Differentiate between upward and downward multiplexing?

The process of multiplexing is carried out at the transport layer. Several conversations are multiplexed in to one connection or physical links or virtual circuit. For example, suppose the host has only one network address available for use. Then it has to be used by all the transport connections originating at that host. For multiplexing the following two main strategies are followed:

Ø  Upward multiplexing and

Ø  Downward multiplexing

Upward Multiplexing 

- In upward multiplexing, the different transport connections are multiplexed in to one network connection. 

- These transport connections are grouped by the transport layer as per their destinations. 

- It then maps the groups with the minimum number of network connections possible.

- The upward multiplexing is quite useful where the network connections come very expensive.

Downward Multiplexing 

- It is only used when the connections with high bandwidth are required. 

- In case of the downward multiplexing, the multiple network connections are opened by the transport layer and the traffic is distributed among them. 

- But for using downward multiplexing, it is necessary that this capacity must be handled well by the subnet’s data links.

Another Technique 

- In either of the cases it is not guaranteed that the segments will be delivered in order. 

- Therefore, another technique is adopted. 

- The segments are numbered sequentially. 

- Each octet is numbered by the TCP sequentially. 

- Segments are then numbered based up on the number of the first octet present in that segment. 

- The segments might get damaged in the transition or some may even fail to arrive at the destination. 

- This failure is not acknowledged by the transmitter. 

- However, the successful receipt of the segment is does acknowledged by the receiver. 

- Sometimes, the cumulative acknowledgements might be used. 

- If the ACK triggers a time out interrupt, the re-transmission of the segment is done. 

- Also the re-transmission is done when an ACK is lost. 

- The receiver must have the ability to recognize the duplicate ACKs. 

- If such thing occurs, the receiver assumes by itself the ACK might have been lost.

- This happens when the ACK duplicate is received before the connection is closed. 

- If the duplicate is received after the closure of the connection, the situation is dealt differently. 

- In this case, the sender and receiver are allowed to know about each other’s existence. 

- They negotiate about the parameters and the transport entity resources are allocated based up on some mutual agreement. 

The connection release is of two types:

Ø Asymmetric release: 

This is the one used in the telephone systems. However it does not works well for the network that use packet switching.

Ø  Symmetric release: 

- This is certainly better than the previous one.

- Here, all the directions are released independently with respect to each other. 

- The host continues receiving data after the disconnection TPDU has been sent. 

- But the symmetric release has another problem which is related with indirection levels and fake messages. 

- There are no proper solutions for this problem in case of the unreliable communication media. 

- Note that this has nothing to do with the protocol. 

- Putting a reliable protocol over an unreliable medium can actually guarantee the delivery of the message. 

- Another thing to be noted is that it the time limit within which the message will be delivered cannot be guaranteed by any protocol. 

- Error conditions might prolong the delivery period. 

- Restarting the connections can lead to the loss of all the state info and the connection might remain as half-open. 

- Since no protocol has been designed to deal with this problem therefore one has to go forward with the risks associated with releasing the connections. 

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

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

What are different types of schedulers and their workings?

Scheduling is an important part of the working of operating systems. 

- The scheduler is the component that provides access to the resources to the processes, threads and data flows. 

- These resources may include time of the processor and the communications bandwidth. 

- Scheduling is necessary for effectively balancing the load of the system and achieving the target of QoS or quality of service. 

- Scheduling is also necessary for the systems that do multitasking and multiplexing on a single processor since they need to divide the CPU time between many processes. 

- In multiplexing, it is required for timing the simultaneous transmission of the multiple flows.

Important things about Scheduler

There are 3 things which most concern the scheduler:

1. Throughput
2. Latency inclusive of the response time and the turnaround time
3. Waiting time or the fairness time

- But when practically implemented, conflicts arise between these goals for example between latency and throughput. 

- It is the scheduler that can make a compromise between any two goals. 

- Based on the user’s requirements and the objectives it is decided to which goal the preference has to be given. 

- In systems such as the embedded systems and robotics that operate in real time environment, it has to be ensured by the scheduler that the processes are capable of meeting the deadlines. 

- This is a very critical factor in maintaining the stability of the system. 

- The administrative back end is used for managing the scheduled tasks that are then sent to the mobile devices.  

Types of Schedulers

There are 3 different types of schedulers available which we discuss below:

Long term Schedulers or Admission Schedulers: 

- The purpose of this type of scheduler is to decide about the processes and jobs to be admitted or added to the ready queue. 

- When a program makes an attempt for executing a process, it is the responsibility of the long – term scheduler to delay or authorize the request for admitting the process to the ready queue. 

- Thus, what all processes will be executed by the system is dictated by this scheduler. 

- It also dictates about the degree of the concurrency and handling of the CPU intensive and I/O intensive processes. 

- Modern operating systems use this for making sure that there is enough time for the processes to finish of their tasks. 

- Modern GUIs would be of very less use if there was no real time scheduling. 

- The long term queue resides in the secondary memory.

Medium term Schedulers: 

- This scheduler serves the purpose of removing the processes from the physical memory and placing them in the virtual memory and even vice versa. 

- This process is called swapping out and swapping in. 

- A process that has been inactive for some time might be swapped by the scheduler. 

- It may also swap a process with frequent page faulting, low priority or more amount of memory etc. 

- This is necessary since this makes the space available for other processes.

Short term Schedulers: 

- These schedulers are more commonly known as the CPU schedulers.

- It decides which one out of all the processes will be executed after the clock interrupt, a system call, an I/O interrupt, hardware interrupt and so on. 

- Thus, we can say that the frequency of the short term schedulers of making decisions is much higher than that of the long term and medium term schedulers since after every time slice these schedulers have to decide.

- There is one more component that is involved in CPU scheduling but is not counted under schedulers. It is called dispatcher. 

What is a thread? What is meant by multi-threading?

About Thread

- A thread is a smaller instance of a process i.e., a sequence of instructions and an operating system scheduler can manage it independently. 

- A thread is sometimes also called as the light weight process. 

- The way these threads and processes are implemented is different for different operating systems. 

- But in majority of the cases threads are contained within a process. 

- The same process can have more than one thread. 

- These threads have to share the resources including memory while different processes may not share these. 

- In simple words, we can say that the instructions or code and the context or the values of the process are shared by its constituting threads. 

In this article we focus on threads and multi-threading.

- Multi-threading is a task of multiprocessor systems.

- But even single processor systems can do it by time division multiplexing just like the multitasking. 

- In TDM context switch occurs between the many threads. 

- This happens many times and so it seems to the user that a number of processors are being executed concurrently.

- However, in multiprocessor systems concurrency can be truly achieved since every processor processes one thread and so many threads are executed simultaneously. 

- Both multiprocessor and time threading is supported by most of the modern operating systems with help from the process scheduler. 

- The threads can only be manipulated through a system and this all is facilitated by the kernel of the operating system. 

- This is why some implementations have been named as the kernel thread. 

- An example of kernel thread is the LWP or the lightweight process and it shares same state as well as info. 

- Some programs even use user space threads when threading with the help of signals and timers etc. 

- These programs perform a kind of ad hoc time slicing. 

- Some may take threads and processes to be the same but there is a considerable difference between the two:

1. Processes are independent whereas the threads are a subset of the processes.
2. More state information is contained in threads whereas the same process state, resources and memory are shared by all the threads contained in that process.
3. Address spaces of different processes are different whereas the same address space is also shared by the threads.
4. IPC or the inter–process communication is the only medium for the processes to communicate with each other.
5. The threads within the same process are switched with context switch which is faster than that of the processes.

Features of Multi-threading

- Multi-threading is now among the widespread programming models. 

- The major characteristic feature of this model is that multiple threads can execute within the same process context. 

- Even though the resources of the process are shared by the threads, they execute independently. 

- The most widespread application of this model is in parallel computing.

- Full advantage of this technology can be taken only when it is applied to a multiprocessor system or a distributed system. 

- This is so because the program threads have a natural tendency to heed to the true concurrent execution. 

- But in these cases, necessary precautions must be taken for avoiding the race conditions and other undesirable behavior. 

- For the correct manipulation of data thread synchronization is also important. - Mutually exclusive operations are another requirement of the threads for preventing the simultaneous modification of the common data. 

- If these primitives are used carelessly, then it can lead the system to a deadlock. 

- Another feature of multi-threading is that it is always responsive to I/P. 

- This can be contrasted with the single threaded applications where if one block occurs, the whole program freezes.