept is described by the two terms. 

- WDMA is classified under channel access methods and is based on wavelength division multiplexing. 

- A system using the WDM consists of a multiplexer at the transmitting end for joining the signals together. 

- Similarly, a de-multiplexer is installed at the receiver’s end.

- The de-multiplexer is used for splitting the joined signal apart.

- It is possible to make a device using the right kind of fiber that can do both multiplexing and de-multiplexing simultaneously as well as an optical add – drop multiplexer.

About Global System for Mobile Communications(GSM)

- Global system for mobile communications is an ETSI (European telecommunications standards institute) developed standard set for describing the protocols for 2G i.e., second generation mobile networks. 

- This standard set now rules the market with around 80 percent of the total market share. 

- GSM describes a digital network operating on circuit switching and that has been optimized for full duplex voice telephony. 

- GSM means a cellular network. 

- For connecting to this network, the cell phone searches for other cells in its surrounding. 

- GSM network offers 5 types of different cell sizes namely:

1. Macro
2. Micro
3. Pico
4. Femto
5. Umbrella cells

- The extent of the area covered by each of the cells varies depending on the environment in which they are being implemented. 

- The base antenna is installed in the macro cells as a mast.

- In the micro cells, the height of the antenna is at the average level of the roof top. 

- Pico cells have a coverage diameter of only few meters.

- Femto cells find use in the small business and residential environments. 

- They are used for connecting to the ISP’s network. 

- Shadowed regions of the cells which are small are covered by the umbrella cells.

Common Properties and Differences between WDMS and GSM

Usually there are a number of differences between GSM and WDMA but they share some common properties also. 
- The base station of the GSM system and that of the WDMA both are connected to the core network of the GSM system in order to facilitate radio connectivity in the handsets. 
- Therefore, the same core network is shared by both the technologies. 
- Principles of the cellular radio system form the basis for the two technologies. 
- There exists a correspondence between the WDMA radio network controller i.e., RNC and the GSM base station controller or BSC. 
- The RBS (radio base station) of GSM system corresponds to the RBS of the WDMA. 
- The basis for developing the lu – interface of WDMA was provided by the A-interface of the GSM technology. 
- The only difference is of the new additional services that are offered by the WDMA. 
- GSM makes use of time division multiplexing along with the radio functionality for the management of the time slots. 
- On the other hand, WDMA makes use of the code division multiplexing for the same. 
- This implies that the control as well as the hardware functions of both are different

Explain CSMA with collision detection?

- CSMA with collision detection is abbreviated as CSMA/CD. 

- CSMA in itself makes use of the LBT technology i.e., listen or sense before talk. 

- But when incorporated with the ability of collision detection, it gets much better. 

- If the channel is sensed to be idle the data packets or frames are transmitted immediately but if not, the transmitter is bound to wait for some time before it can re-transmit. 

- Sensing the channels prior to transmission is absolutely necessary if the collisions are to be avoided. 

- Sensing the channel is the most effective way of avoiding the collisions. 

- There are two types of CSMA protocols namely persistent and the non-persistent CSMA.

- In CSMA/CD protocol all the hosts have freedom for transmitting and receiving the data frames on one and the same channel. 

- Also, the size of the packets is variable.

CSMA/CD comprises of two processes:

Carrier Sense: In this process the transmitter or the host checks if the channel or the line is not occupied before starting the transmission.

Collision Detection: CSMA/CD tries to detect the collisions in the shortest possible time. If it happens to detect a collision, it stops the transmission then and there and waits for a random amount of time which is equal to the binary exponential back-off. It then again senses the channel.

- For ensuring there occurs no collision during the transmission of a packet, a host must have the capability of detecting the collision before the transmission process is complete. 

- What happens is that the host A sensing the line to be idle starts transmitting a frame. 

- Just before the first unit of this frame reaches host B, it also senses the line to be idle and starts its transmission. 

- Now the host B receives data while its transmission is still in progress and so it detects that a collision is about to occur. 

- A collision occurs close to the host B. the host A also receives data in midst of its transmission and therefore detects the collision. 

- For making the hosts detect collision before transmission, a minimum length has to be decided for the packets that are transmitted via CSMA/CD networks. 

There are 3 states for a CSMA/ CD channel namely:

1. Contention
2. Transmission
3. Idle

- Ethernet is the most popular example of the CSMA/CD networks. 

- A minimum slot time is required for collision detection between the stations.

- This slot time must equal twice the maximum value of the propagation delay. - The host acquires the channel on the basis of the 1 – persistence. 

- Also, a jam signal is transmitted if a case of collision detection occurs. 

- CSMA/CD make use of the binary exponential back-off algorithm. 

- It is obvious that the idle time of the channel will be small if the load is heavy. 

- It normalizes all the packets with respect to the time of the packet transmission.

- CSMA/CD represents a very effective method for media access control. 

- There are different methods available for detecting the collisions. 

- Which method is to be followed depends largely on the transmission medium that exists between the two stations. 

- For example, if the two stations are connected via electrical buses, the collision can be detected by making comparison between the transmitted and the received data. 

- Some other way involves recognition of a signal of higher amplitude than the normal one. 

- The jam signal used in the CSMA/CD networks is constituted of 32 bit binary pattern.

Explain Carrier Sense Multiple Access Protocols - CSMA

- CSMA or Carrier Sense Multiple Access is defined as a probabilistic MAC (media access control) protocol. 

- In this protocol the absence of the traffic is verified by the node prior to start transmitting through a transmission medium that is shared by a number of channels. 

- For example, a band of EM spectrum, electric bus and so on. 

- Feedback from the receiver is used by the transmitter for determining whether the channel is already occupied by some transmission process or not. - This is termed as the carrier sense. 

- The transmitter before starting a transmission starts detecting whether a carrier wave is present their transmitted by some another station. 

- If a carrier wave is present, the transmission has no other option other than waiting for the transmission in progress to end before starting its own transmission. 

- To put it simply we can say that the carrier sense is what the CSMA protocol is based up on. 

- By ‘multiple access’, it simply means that a number of stations can transmit and receive data through the same medium. 

- But in this case all the other stations receive the data transmitted by one node using that same medium. 

There are many types of CSMA protocols as mentioned below:

1. CSMA with collision detection: 

- In short, this protocol is known as the CSMA/CD. 

- This protocol is used for improving the performance. 

- This it does through the termination of the transmission once a collision occurs along making reductions in the probability of occurrence of the other collisions upon re-transmissions.

2. CSMA with collision avoidance: 

- CSMA/ CA is the abbreviated form of this type of CSMA protocol. 

- This protocol avoids collision for improving the performance of the same.

- It tends not to be greedy on the transmitting channel.

- If it happens to sense the channel as busy prior to transmission then it is said to defer to a random interval.

- This is a measure for reduction in the probability of the packet collisions occurring on the channel.

3. Virtual time CSMA: 

- In short, known as the VTCSMA.

- This type of CSMA protocol has been designed for avoiding the collisions that are generated by the nodes. 

- These are the nodes that translate the signals at the same time. 

- This CSMA protocol is employed in the hard real time systems. 

- Two clocks are used by each node namely a real clock (rc) and a virtual clock (vc) for telling ‘real time’. 

- If the channel happens to be busy, the virtual clock freezes and if not it is reset. 

- vc is not initialized if the channel is busy and it runs faster when compared to the real clock.

CSMA happens to operate in various modes as discussed below:

1-Persistent: 

- The sender stations keeps on sensing the transmission channel until it becomes idle if previously it was busy. 

- When it finally becomes, then it starts transmitting the data packet i.e., the frame. 

- If a collision is detected, the transmitter has to wait for a random period of time before attempting to re-transmit the signal. 

- It finds use in systems such as Ethernet i.e., the systems using CSMA/ CD.

P-Persistent: 

- This mode lies somewhere between the non–persistent and the 1–persistent CSMA access modes.

- Once the data is ready to be transmitted, it checks for the status of the medium and starts transmitting the data packets with a certain probability.

Non-persistent: 

Less aggressive when compared to the previous one.

O-Persistent:

It works based according to the assigned transmission order. 

What is Dynamic synchronous transfer mode (DTM)?

Dynamic synchronous transfer mode or the DTM is a technology developed for optical networking. The ETSI (i.e., the European telecommunications standards institute) standardized this technology in the year of 2001 marked with the following beginning specification ‘ETSI ES 201 803 – 1’. 

This is a circuit switching network technology that doubles as a time division multiplexing technology too. Actually, this technology is built up on a combination of the switching and transport.

This technology guarantees to provide QoS or quality of service for services that are involved with the streaming of videos. 

However, it might be used for packet – based services also. It is marketed for the following:

1. Professional media networks
2. Mobile TV networks
3. DTT or digital terrestrial television networks
4. Content delivery networks
5. Consumer oriented networks (for example, triple play)

What is Switching?

- Switching of the channels is specified by DTM. 

- This is what that makes it different from the other transmission techniques that we have, for example, SONET (synchronous optical networking), SDH (synchronous digital hierarchy) and so on. 

- End to end provisioning is done for the DTM channel over a network with general topology through the use of control signaling.

- DTM therefore represents a circuit switched system. 

- The switches are nothing but time space switches that guarantee the QoS property. 

- The allocation of the resources is done physically for each channel in the switch. 

- This is quite contrary to the switches that are based up on packets or cells. 

- In those kind of switches there is always a competition for resources between the packets and cells. 

- Such a competition leads to delaying and discarding of the packets and cells. - Other methods offer a shared resource allocation mechanism that draws a limit for the packet and cell switches regarding their utilization of the network in such way that the QoS is maintained at a certain level. 

- But DTM does not follow this shared allocation mechanism rather it implies that a network can be loaded up to full limit theoretically and still can guarantee the QoS. 

- Thus, here real utilization is more like a question of adaptation of the network topology as well as its link capacities considering the actual traffic matrix.

- Packet and cell based switching technologies are more suited to statistical multiplexing.

- It means whenever a packet streams in a router come at an outgoing link that is common to all of them, buffering is carried out until the resources are free on that particular link.

- In this way, it becomes possible to make use of the outgoing link to the maximum degree possible without causing many delays. 

- This also proves fitting for the best effort traffic. 

- But there are certain QoS requirements of the streaming media that cannot be ignored. 

- Streaming traffic is by nature not statistical and therefore is better maintained by end to end resource allocation.

- This category is applicable for audio and video services.

- This is not exclusive of the IP traffic gained via guaranteed QoS transport if majority of the content is audio and video. 

- Some other technologies such as that of IP and Ethernet were also adopted for the same purpose. 

- Multi protocol label switching or MPLS can be applied to the carriage network for improving the reliability as well as determinism that is required by most of the streaming media. 

- This technology is applied along with the techniques such as the forward error correction.

- Ethernet has been made supportive for audio and video transmission by improvement in technologies such as the provider backbone bridge traffic engineering. 

- The development of dynamic synchronous transfer mode took place at the royal institute of technology.