Optimizations for Push-To-Talk in Wireless Networks Efficient Management of Call Setup Latency

Krish Pillai, PhD Department of Computer Science

Lock Haven University of Pennsylvania Lock Haven, PA 17745, United States

e-mail: kpillai@lhup.edu

Haseeb Akhtar, Senior Member, IEEE Applications and Services Architect

Nortel Networks Richardson, TX 75080, United States

e-mail: haseebak@nortel.com

Abstract— Push-to-Talk or PTT is ideal for group communication in a cooperative work environment, since conversations tend to be terse, and single-point to multipoint communication is the norm. PTT is generally run using simple two-way devices communicating on a common channel. However, field experience has repeatedly exposed the need for better range and higher robustness, particularly since Departments of Public Safety and Medical Emergency Response Teams have come to rely heavily on this technology. PTT over a stable regulated carrier-grade wireless network such as 1xEVDO, GSM, or TDMA is therefore highly desirable. Unlike peer-peer half duplex communications and other low-cost alternatives run over unlicensed spectrum, providing PTT over regulated technologies such as 1xEVDO-Revision A is challenging. The foremost stumbling block is temporal resource allocation overheads during call setup. Call setup can take considerable time owing to network complexity and the state changes involved in setting up a Traffic Channel (TCH).

This paper surveys a repertoire of optimization techniques that can be used to improve network performance metrics to acceptable limits for PTT deployment.

Keywords-1xEV-DO Rev. A; Push-to-Talk; Differentiated Services; Wireless; Wide Area Networks.

I. INTRODUCTION PTT, or walkie-talkie as it is informally referred to, is a

peer-to-peer half-duplex communication technology generally implemented using low power transceivers over limited range. But field experience has shown that PTT requires wider coverage and ought to be more robust during emergent situations. Subsequently, PTT has been successfully offered over wireless WAN technologies such as TDMA, CDMA and GSM over the past decade, leveraging the stability and robustness that carrier-grade regulated networks offer. Ideally, PTT services can be deployed over any technology that supports dormancy in Access Terminals (AT). Dormancy is the ability of ATs to temporarily relinquish air resources when idle for prolonged periods. Dormant to Active (D to A) transitions, which causes the AT to reacquire its air resources, take time and a network that is not optimized may appear too sluggish to the user.

In the United States, Integrated Digital Enhanced Network (iDEN™) is one such PTT service, which was developed by Motorola in the mid-nineties. This technology was deployed with extensive coverage over TDMA by Nextel communications in 1996. Since then

Qualcomm has offered similar services over CDMA 2000 (BREWChat™), and has more recently provided a packet switched solution over 1xEVDO (QChat™). 1xEVDO based PTT is different from previous technologies in that it is entirely based on Internet Protocol [2] at the level of the application server. Infrastructure manufacturers such as Nortel Networks, Alcatel-Lucent, and Motorola notably, provide PTT solutions based on Qualcomm’s QChat™, with heavy optimizations for reduced latencies that compare with circuit switched technologies. In this paper, we specifically address PTT optimizations for 1xEVDO, but the philosophy can be extended to other wireless technologies.

1xEVDO Rev. A [5] (DOrA) is basically an evolution of CDMA 2000 [6] providing high speed IP based data service and improved carrier capacity over CDMA 2000. Unlike other circuit switched implementations of PTT, this packet-switched version is highly scalable, and is therefore more cost effective. Given all of its apparent advantages over CDMA 2000 [7], it is seen that the foremost constraint in DOrA is temporal overheads for resource allocation during call setup, which needs to be wrestled down to meet stringent PTT performance requirements. The techniques specified here seek to optimize call setup delays in the following ways:

• reduced sleep time for Access Terminals • prioritization of PTT call attempts over regular

calls • proactive transmissions prior to TCH allocation • preferential treatment of PTT flows using tags

In this paper we define pertinent performance indicators, provide a survey of optimization options, and finally present call flows to demonstrate the impact of the indispensable ones.

II. SYSTEM ARCHITECTURE

A. Overview A major change in the evolution from CDMA 2000 [1]

to 1xEVDO [5] is the removal of the Mobile Switching Center (MSC) from the control plane. The resulting distributed architecture is more scalable and reliable.

The topology is simplified considerably from the original reference model with links from Base Transceiver Systems (BTS) being aggregated through a router and managed by a Base Station Controller (BSC).

2009 Third International Conference on Next Generation Mobile Applications, Services and Technologies

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DOI 10.1109/NGMAST.2009.37

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Figure 1. Topology of a PTT system layered over 1xEVDO

Particularly notable are the conspicuous absence of the Mobile Switching Center (MSC), the Home Location Register (HLR) and other centralized elements of CDMA 2000. Resource allocation and control of the radio network falls entirely under the purview of the BSC, which works in conjunction with the Packet Data Serving Node (PDSN) on the back-end, and the BTS on its front-end. The distributed nature of this topology, however, introduces certain unique challenges. Network latency, which up until now was a negligible component in centralized architectures affected only by backplane delays, becomes an important aspect of call setup. Unlike previous architectures that use statically reserved resources per call, transit delays in relaying messages between the BTS and the BSC during D to A transition can add precious milliseconds. As mentioned earlier, this is unavoidable for the case in point, since dormancy support in ATs is necessary for the deployment of PTT.

The general topology of a PTT system is shown in Figure 1. The BTS and the BSC together constitute the Access Network (AN). The AN connects to the Public Data Network through the PDSN, which basically serves as the gateway. Various features can be layered over this infrastructure through the addition of specialized servers to the topology. A commonly used approach in implementing PTT is to deploy an application server in the network that cooperates with existing 1xEVDO components to manage PTT calls.

B. Push-To-Talk Service The PTT application server manages call activation,

group membership tracking, media duplication and distribution, and conversation moderation for sessions. The general use-case scenario is that of an AT communicating with a group, not much different from that observed in Instant Messaging. Media micro-flows have to be mirrored and distributed to each active member of the group. Therefore, in addition to being in the control plane, the PTT application server needs to be involved in the data plane as well, for the sake of media bridging. PTT

related layer 3 messaging originating from or terminating at the AT is routed as IP packets by the PDSN, and processed by the PTT application server.

For scalability, the PTT application server itself may be built as a distributed system composed of multiple functional entities, which in turn could adversely impact some of the performance metrics discussed here.

III. PERFORMANCE METRICS In order to quantify the impact of various optimizations

on system response time, four important indicators are identified. These performance metrics serve to represent the user’s perception of network latency.

A. Initial Dispatch Latency Initial Dispatch Latency (IDL) is the time interval from

the push of the button on the AT, to the reception of an auditory signal (beep) that indicates a floor grant from the PTT application server. At that point, the AT has an official permit to talk. The permit to talk can be granted in one of two ways. It can be guaranteed, in which case media delivery to the group is fully assured by the PTT application server.

Alternately, floor grant may be optimistically granted by the application server, in which case the confidence factor in TCH allocation is only as good as the paging success rate for the network. Optimistic allocation of floor grant will speed up PTT response time and is a feasible option in deployments that have a very high call success rate. IDL specifically applies to use-case scenarios involving dormant ATs. When a call involving dormant ATs is initiated, D to A transitions will become a major component of IDL measurements. In this scenario, both originating and terminating ATs will go through TCH acquisition. The IDL metric, therefore, represents the worst-case scenario for call setup, where both ATs are in a dormant initial state. The unwritten rule is to maintain IDL to a value less than one second.

B. Initial Media Latency This metric, IML, is the time taken from the moment

floor grant is received by the originating AT to when media from the originating AT is available for playback at the terminating AT. This performance indicator accounts for the PTT application server processing overheads for media duplication and group activation. Vocoding delays at the ATs and at the PTT application server are the primary components of this delay and is not the focus of the optimizations discussed in this paper.

C. In-Session Dispatch Latency In-Session Dispatch Latency or ISDL is the time

elapsed from the push of the button to receiving floor grant or an audible beep at an AT, when it is already in an active PTT session. Since both originating and terminating ATs are in active PTT sessions, ISDL offers a best-case timing measurement. This metric is particularly useful for studying the impact of scheduling delay

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optimizations on the BTS.

D. In-Session Media Latency This performance indicator, ISML, is the interval from

the instant floor grant is received at the originating AT to the instant media delivery starts to the terminating AT when both ATs are in active PTT sessions. It is assumed that the originator starts talking without delay on receiving floor grant. This metric again is ideal for studying forward link scheduling optimizations on the BTS.

It may appear at this point that IML and ISML are essentially the same, since under normal circumstances, both ATs would have gone through state transitions and be active by the time the floor is granted to the originator. But the introduction of optimizations and ad hoc resource allocation may shorten IDL well enough, causing a talk permit to be granted to the originator while the terminating AT is still transitioning from a dormant to an active state. This possibility necessitates the definition of two independent metrics to measure media latency.

IV. OPTIMIZATIONS DOrA [5] standards incorporate a host of features that

improve on previous implementations. The Enhanced Idle State Protocol (EISP) feature, support of sub-synchronous Control Channel, and improved rates on the Access Channel all provide considerable relief to the aforementioned performance indicators. The Access Channel is the uplink signaling channel used for sending signal messages from the AT to the AN. Higher rates on the access channels make it possible to piggyback data on signaling messages. In addition to this, a change in the interpretation of call flows may facilitate an improvement in response time, as is explained in the following sections.

A. 1xEVDO Rev A Enhanced Idle State Protocol EVDO Rev-A [5] introduces support for reduced sleep

states for ATs through the EISP feature. In previous versions of the standard, the sleep period was statically defined to be of the order of 5.12 seconds. Taking limits, this will translate to a 2.5 second delay for an AT to even detect a page intended for it. As a result of this sleep state delay, PTT calls destined for a dormant member can take a major impact on turn-around time. Reducing sleep time arbitrarily to soften latency is not an option since lower sleep time translates to shorter battery life.

EISP offers a flexible sleep profile for ATs as a function of its idle time. By nature, PTT call arrivals tend to be clustered. Therefore, an AT that has recently received a page needs to be more alert or sleep less than one that hasn’t been paged in a long time. The ATs and the AN implementing EISP, keep track of how many times an AT has had to wake up consecutively and not see a page. If the futile wakeup count is high, the AT can sleep longer intervals. If the count value falls below a certain range, the AT naps for shorter intervals. Up to

three thresholds for sleep periods are supported in the current standards. EISP is therefore essential for PTT deployment since it reduces IDL considerably by shrinking D to A transition times for terminating ATs to tolerable limits. EISP achieves this without compromising battery life of the ATs.

Figure 2. Traffic Channel Setup messages

B. Shorter Paging Cycle In addition to EISP, one way to speed up response time

is to use a shorter paging cycle. This again, has an impact on the battery life of ATs since now a higher rate of general page messages are broadcast to all ATs. Location based paging or a more sophisticated paging scheme may bring some relief by limiting the cell sites that get paged. Currently the architecture [5] supports a sub-synchronous control channel paging cycle of 213.33 ms.

C. Optimizing Traffic Channel Setup messages In the worst-case scenario, an AT would be dormant

with no TCH resources allocated to it. Establishing a TCH from the AN to an AT conventionally requires the last message, Traffic Channel Complete (TCC), to be sent over a successfully established channel. TCC therefore semantically indicates to the BSC that full duplex connectivity has been successfully established to the AT from the network. In situations where TCH setup success rates are high, apriori traffic flow before all conventional handshaking completes is an option. The existing four-way handshake that straddles DRC and Pilot acquisition can therefore be re-interpreted to achieve an improvement in IDL. Figure 2. shows the various phases during TCH setup, with the utilizable phase identified.

D. Data over Signaling Channel (DoS) DOrA provides a feature by which data bits can be

carried over the signaling channel as part of the Multi-flow packet application protocol. This allows the AN and AT to exchange data even when a TCH is not available or

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is being set up. Retransmission and duplication capability of the signaling channel gives more reliability to this transmission mechanism, but this can be selectively turned off to improve turn around time. The sender can set the Ack Required field to force the receiver to send a DoS acknowledgement by standards.

For PTT applications, Ack Required field can be turned off to decrease turn around time when sending data over the signaling channel. The DoS application protocol provided by DOrA is an improvement over the Short Data Burst (SDB) feature provide in CDMA 2000 [8] and can be exploited to circumvent D to A transition times on ATs for PTT. Session Initiation Protocol (SIP) messages, particularly the INVITE message, do not have to wait for TCH set up to complete, and can be carried over DoS protocol to provide improved IDL[4].

E. Persistence Override Feature The Persistence Test feature allows the AN to control

AT usage of the Access Channel (uplink signaling channel) especially during times of overload. The AN can arbitrarily increase the persistence value sent out to different users. The ATs would then have a different success rate to be able to transmit on the next access probe [5]. The AN can even block the Access Channel entirely by setting persistence vectors appropriately. Overriding the persistence setting is recommended for PTT capable ATs, since this would give them a higher success rate at transmitting on the very first access probe.

F. Prioritization of Flows DOrA expands on the Radio Link Protocol (RLP)

specifications defined for revision 0 [5]. RLP can now support multiple simultaneous data flows up to a limit. Differing Quality of Service (QoS) requirements can therefore be mapped to different RLP flows over the same session stream. A flow identifier that allows micro-flows to be bundled within a DOrA stream will then uniquely identify each RLP flow.

In this approach, the PTT Application Server can mark SIP messages [9] used for call setup for selective prioritization. The Application server simply utilizes the existing Differentiated Services Code Point (DSCP) scheme to tag individual packets [10]. The Access Network can then map these packets to the appropriate RLP flow for prioritized scheduling. Typically the AN, in decreasing order of priority, handles at least four RLP flows:

• PTT Session Signaling (PSS) • Delay Sensitive (DS) • Rate Sensitive and (RS) and • Best Effort (BE)

SIP messages associated with PTT would ideally be mapped to the PSS class for a quicker response.

G. Relaxing Talk Permit Requirements As mentioned earlier, the PTT Application Server moderates all sessions. The PTT server sends a floor grant

message or a talk permit to a requesting AT once a session has been approved. The PTT server issues a Guaranteed Talk Permit (GTP) to the talker only if it has heard back from at least one member of the group, subsequent to forwarding a session invitation to the entire group. This approach ensures that the Digital Signal Processing (DSP) resources are not allocated prematurely when there are no responders to the call. This approach is termed Optimistic Talk Permit (OTP) and reduces IDL by a substantial amount. But under overload conditions, this relaxation of talk permit criterion may lead to poor network utilization.

Figure 3. Push-to-Beep call flow for GTP with no DoS

H. Embedding PTT Session Initiation into Page message The conventional approach to delivering a PTT call is

to first locate the terminating AT by paging it, and then to deliver PTT Session Initiation (PSI) Message over the TCH once it has been established, as shown in Figure 3. Considerable savings in IDL can be achieved if the initial PSI Message can be compressed into the page message itself. The special page message would have to be constructed only for PTT calls. The PTT server may insert DiffServ code points in PSI Messages, indicating to the BTS that these are related to PTT. The BTS can then do the necessary encapsulation. When used in conjunction with DoS, the acknowledgement to the PSI Message can then be transmitted back through the return link using DoS, moving D to A transition for the terminating AT entirely out of the critical path. This is discussed in more detail in the following section. The discussion of empirical data related to this optimization, however, falls outside the scope of this paper.

V. CALL FLOWS As mentioned earlier, the use of DoS for GTP sessions

helps remove D to A transitions from the critical path for call setup. The metric under observation in these charts is IDL, also referred to as Push-to-Beep timing. The PDSN is intentionally left out of the flow diagram for the sake of

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brevity. Link budget analysis will normally have to take backhaul latency into consideration, but is also ignored here. The various call set up messages shown in the following figures are in generic form, and may be either standards based SIP messages or a propriety one.

The call flow in Figure 3. pertains to a network with no support for DoS. A PTT session starts off with the originator depressing the PTT button, at which point a PSI Message is sent out. In the worst-case scenario (IDL), the originating AT is in a dormant state, causing it to first get TCH established, before a PSI Message can be transmitted to the PTT server. In step 2, D to A transition occurs and this process may well be to the tune of 300ms, with possibly large variance. Once TCH is established in step 3, PSI Message is sent over the RTC. The PSI Message then gets forwarded to the PTT server through the PDSN. On receiving the PSI Message, the PTT server would activate the group to which the message needs to be sent. A PSI Message is then sent from the PTT server to all members of the group. Only one group member is shown in this figure for brevity. Again the worst-case scenario has the far-end AT in a dormant state, which means it has to be paged and located initially. The AT, upon receiving the page, first sets up its TCH and then receives the PSI Message over the newly established channel, all the while incurring another impact in IDL due to its D to A transition. The Acknowledgement that the far-end AT sends back is then routed to the PTT server. To minimize latency, upon receiving the very first Acknowledgement from the group, the PTT server sets aside DSP resources for the entire group and sends a floor grant message to the originator, permitting the originator to start talking. With no DoS support, it is seen that D to A transition falls into the critical path for call set up on both ends of the call. This could potentially add up to a second or more to the IDL metric pushing IDL over the 1-second limit beyond which users may sense system sluggishness.

The second scenario, shown in Figure 4. illustrates the advantages of using DoS protocol. When a PTT request is made, the PSI Message is sent over the uplink using DoS even before TCH is established. On the terminating side, the page message is modified to carry PSI information embedded within. The Acknowledgement from the terminating AT is sent back using DoS. D to A transition delay is therefore fully removed at both ends of the call. Empirical studies show that an improvement of 19.48% for the mean value of Push-To-Beep delay can be achieved through this optimization.

It should be noted that D to A transitions could take an inordinate amount of time and its variance needs to be fully categorized. Under unique circumstances, an originator may get her talk permit before the terminating AT has completed its D to A transition. Voice packets may therefore arrive at the far-end AT destined for a terminal that still has no TCH established to it. This would require buffering mechanisms at the AN to ensure loss free media transmission. As mentioned earlier, it is therefore entirely

possible that excessively reducing IDL may have the effect of increasing Initial Media Latency or IML.

Figure 4. Push-to-Beep call flow for GTP with DoS

VI. DATA DELIVERY AND SCHEDULING DELAYS By definition, in-session data delivery and scheduling

delays preclude call setup latencies. The aforementioned optimization techniques focus on reducing call setup latencies by sending traffic apriori to traffic channel allocation state change, or by making use of the signaling channel to overlap communications while D-A transition is in progress at the AT. Improving in-session data delivery involves enhancing processing power. A typical intra-region call flow is shown in Figure 5.

The call originates from AT1 and terminates at AT2. The same RAN, PDSN and PTT Server serve them both. In this scenario, the transaction originating at AT1 goes through initial vocoding delays, followed by a reverse traffic channel delay leading up to the Radio Access Network (RAN). This delay is aggregated with BTS processing overheads, after which the assembled packets are transmitted to the PTT Server via the PDSN incurring additional propagation delays. Signal processing modules within the PTT Server does media replication if needed (to support group-wide communications), and the packets are then routed back to the RAN via the PDSN for forward link transmission.

These packets are subject to additional buffering and playback delays at the receiving AT. It is seen that with these optimizations in place, the worst case is constrained to 40 ms of the typical in-session delay.

The flow diagram illustrates that vocoding, buffering and media replication, and scheduling delays overwhelmingly influence in-session latency. These delays can be improved by faster signal processing, improved scheduling to cut down transmission delays, and higher link speeds leading to lesser propagation delays.

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Figure 5. In-session data delivery and scheduling delays

VII. CONCLUSION It is seen that unlike conventional VoIP calls, PTT

offers a unique challenge along both the control and data planes. Some of the optimization choices discussed in this paper may impose implicit constraints that are not quite intuitive. For instance, SIP messaging, even under compression [3], may run into several bytes, which may render it unsuitable for encapsulation within DoS and/or general page message. Dynamic dictionary based signaling compression framework that may allow SIP signals to be accommodated within DoS and/or page messages have been proposed in [11][12][13]. An alternative, of course, is to use a proprietary protocol for signaling. Other optimizations discussed in this paper are those that can largely be implemented without deviating from open standards. Most service providers often hold an IDL of less than one second as a golden standard. Keeping call set up times low enough to meet this de-facto requirement in an increasingly complex topology is an ongoing area of research.

ACKNOWLEDGMENT The authors wish to thank the members of the CDMA

Systems Design Group at Nortel for their valuable suggestions.

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