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When customer traffic exceeds or ‘violates’ its contracted peak cell rate

(PCR), taking into account its cell delay variation tolerance (CDVT),

cells are discarded; when it exceeds the sustainable cell rate (SCR)

parameters, taking into account its maximum burst size (MBS)

allowances, cells may be discarded or tagged (cell loss priority (CLP) bit

changed to 1), depending on the type of traffic contract. When congestion

occurs in the core network, low priority cells (CLP = 1) are discarded

first by core ATM switches before the cells of normal priority (CPL = 0).

Policing enables networks to assure a consistent quality of service within

the core network, preventing the overall network performance to be

degraded by an individual user.

The ATM software’s policing measurement feature is a real time

measurement that checks a virtual connection’s conformance to the

traffic contract at the ingress to the public network; this could be a

virtual channel connection or virtual path connection. The user specifies

the traffic contract type (e.g. VBR.3/SBR3) and parameters (PCR, CDVT,

SCR and MBS) for the virtual connection and the ATM software counts

the non- conforming test cells and delivers a nonconformance count and

non- conformance ratio for active traffic on the network. This

measurement would be used typically to check a user’s traffic either side

of the boundary to the public network when quality of service is suspect

(the user experiences unexpected levels of cell loss). If the user traffic

entering the network does not conform, the network cannot be blamed

for the resulting poor QoS.

Traffic Contracts Supported

As shown in figure 3, the user may specify one of the following types

of traffic contract to determine how many cells in the virtual connection

are conforming or non- conforming:

CBR.1/DBR/UBR.1: GCRA(1/PCR0+1, CDVT0+1)

VBR.1/SBR1: GCRA(1/PCR0+1, CDVT0+1), GCRA(1/SCR0+1, BT0+1),

no tagging

VBR.2/SBR2: GCRA(1/PCR0+1, CDVT0+1), GCRA(1/SCR0, BT0),

no tagging

VBR.3/SBR3: GCRA(1/PCR0+1, CDVT0+1), GCRA(1/SCR0+1, BT0+1),

tagging supported

(note that in this notation, subscripts ‘‘0’’ and ‘‘1’’ means high and low

priority cells respectively, so ‘‘0+1’’ means all cells; note also that BT

(burst tolerance) is derived from the MBS, PCR and SCR values)

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Agilent Technologies 37907A Manual do Utilizador Página 16