Let’s have a talk about QoS

Posted: February 16, 2010 in technical
Tags: , , , , , , , , , , , , , , , , , , , , , ,

Everybody knows about this Quality of Service thinggie that helps admins to classify traffic and better allocate network and system resources to accommodate traffic needs and also to enforce the policies that exist for each user/groups of users – basically according to the amount of money they pay for this service 😛

In order to enforce a QoS on SAE entities, there are a few things to keep in mind:

A. PCRF – Policy Charging Rules Function device, which basically is a database that holds specific service QoS associations for each UE – this device is interrogated by the PGW in order to find out which QoS is applied on which traffic for each UE that requests a dedicated bearer

B. HSS – Home Subscriber Server, which has _nothing_ to do with QoS; this is a static database that holds information about the UE as is, and no information about any SLA of that UE with the provider

C. Only dedicated bearers hold a QoS level as part of their TFT association

D. The QoS on SAE has multiple variants, dividing the bearers into 2 groups:

GBR bearers (Guaranteed Bit Rate)

non-GBR bearers

E. The TFT (Traffic Flow Template – containing the filtering components for the traffic) that is associated with a bearer is not per flow, but per direction:

TFT-UL (TFT uplink)

TFT -DL (TFT downlink)

Also, the TFT can be:

bearer-level

SDF-level (service data flow level)

Now, we are interested in the GBR bearers. The GBR profile includes the following parameters:

1. QCI – QoS Class Identifier

Rules:

a. 1 QCI corresponds to 1 bearer only

b. 2 services having different QCI values can NOT be on the same bearer (TFT)

c. 2 services having the different QCI values can NOT have the same ARP (see below) value

d. QCI values must belong to [1..255]

2. ARP – Allocation & Retention Priority

Rules:

a. this value has NO influence on QoS

b. this value is set per eNB, following the PGW’s decision

c. it is established by PCRF according to a tuple of —activity type — subscription information — admission policies—

3. GBR – Guaranteed Bit Rate

4. MBR – Maximum Bit Rate

** There are also aggregated GBR bearers: AMBR (this is per APN) and UE-AMBR (the per-non-GBR, per UE rate) — [note to myself] to study more on this!

***Things to consider:

Thinking about the Mobility/Handover scenarios, keep in mind that, when a UE moves from one eNB to another, or from one MME to another, or from one SGW to another, the QoS is enforced on ALL the EPS components. This means that the QCI, ARP, GBR, MBR rates will all be verified on the resources of the destination (destination eNB, destination MME and so on). This means further on that, should at least one of the components not have enough resources to sustain all the QoS of a specific UE, some bearers will be dropped – this, again, is a decision of the SGW, taking into consideration, of course, the signaling came from the rest of the components.

[courtesy of my LTE guru colleagues]

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Comments
  1. Sriram says:

    Hi Cristina,
    Thanks for the useful posts. One thing i am not clear about is QCI.

    Is QCI value unique for each bearer ? If so how is it possible to create 11 bearers per UE, when only 9 values are defined.

    And it should also be possible to have same kind of packet forwarding treatment to different kind of SDFs based on QCI(by reusing the QCI value)

    Please share your inputs.

    Sriram

    • @Sriram: I am not sure about this, but I don’t think there are only 9 QCI values. If I remember correctly, they are unique per UE, so they should be different. Right now I am on a tight customer case schedule, but when I get back to Romania I will come back with a more accurate response.
      Thank you very much for your interest.

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