May 14, 2007

Violeta Cakulev, Mike Dolan, Frank Alfano, Nancy Lee - Alcatel-Lucent

ABSTRACT: This contribution discusses the benefits on several features of

idle mode buffering in the RAN. It shows that idle mode buffering in the RAN

can be a necessary option available to operators.

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Benefits of Idle Mode Buffering in the RAN

3GPP2 TSG-A.4

3GPP2 A40-20070514-xxx

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Outline

Push-to-talk benefits form idle mode buffering in the RAN

Alignment with other standards

Race conditions

Network architecture

Scalability

Intelligent paging strategies

Impact of future changes to the air interface

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Push-to-Talk

Half duplex voice service in cellular network environment that allows one-to-one or one-to-many group communication within a service area

PTT requires instantaneous set up times

PTT is based on SIP signaling which is already delay intensive

PTT supports one-to-many group communication

PTT server sends multiple copies of each packet – one to every group member

Contributes to congestion

Buffering at AGW increases delay

Extra delay: 2 x delayAN-AGW

For PTT it is essential to buffer at the RAN

Keep set up times and amount of signaling over the backhaul low

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Alignment with Other Standards

Align with HRPD

Buffering in the RAN is an approach used and tested in HRPD

The PDSN is not aware if AT active/dormant – Idle mode buffering is done in the RAN

The incoming data is always sent via A10 connection

If decision is made to develop a paging strategy that works over different technologies alignment on idle mode buffering is important

Align with 2G (GPRS) and 3G (UMTS) 3GPP networks

Align with WiMAX and LTE

Buffering in the ASN-GW (SAE-GW) is an approach accepted in WiMax (LTE)

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Race Conditions

Buffering at AGW introduces numerous race conditions

Example: AGW believes AT is active while instead AT is idle

Two options

Sender needs to retransmit – Increases delay – for some applications can be

unacceptable RAN does buffering and paging

– Greatly improved delay and efficiency

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Network Architecture

For certain deployments buffering in the RAN significantly reduces paging delay and signaling overhead

Example: Home femtocells

If a user is last registered in a particular femtocell it is highly likely that it remains in the coverage of that femtocell

Idle mode buffering in the RAN

Significantly reduces delay

Significantly reduces amount of signaling

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Scalability

Scalability imposes a severe problem for AGW

AGW need to support possibly millions of users

Number of users is not likely to grow rapidly

Amount of user’s data is expected to grow rapidly

Devise solutions that keep AGW as simple as possible

Handling idle mode buffering and paging in the RAN allows for better scalability

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Intelligent Paging Strategies

QoS Paging Adds capability to specify different paging strategies for different

QoS flows

“Best Effort” vs. VoIP vs. PTT Allows trade-off of paging effectiveness, efficiency, and latency for

different applications

Enables priority paging

Ability to order the paging messages within the control channel queue based on an assigned priority

Relies on the knowledge of subscription information (QoS profile) of the incoming page

If idle mode buffering is done at the AGW, AGW needs to ‘learn’ about QoS paging

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Intelligent Paging Strategies – Examples

PTT

Aggressively page a large area on the initial page to increase likelihood of finding the AT on the 1st attempt

Inefficient use of control channel resources is a tradeoff to provide high QoS

Make 2nd Page attempt after a relatively short interval

Abandon paging after a few seconds, since the application server waits for a response only a short interval

Best Effort

Initial page can only cover the area where the likelihood of finding the AT is largest

Subsequent page attempts can cover an expanded area to increase effectiveness

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Future Changes to the Air-Interface

What if AT modes defined in air-interface spec change in future?

If only AGW is doing buffering

Change in the air-interface spec will affect AGW

If option to do buffering in both RAN and AGW

Opens a possibility to make changes to the air-interface spec without any effect on AGW

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Recommendation

Discuss and adopt buffering in the RAN as an option necessary to support various features efficiently, and offering observable benefits.