Reliability issue in telecommunication network

Reliability issue in telecommunication network

The design and analysis of the reliability and availability issues are considered for

interconnected networks such as communication and telephony networks.

In telephony networks each subscriber is considered that is able to place a call to any

other subscriber in the network. Generally, the manner in which communication links

are provided depends on a number of factors.

Reliability is primarily concerned with ensuring that some routes in the network

connecting intended source and destination are operationally. Availability is

concerned primarily with ensuring that some routes are not only operational but also

unblocked or idle (avoiding congestion in the network or applied to have good radio

resources allocation).

2.1 Telecommunication network

Given a set of geographically distributed sites, information about the traffic between

them and information about the means by which they can be connected (links) is

given. Network design problem is to select some candidate connections. The primer

goal is to keep the cost within a specified budget and to provide sufficient connections

to support the traffic offered at a specified capacity, speed or throughput.

How can be improved the performance of the network? The network design has to

improve the performance for example maximizing the fraction of the messages

delivered within a time limit specified by the designer or client. The network design is

a challenging problem. The network planner must therefore address reliability issues:

How does one anticipate and deal with faulty behavior of the network?

The same question can be considered for the availability issue as well.

Satisfactory techniques to anticipate and to accommodate component failures are in

many ways few well understood than the design and performance in a failure free

system.

One of the hardest tasks is to anticipate the failures in the system and how much one

can reduce the performance of the network.

Reliability together with availability becomes the central problem for network design.

In this case the performability term is defined as a measure of performance and

reliability requirements.

2.2 Reliability and performability

Network reliability concerns the capability of the network to provide connections to

support required network functionality. Connectivity plays an important role. A dual

effort is done to extend reliability measures to treat performance measure more

complex than connectivity, and to adapt performance analysis techniques to handle

failures in the system.

Basic knowledge of network planning and design are introduced.

For example the binary situation where each link is either operating as intended, or

failed completely. The probability for the connection is known. A network design is a

selection of E links connecting V nodes.

A state of the network is a subset of S_E containing precisely the links that are

operational. A network is considered functional if there is a set of terminal nodes in

the network that must all be reachable from a specified source node.

The reliability can be measured in this case as a probability of connectedeness : s,Tconnectedeness

is the probability that all the terminal nodes in T are reachable form a

source node s in the network state , when each link operates independently with

known probability.

For a network with reliable nodes V and unreliable two links E , each state S_E has an

associate probability Pr[S] of arising and a value _(S) of some performance measure

_. Then the expected performance is

Perf(V,E)= Pr[S]_(S);

This performance is perhaps the most commonly studied class of performability

measures.

Few classes of algorithms are used to measure the performability.

These methods are exact algorithms, Monte-Carlo methods, efficiently computable

bounds, most probable states, most relevant states and hybrid methods.

2.3 Exact algorithms

This type of algorithm is applied to analyze the coherent performability of measures.

Coherence states that for any two states S and S’ if S_S’ then _(S)_ _(S’). When the

statistical independence holds then the performance measure is coherent. The

performance, in this case, is an indicator factor for the network. The inconvenience of

the algorithm is that as the number of links is getting larger the complexity is

becoming higher.

2.4 Monte Carlo methods

One can argue that the performance should not be exactly measured. Estimation of the

value can be enough.

Some assumptions about performance and/or the link failures are made. The

estimation possibility is the weakness point and the strongest one of this method. The

strength consists of typically one sampling plan serves for a wide variety of

performability measures .The weakness part comes because that is not possible to

incorporate some features about a specific network.

A trade off between time and accuracy is done using these methods. The more time is

used the better results of the estimation are obtained.

2.5 Efficiently computable bounds

If the time to get the estimation is limited then another technique is used to get the

estimation of the performance of the network.

One can compute upper and lower bounds of performability efficiently. If the

performability measure of the interest is connectivity-based, efficiently computable

bounds methods are sufficient accurate. In a contrary situation, much faster methods

can be used such as Monte Carlo methods. The bounds that are the most accurate for

reliability measures appear to be those that are most dependent on the measure being

connectivity-based.

2.6 Most probable states and most relevant state

These methods are used in case when the performability is measured based on the

most probable state or most relevant state, which can influence the performance of the

network.

The accuracy of the most probable state technique is difficult to compare, as one

delivers absolute bounds while the other yields point estimates. Most probable state

method makes some restriction on the performability measure examined.

The most relevant state method is used in such cases when some states with lower

probability could influence seriously the performance of the network.

Specifically Pr[S] might be relatively small, but _(S) can be very large.

All the presented methods are used to estimate the performance of the network from

the hard point of you. Application to the multi-interconnecting networks is hot

research area. Flexibility and reliability of the network are main points for designing

and planing network behavior.

As a conclusion of this part, which can be pointed out, is that in practical situation the

failures of the hardware in the interconnected networks are very difficult to be

estimated. The performance of the network can be predicted using different methods.

The main goal of the operator is to design a network where the number of failures in

the system (hardware failures) is low. The flexibility of the system assures the

reconfigured possibility of network and has also economical benefits.

Another analysis of the network from the availability issues can be discussed. The

addressed issue represents to avoid the congestion in the network.

If an analysis of the network from radio resources allocation point of view and

admission control is done then the performance of the network is considered the

capacity of the system and the quality of the service.

Next paragraph presents the performance of network in case that the considered radio

resource used for allocation is power, which is one of the critical resources in the third

generation mobile communication. First the radio network simulator is briefly

described and after that power control algorithms are presented. Simulation results

and performance of the network are presented. The practical constraints are taken into

account.

Conclusions and future research area are considered in the final part of the paper.