Packet Data Channel: group data channel.

Packet Data Channel: group data channel.

In GSM/GPRS/EDGE networks, physical channels used by PS (packet switched, group switching) services are called pdch. The function module PCU (packet control unit) in BSC (Base Station Controller) allocates pdch to PS business users.

There are three types of pdch:
· Dedicated pdch: This type of pdch can only be used for PS services. Even if the physical channel of CS (circuit switched, Circuit Switching) services is congested and pdhc of this type is idle, the CS service cannot use this type of pdch either.
· On-Demand pdch: This type of pdch will be used when physical channels need to be allocated for PS businesses and there are physical channels for idle CS businesses, the physical channel of the idle CS service is converted to the on-demand pdch required by the PS service. If no PS service is performed on a certain on-demand pdch, The on-demand pdch will be changed back to the physical channel of the CS service after a certain period of time. However, if the physical channel of the current CS service is congested, The on-demand pdch will be pre-cleared even if a PS service is in progress on-demand pdch, returns the physical channel of the CS service.

There are two types of GPRS networksPdchAllocation, dedicatedPdch(DedicatedPdchOr fpdch), one is dynamicPdch(On-DemandPdch). DedicatedPdchIs only used to carry GPRS traffic, not to carry voice services, in the definition of dedicatedPdchLater, dedicatedPdchThe channel will be fixed to the GSL (GPRS signalling link) device that occupies PCU. DedicatedPdchIt cannot carry voice services, so it adds dedicated community services.PdchThe capacity of the voice TCH channel will be reduced. When the CS traffic volume increases, the congestion of the residential area will be affected.FpdchParameters are used to define the dedicated cellPdchNumber of channels. The value ranges from 0 to 8. That is, up to 8 channels can be set for a cell.Fpdch. Increasing this parameter can increase the fixed Use channel of GPRS/edge and decreasePdchAllocate failure rate.

Allocation ParametersPdchallocAssign a dedicatedFpdchTime plays a very important role. The Allocation AlgorithmFpdchIn the following channels:

1. First, it is allocated to channels with edge capabilities;

2. The second step is based onPdchalloc;

3. The third is the TCH channel with the maximum number of consecutive idle channels.

The most serious problem with grouping data is not fixed.Pdch(Fpdch ). A fpdch in a cell will activate the channel management function, which can retain continuous time slots for grouping business use. This means that CS traffic will try to avoid the potential pset and use other channels. If fpdch is not allocated in the cell, channel management will randomly allocate time slots to CS, which makes it difficult for subsequent grouping businesses to find continuous time slots.

DynamicPdchThe allocation is triggered by the system under certain conditions. When morePdchWhen it is used to carry PS services, the system allocates idle tch from the CSD (circuit switch domain) as dynamicPdch.
DynamicPdchIt is only temporarily allocated to GPRS users. When no TBF is transmitted above, it will return PSD (Packet Switch domain). If no data is transmitted within a certain period of time, it returns the circuit field.

InPdchA maximum of eight consecutive time slots can be allocated.PdchSet, that is, pset. Pset can be composed of dedicated or on-demandPdch. All channels in the same pset are of the same frequency or use the same frequency hopping sequence. Only channels of the same pset can be allocated to the same mobile phone. The maximum number of channels that a cell can allocate is determined by the available cell TCH channel and GSL in PCU.

When a GPRS/EGPRS user requests a channel, these selected channels belonging to the same pset will be reserved for this TBF. If the phone is already reserved in the opposite direction (upstream or downstream)PdchAccording to the mobile phone's multi-time slot type and QoS requirements, reserve new channels for this direction in the same pset (the number of reserved channels does not always reach the maximum number of channels supported by mobile phones ). ReservedPdchIt will be continuously monitored. Any TBF may be upgraded to meet QoS requirements or mobile phone type.

The reserved channel is determined by the following factors:

Mobile phone type (EGPRS or GPRS), frequency capacity, multiple time slot type

Ø Qos attributes of the Data Service to be established

Ø available channel resources (e-, G-and B-pdchs)

Ø channel load of ongoing traffic

On-DemandPdchCSD is assigned temporarily. There are two ways to return CSD. The first is whenPdchIt is no longer used by PS calls and placed in the PSD idle list. after a certain period of time (piltimer), if it is not usedPdchIt will be reassigned to CSD. The second approach is pre-emption. When there is a shortage of tch channels in the circuit switching traffic, the appropriate On-DemandPdchIt will be reassigned to CSD, and the returned principle is set by pdchpreempt.

The piltimer parameter controls the timeout time of the idle on-demand channel and re-allocates the channel to CSD. this parameter is a BSc parameter and the increment is 1 second. The setting of this time can be seen as a compromise. On the one hand, we need to maintain the On-DemandPdchTo reduce the number of GSL devices used in PCU. On the other hand, data services must have good performance in traffic and latency. If this time setting is short, the number of GSL devices in use will be reduced. However, whenPdchAfter being released to CSD again, on-demandPdchThe system will be allocated more frequently, resulting in system latency.

Fpdch parameter settingsPdchIs equally important. This parameter is the number of fpdch values. The default value is 0. We recommend that you set fpdch to 1 for all cells.