error performance in telecommunication

8/10/2019 Error Performance in Telecommunication

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MM3 - Reliability and Fault tolerance

in Networks

Service Level Agreements

Jens Myrup Pedersen


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The General Problem

The Quality of Service (QoS) parameters for the Communication Network

specifies the minimum requirements to e.q.:

Network accessibility

Network availability

Network performance (capacity, delay etc.)Network operation and maintenance

The Quality of Service (QoS)parameters for the Communication Network will

normally be specified in a Service Level Agreement (SLA)between the

Network Clients and the Network Operator.

The SLA also specifies how the QoS parameters should be measured in order

to validate the fulfilment, as well as actions to be taken in case of service

degradation.


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The General Problem

In critical Distributed Real-Time System applications these parameters will

have a high impact on the network design in order to meet the overall system

requirements.

The basic parameters is concerned with accessibilityand availability and are

expressed as a percentage of the agreed network service period.

Accessibilityis the parameter for potential access to the network Availability is

the parameter for the ability to communicate across the network.

The ability to meet the demands is based on the network architecture and the

Mean Time Between Failures (MTBF) and the Mean Time To Repair (MTTR)of the individual nodes, lines and management system.


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The General Problem

In critical Distributed Real-Time Systems the network QoS must be ensured by

a sufficient degree of redundancy and fast reaction in the network management

system.

The cost in the network design can be very high. There is a potential base for

relatively high savings in applying optimisation in the choice of network

topology in the design phase.

For large networks this optimisation problem is unsolved in general as the

complexity in the network grows exponentially with the number of nodes.

Applying some basic principles may not lead to optimal solutions, but at leastsolutions solving the problem with predictable parameters.


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Example from a general purpose network JANET, UK

Network availability for at least:

Availability of 99.6% to more than 90% of clients

Availability of 99% to more than 96.5%of clients

Availability of 97% to more than 98.5% of clients

Availability of 93% to more than 99.5% of clients

Mean time between failures of the service of at least:

1000 hours provided to 99% of clients

The target rate is less than 0.001 incidents per hour,

calculated each month by dividing the number of failures in

the best 99% access points by the number of access points and

the number of hours in the month.


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End-to-end latency between any pair of clients for

128 octet packets, measured as the time of entry on to the first access

line of the last bit of the packet to the time of exit from the second

access line of the first bit of the packet, of less than a stated targettime, which depends on the transmission technology used for 95% of

transmissions over any thirty minute period.

Clients shall normally expect to be able to

transmit and receive traffic (from a number of sources) which, over

any thirty minute period, uses at least 40% of the nominal capacity oftheir access line, once the overheads of the data solely concerned with

the transmission technology in use have been discounted


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Performance Indicators and Service Levels for Domain Name Service :

Availability of the primary name server for the target domain of

99.5%

Availability of service from an available officially supported name

server of 99.95%.

Performance Indicators and Service Levels for NTP Time Service:

This service is intended for use by access points in constructing their own

distributed time services (RFC 1305).

Availability of each time reference of 98%,

MTBF of 800 hours.


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International Standards

To estimate and verify the quality of the various components in the network

a number of measurement are specified in international agreed standards.

The ITU Recommendations G.821 and G.826 specify a set of communication

line parameters for SDH networks, primarily based on Bit Error Rates and

derived numbers.

The values will be part of the SLA between the end user and the network

service provider.


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International Standards, SDH

The recommendation G.821 has the following definitions:

Errored second (ES),a one-second time interval in which one or more bit

errors occurs.

Severely Errored second (SES),a one-second time interval in which the bit

error rate exceeds 10-3.

Unavailable second (US),a circuit is considered to be unavailable from thefirst of at least 10 consecutive SES. The circuit is available from the first of

at least 10 consecutive seconds which are not SES.

Degraded minute (DM),a one-minute time interval in which the bit error

rate exceeds 10-6.

Error free seconds (EFS),a one-second time interval without any bit

errors.

In recommendation G.821 similar definitions are specified based on the block

level.


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International Standards, SDH

The recommendation G.826 has the following definitions:

Errored second (ES),a one-second time interval containing one or

more errored blocks.

Errored block (EB),a block containing one or more errored bits

Severely Errored second (SES),a one-second time interval in whichmore than 30% of the blocks are errored.

Unavailable second (US),as for G.821

Background block error (BBE),an error block that is not a SES

A measurement time interval has to be specified, and the derived ratios for

ES, SES and BER are the base for the QoS parameters.

The recommended measurement time for G.821 and G.826 is 30 days.


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International Standards, ATM

The recommendation I.356 has the following definitions:

Cell Loss Ratiothe number of cells lost divided by the number of cells

transmitted.

Cell Error Ratio (CER),the number of errored cells divided by the

number of cells transmitted.

Cell Misinsertion Rate (CMR)the number of wrongly inserted cells in

a specified time interval.

Cell Transfer Delay (CTD)the time from a cell enters a device under

test to it leaves the device.

Mean Cell Transfer Delay (CTD)is the arithmetical mean of a numberof CTD values in a specified period.

Cell Delay Variation (CDV)is the degree of variation in the cell

transfer delay (CTD) of a virtual connection.

error performance in telecommunication

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