SDH Networks : Performance analysis standards

G.821,G.826 and M.2100 performance analysis

The quality of digital links is determined with the aid of bit error ratio tests (BERT).The results of such measurements must, however, be classified in someway, not least because the quality of a transmission Path is often the subject of a contract between the network provider and the telecommunications user. 

For this reason, an objective means of classifying a line as either  “good” or “bad” is required. The ITU-T Recommendations G.821, G.826 and M.2100 are internationally recognized standards that specify these parameters.

G.821 :  This Recommendation was originally specified for international circuit- switched nX64kbit/s connections and expanded to include higher bit rates as time went on. A hypothetical reference connection is the basis used for determining quality parameters; this comprises an international long distance segment, a national segment and a subscriber access segment.

G.821definitions:

*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 ratio exceeds 10^-3

*Unavailable second(US):  A circuit is considered to be unavailable from the first of at least ten consecutive SES. The circuit is available from the first of at least ten consecutive seconds which are not SES.

The original version of G.821 also included:

*Degraded minute(DM): A one-minute time interval in which the bit error ratio exceeds10^-6

Derived parameter:

*Error-free second(EFS):  A one-second time interval in which no bit errors occur. The disadvantage of this method is that it relies on the evaluation of bit errors and so the test channel must be taken out of service to perform the measurement.

G.826:

 This Recommendation, issued in 1993,takes higher bit rates into account and allows in-service measurement as it relies on the evaluation of block errors.

G.826 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 or which contains at least one severely disturbed period(SDP).

*Background block error(BBE): An errored block that is not a SES.

*Unavailable second(US):  seeunderG.821, above.

The results are referred to the measurement time. 

This gives the following error parameters: Errored seconds ratio (ESR), severely errored seconds ratio(SESR) and background block error ratio (BBER). The specified quality requirements refer to  a particular path.

There commended measurement time for G.821andG.826is 30days.

M.2100 Recommendation:

M.2100 specifically applies to commissioning and maintenance. Commissioning consists of a 15-minute line up phase followed  by a 24-hour in-service measurement.  Once the line up phase is completed successfully, errors may occur within certain limits. If this is the case, the line remains in service, but must continue to be monitored for a further 7days.The measurement procedures are defined in M.2110 and M.2120. The limit values are derived form the performance parameters specified in G.821 and G.826.

Jitter:

The term jitter refers to phase variations in a digital signal. Put another way, the edges of the digital signal may differ from the expected ideal positions in time. Jitter is described in terms of its amplitude(expressed in unit intervals, UI)and its frequency. If the jitter frequency is below 10Hz,the term used is wander. 

Signals that are affected by jitter can not be sampled accurately; in an extreme situation, this might result in misinterpretation of the input signal. This results in single errors or error bursts and a corresponding degradation in transmission quality. 

Jitter and wander can also be the cause of buffer under flow or overflow, which leads to bit slips. The theoretical limit of correct sampling at high jitter frequencies is half the bit width. Distortion and additive noise means that the actual limit must be set much lower than this.

What causes jitter? The clock sources for network elements such as regenerators and add/drop multiplexers are one possible cause. Various types of jitter are differentiated as shown in the following table:

Other causes of jitter are interference signals and phase noise. Jitter caused by interference signals is  also called non-systematic jitter. Phase noise occurs despite the use of a central clock as a result of thermal noise and drift in the oscillator used. Various measurement methods have been developed for the different causes of jitter.