1 MM3 - Reliability and Fault tolerance in Networks Service Level Agreements Jens Myrup Pedersen

2 Service Level Agreements 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.

3 Service Level Agreements 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 accessibility and availability and are expressed as a percentage of the agreed network service period. Accessibility is 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.

4 Service Level Agreements 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 least solutions solving the problem with predictable parameters.

5 Service Level Agreements 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 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.

6 Service Level Agreements Example from a general purpose network JANET, UK

7 Service Level Agreements Example from a general purpose network JANET, UK

8 Service Level Agreements Example from a general purpose network JANET, UK

9 Service Level Agreements Example from a general purpose network JANET, UK

10 Service Level Agreements Example from a general purpose network JANET, UK 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 target time, 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 of their access line, once the overheads of the data solely concerned with the transmission technology in use have been discounted

11 Service Level Agreements Example from a general purpose network JANET, UK 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.

12 Service Level Agreements 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.

13 Service Level Agreements 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 Unavailable second (US), a circuit is considered to be unavailable from the first 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 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.

14 Service Level Agreements 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 which more 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.

15 Service Level Agreements International Standards, ATM The recommendation I.356 has the following definitions: Cell Loss Ratio the 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 number of 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.