Atec Agilent-J2300E - Manual (Page 14)

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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 may be
discarded or tagged; 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 Agilent Advisor’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 Agilent Advisor counts the non-conforming test cells and
delivers a non-conformance 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/PCR
0+1
, CDVT
0+1
)
VBR.1/SBR1: GCRA(1/PCR
0+1
, CDVT
0+1
), GCRA(1/SCR
0+1
, BT
0+1
),
no tagging
VBR.2/SBR2: GCRA(1/PCR
0+1
, CDVT
0+1
), GCRA(1/SCR
0
, BT
0
),
no tagging
VBR.3/SBR3: GCRA(1/PCR
0+1
, CDVT
0+1
), GCRA(1/SCR
0+1
, BT
0+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)
ATM Testing (continued)