VoLTE Deployment and the radio access network The LTE User Equipment Perspective

Transcription

1 VoLTE Deployment and the radio access network August 2012 Rev. A 08/12

2 SPIRENT 1325 Borregas Avenue Sunnyvale, CA USA Web: Americas SPIRENT Europe and the Middle East +44 (0) Asia and the Pacific Spirent. All Rights Reserved. All of the company names and/or brand names and/or product names referred to in this document, in particular, the name Spirent and its logo device, are either registered trademarks or trademarks of Spirent plc and its subsidiaries, pending registration in accordance with relevant national laws. All other registered trademarks or trademarks are the property of their respective owners. The information contained in this document is subject to change without notice and does not represent a commitment on the part of Spirent. The information in this document is believed to be accurate and reliable; however, Spirent assumes no responsibility or liability for any errors or inaccuracies that may appear in the document.

3 Contents Introduction Dedicated Bearers Semi-Persistent Scheduling... 6 Robust Header Compression... 8 Discontinuous Reception Transmission Time Interval Bundling LTE Voice and Legacy Voice Services Considerations for LTE UE Developers Summary Spirent white paper i

4 INTRODUCTION One promise of Long Term Evolution (LTE) is the availability of a relatively flat, all-ip access technology that provides a bandwidthefficient method of delivering multiple types of user traffic simultaneously. Indeed, the ability to deploy Voice over IP (VoIP) services such as Voice over LTE (VoLTE), while also allowing high-rate data transfers, is one of the principal drivers for the evolution to LTE. In the context of deploying VoLTE, a lot of emphasis has been placed on the realization of an IP Multimedia Subsystem (IMS) and its associated Session Initiation Protocol (SIP) in a wireless environment. Undeniably, IMS and SIP are key to deploying VoIP services such as VoLTE in LTE networks. corresponding literature WHITE PAPER IMS Architecture: The LTE User Equipment Perspective Reference Guide IMS Procedures and Protocols: The LTE User Equipment Perspective Posters LTE and the Mobile Internet IMS/VoLTE Reference Guide It is IMS that provides the interconnect and gateway functionalities that allow VoIP devices to communicate with non-voip devices or even non-wireless devices. SIP defines the signaling necessary for call establishment, tear-down, authentication, registration and presence maintenance, as well as providing for supplementary services like three-way calling and call waiting. Without SIP signaling, or at least a proprietary equivalent, it would not be possible to provide VoIP telephony services. Without IMS or its equivalent, VoIP services would be limited to establishing calls between two VoIP users on the same network, and would not allow calls to users on parallel or legacy technologies. Therefore, it is no wonder that recent User Equipment (UE) testing and measurement has focused on two areas: The UE s ability to establish and maintain connectivity with an IMS network, including all of the registration, authentication, security and mobility associated with this connectivity The UE s conformance to SIP signaling protocol and SIP procedures/call flows, including any number of extensions that may be used in different deployment scenarios 1 Spirent white paper

5 However, to focus development and testing only on these two areas would overlook the most significant goal of VoLTE: to delivery carrier-grade (or telco-grade ) voice services that are perceived by subscribers to be as good as, if not better than, legacy circuit-switched voice services. This concept fundamentally differentiates VoLTE from other VoIP services. Deploying IMS and SIP will provide VoIP service in an LTE network, but VoLTE raises the bar to provide the carrier-grade voice service that is the vital objective of LTE networks and operators. Ensuring carrier-grade voice requires the marriage of IMS and SIP with a number of LTE Radio Access Network (RAN) features. It is this combination of IMS, SIP and RAN features that ultimately provides the carrier-grade VoLTE experience. The remainder of this white paper will identify this set of RAN features and how each of these features improves the quality of VoLTE service. Voice over LTE Objective: To deliver carrier-grade (or telco-grade ) voice services that are perceived by subscribers to be as good as, if not better than, legacy circuit-switched voice services. Spirent white paper 2

6 Dedicated Bearers One might ask why any of the many existing VoIP clients could not be installed on an LTE UE and used to provide carrier-grade voice services. The answer is competition for resources. As we all know, over-the-air bandwidth is a finite and precious commodity, even with the increased spectral efficiency offered by LTE. We also know that the number of applications using IP data and the total amount of data bandwidth these applications consume continues to grow at an exponential rate. Each of these applications and their associated data must compete for that finite bandwidth. From a network s perspective, the encoded voice packets generated by an off-the-shelf VoIP client are notionally indistinguishable from the data traffic associated with an download, viewing a YouTube video, web browsing, or any number of a host of other applications. The network will attempt to multiplex all of this generic packet data traffic, not only from a single user but from all users, onto a single shared channel. In LTE, these channels are the Physical Downlink and Physical Uplink Shared Channels (PDSCH/PUSCH). Residing in these physical channels will be at least one Evolved Packet System (EPS) bearer. The EPS bearer provides a logical connection between the UE and a Public Data Network (PDN) Gateway (PDN-GW). Typically, a Default EPS Bearer will be established to provide a logical connection between the UE and an Internet PDN-GW for the purpose of delivering this generic data traffic between the UE and one or more application servers (e.g. web server). One downside of the Default EPS Bearer is that there is no control over quality of service. A best effort strategy is used to deliver all of the generic traffic between the UE and the Internet PDN. When the finite resources of the network are overwhelmed, data traffic queuing takes place, leading to unforeseeable latency or dropped packets. This is obviously undesirable, or even unacceptable, for real-time applications such as a voice call. Dedicated Bearers Benefit: Dedicated Bearers allow VoLTE audio traffic to be separated from all other traffic and delivered with a higher QoS level 3 Spirent white paper

7 To overcome the best effort delivery of all indistinguishable traffic over a single EPS Default Bearer, LTE introduces the concept of an EPS Dedicated Bearer. A Dedicated Bearer allows certain types of data traffic to be isolated from all other traffic (for example, VoIP traffic from FTP file download). Each Dedicated Bearer (there can be multiple Dedicated Bearers establishing virtual connections to one or more PDN-GWs) is associated with a Traffic Flow Template (TFT). A TFT defines which traffic, based on source/destination IP addresses and TCP/UDP ports, should be delivered on a particular Dedicated Bearer. Typically for VoLTE, after SIP signaling is used to establish a voice session and negotiate the session parameters (e.g. which audio codec, bit rate, transport protocols and ports will be used for audio), an EPS Dedicated Bearer between the UE and an IMS PDN-GW is established for the express purpose of transporting encoded voice packets. Refer to Figure 1 for an example of the traffic usage of a Default Bearer vs. a Dedicated Bearer. Figure 1: EPS Bearer: Default vs. Dedicated Further, each Dedicated Bearer can have different service quality attributes specified. In LTE, a combination of Resource Type (Guaranteed Bit Rate vs. Non-Guaranteed Bit Rate), Packet Delay Budget (the maximum acceptable end-to-end delay between the UE and the PDN-GW), Priority (which can be dropped when network resources become scarce) and Packet Error Loss Rate (the maximum acceptable rate of IP packets that are not successfully received by the PDCP layer) are used to define a set of QoS (Quality of Service) Class Identifier (QCI) levels, refer to Table 1. Spirent white paper 4

8 Attributes for QoS Class Identifier (QCI) Attribute Resource Type Packet Delay Budget Packet Error Loss Rate Allocation Retention Priority Description Guaranteed Bit Rate vs. Non-Guaranteed Bit Rate Maximum acceptable end-to-end delay between the UE and the PDN-GW Maximum acceptable rate of IP packets that are not successfully received by the PDCP layer Value assigned for scheduling when capacity is reached, with 1 being highest level Table 1: QoS Class Identifier for LTE Table 2 provides a definition of standardized QCI values. A Dedicated Bearer established to carry VoLTE traffic may typically be assigned a QCI value of 1, indicating a guaranteed bit rate (largely consistent with the fairly predictable output of a vocoder), a maximum end-to-end latency of 100ms and a maximum tolerance 10-2 for IP packet loss. Traffic on a Dedicated Bearer with QCI=1 would be prioritized over all best effort traffic on the Default Bearer. As other applications are deployed in the future, multiple dedicated bearers may be used, each with a different QCI value. For example, a video telephony implementation may choose to transport audio using a Dedicated Bearer with QCI=1 and place video on a different Dedicated Bearer with QCI=2. This would indicate that both audio and video should be prioritized over best effort traffic. It also indicates that audio traffic is more important to deliver with lower latency (100ms packet delay budget vs. 150ms) while video traffic is more sensitive to packet errors (10-3 packet error loss rate vs ). QCI Resource Type Priority Packet Delay Budget (ms) Packet Error Loss Rate Example Services 1 GBR Conversational Voice 2 GBR Conversational Video (live streaming) 3 GBR Non-conversational video (buffered streaming) 4 GBR Real-time gaming 5 Non-GBR IMS Signalling 6 Non-GBR Voice, Video (live streaming), interactive gaming 7 Non-GBR Video (buffered streaming) 8 Non-GBR TCP-based (WWW, , FTP) 9 Non-GBR Table 2 Standardized QCI Values 5 Spirent white paper

9 Semi-Persistent Scheduling As mentioned above, shared channels (PDSCH/PUSCH) are used at the physical layer to transport the data carried by the logical bearers. Since these channels are shared amongst all of the users on an enodeb, there must be a way to allocate these channels to avoid multiple users trying to simultaneously use the same resource. In the frequency domain an LTE carrier is divided into a number of subcarriers (currently anywhere from six to one hundred depending on the bandwidth of the LTE carrier). In the time domain each subcarrier is grouped into 0.5ms time slots during which either six or seven of OFDM symbols can be delivered, depending on whether the system is using normal or extended cyclic prefixes (inter-symbol guard periods). See the 3GPP s TS document for details. This results in a time-frequency grid of subcarriers and time slots (refer to Figure 2). A grouping of twelve subcarriers in one time slot duration is known as a Resource Block (RB). An RB is the minimum allocation of the LTE physical layer resource that can be granted to a UE. A pair of physical control channels is used to grant RBs to UEs operating on the network. The UE uses the Physical Uplink Control Channel (PUCCH) to request allocation of the PUSCH, and the UE is granted both uplink and downlink allocations via the Physical Downlink Control Channel (PDCCH). The PDCCH identifies which subframes (a subframe is two slots) a UE should decode on the PDSCH, and which UEs are allowed to transmit in each uplink subframe on the PUSCH. Since every RB on the downlink and uplink must be granted, VoLTE introduces a challenge: granting control channel overhead becomes too great for the necessary persistent and near continuous allocation of RBs to deliver the relatively small packets typical of a VoIP-based conversation. Figure 2: LTE Physical Layer Resource Block Spirent white paper 6

10 Semi-Persistent Scheduling (SPS) was introduced to minimize granting overhead for applications such as VoLTE. SPS takes advantage of the fairly consistent and predictable transmission pattern of VoLTE packets (e.g., a VoLTE implementation might typically be sending an encoded voice packet every 20ms) to make a persistent grant of uplink and downlink RBs rather than scheduling each uplink and download RB individually. A persistent grant removes the need to make a separate grant for each 20ms of encoded audio. A Radio Resource Control (RRC) message is used to establish the periodicity of the recurring RB grant. The green boxes in Figure 3 illustrate the SPSscheduled RBs for a VoLTE call. As shown by the orange box in Figure 3, additional RBs can be dynamically scheduled for data traffic while SPS is enacted (e.g. enable a web page download while on a VoLTE call). Figure 3: Semi-Persistent Scheduling One potential downside of SPS could occur in situations where there is silence during a VoLTE conversation. If the SPS grant is maintained during silent periods, physical layer resources are wasted. That is why SPS is semi-persistent ; when it makes sense, an SPS grant can be cancelled. If the UE does not transmit audio packets over a number of network-defined transmission opportunities, the uplink grant will implicitly expire. On the downlink, the network has the option of using an RRC message to cancel the grant. Thus the right balance can be struck between reducing control channel overhead and maximizing efficiency in the use of shared data channels. Semi-Persistent Scheduling Benefit: SPS greatly reduces the overhead associated with scheduling small and periodic VoLTE audio packets, thus reducing processing overhead and providing more bandwidth to accommodate additional users 7 Spirent white paper

11 Robust Header Compression Typical VoLTE calls consist of relatively small encoded audio packets being transmitted every 20ms. In fact, the size of the encoded data is smaller than the headers for the protocols that are used to transport the encoded data. Real-time Transport Protocol (RTP) is a standardized packet format used for media streams such as VoLTE audio. The User Datagram Protocol (UDP) then provides a transport-layer mechanism for the RTP stream between two Internet Protocol (IP) endpoints. In the case of VoLTE, this would be between the IMS voice client in the UE and a Media Gateway in the IMS core. Finally, an IP layer is used to establish network interworking. In the case of IPv6, which is typically used for VoLTE deployments, the combination of RTP, UDP and IP headers can be around 40 to 60 bytes long. To reduce the size of headers used to deliver VoLTE audio, Robust Header Compression (RoHC) is employed. RoHC is used over the air interface to conserve the precious bandwidth of the radio access network (refer to Figure 4.) RoHC takes advantages of the redundancy of some headers in various protocol layers, as well as the redundancy of information contained in the headers of subsequent packets in the same audio stream, to greatly reduce the size of the header overhead. The 40 to 60 bytes of header length can be reduced to as little as 3 to 4 bytes. With RoHC enabled, a VoLTE encoded audio transmission using the Wideband-AMR codec is reduced from around 75 bytes to around 35 bytes. Figure 4: RoHC Compression and Decompression at the UE and enodeb Spirent white paper 8

12 It should be noted that there are actually multiple usage profiles defined for RoHC: Profile 0: Uncompressed Packets that cannot be compressed with the following profiles Profile 1: RTP Compress packets using IP/UDP/RTP protocol headers Profile 2: UDP Compress packets using IP/UDP protocol headers Profile 3: ESP Compress packets using IP/ESP protocol headers Profile 4: IP Compress packets using IP protocol headers Profile 7: RTP/UDP-Lite/IP Compress packets using RTP/UDP-Lite/IP protocol headers Profile 8: UDP-Lite/IP Compress packets using UDP-Lite/IP protocol headers The above example of VoLTE transmission compression ratios assumed the use of RoHC Profile 1. Robust Header Compression Benefit: RoHC can achieve a nearly 50% reduction in the size of VoLTE audio transmissions, thus decreasing bandwidth needed for any single call and increasing the overall number of users on an enodeb site 9 Spirent white paper

13 Discontinuous Reception Packet-based voice services such as VoLTE encode periods of audio conversation (VoLTE is typically 20ms periods) and then rapidly burst-transmit the encoded period of audio to the receiver for decoding and playback over the 20ms period. When viewing over-the-air transmissions, it is apparent that each encoded audio packet transmission is followed by a period of no transmission. Discontinuous Reception (DRX) takes advantage of these silent periods to turn off the RF receiver of the UE, as well as other entities such as A/D converters and digital signal processors associated with downlink demodulation. This reduces the drain on the device s battery and increases talk and standby usage time. RRC messaging is used to enable DRX and establish the UE receiver s on/off pattern. Given that the network established the DRX pattern, it will know when the UE is monitoring the PDCCH and know when to schedule downlink data to the UE. Selection of the DRX pattern must carefully be determined based on the latency requirements of the application and the need to receive any possible retransmissions. Having too long of a sleep period may lead to latency greater than the desired performance based on the QCI value in use. Refer to Figure 5 for an illustration of a DRX pattern. Figure 5: DRX Pattern Spirent white paper 10

14 DRX can also operate in one of two different modes: Long DRX and Short DRX. Long DRX has the UE receiver disabled for a longer period of time, and could be applicable during periods of silence in the conversation when audio packets are sent less frequently. However, when audio is consistently present, Short DRX can be used and a cycle can be mapped to the periodic arrival of audio packets. Switching between Long DRX and Short DRX is controlled by the enodeb s MAC Layer and/or an activity timer at the UE. Refer to Figure 6 for an illustration of Long and Short DRX. Figure 6: Long and Short DRX Discontinuous Reception Benefit: DRX helps save the UE s battery life during a VoLTE call by allowing the UE to turn off its receiver in between reception of audio packets 11 Spirent white paper

15 Transmission Time Interval Bundling LTE introduces a shorter Transmission Time Interval (TTI) than was offered in previous cellular technologies. Specifically, a 1ms subframe is defined as the TTI. Since resource scheduling is done for each TTI, a smaller TTI facilitates low over-the-air latency for real-time applications. VoLTE is an example of an application that benefits from this short 1ms TTI. However, the short TTI does lead to uplink issues in select scenarios, most notably at the edges of enodeb coverage. When an enodeb detects that a UE is at a cell edge where reception is deteriorating and the UE cannot increase its transmit power, the enodeb can initiate TTI bundling via RRC messaging. In essence, this means the UE will increase the error detection and correction associated with each data transmission by transmitting over multiple TTIs (for example, bundling four consecutive TTIs). With this enhanced error detection and correction, overall latency is less than when using a single TTI. Figure 7 shows how TTI bundling helps deliver lower-latency VoLTE data at cell edges, where data errors are expected. Rather than wait for the HARQ process (normal HARQ interlace period is 8ms) to ask for a retransmission of data with new error detection/ correction bits, TTI bundling assumes that data will need to be retransmitted. In TTI bundling a number of data packets are pre-emptively packed into a single HARQ interlace period. Each packet contains the same source data coded with 4 different sets of error detection/correction bits. Also, HARQ retransmission adds HARQ ACK/NACK overhead that TTI bundling does not. Figure 7: Effect of TTI bundling on latency Transmission Time Interval Bundling Benefit: TTI Bundling increases the uplink efficiency at cell edges by using multiple bundled TTIs to transmit increased error detection and correction data Spirent white paper 12

16 LTE Voice and Legacy Voice Services While IMS-based VoLTE, deployed with the RAN features mentioned above, will provide a high-quality voice experience when a user is in LTE coverage, consideration must also be given to these same users when not in LTE coverage or when leaving LTE coverage. This is especially important given that most initial LTE deployments will not be as ubiquitous as the underlying 3G coverage. This will certainly lead to situations in which a UE on an active VoLTE call will need to transition that call to a legacy network as the UE roams out of LTE coverage. In early deployments of LTE, there are two general approaches to handling scenarios when the UE moves out of LTE coverage: single radio solutions such as Circuit-Switched FallBack (CSFB) and dual radio solutions such as Simultaneous Voice-and-LTE (SVLTE). With either interim approach, voice traffic is being handled by the legacy circuitswitched networks and they are not, at the root, LTE solutions. A second phase in LTE voice evolution introduces VoLTE and utilizes a single radio solution that seamlessly maintains voice service as the UE moves in and out of areas with LTE coverage. This involves completing a seamless handover from VoLTE to legacy circuit-switched voice technology. Often referred to as Single Radio Voice Call Continuity (SRVCC), this allows a UE, at the proper time and with the proper direction from the network, to handover and retune from LTE to a legacy GSM or UMTS network (or even a 1X network in the case of legacy 3GPP2) and simultaneously transition the audio stream from VoLTE packet-switched delivery to GSM/UMTS (or 1X) circuitswitched delivery. This provides for a cost-effective UE (a single radio design is used) that can perform voice services in the most efficient manner (VoLTE when in LTE coverage; circuit-switched otherwise) and deliver a positive user experience (calls are maintained even when the UE moves out of LTE coverage). Refer to Figure 8 for an illustration of a network topology supporting SRVCC. Single Radio Voice Call Continuity Benefit: SRVCC provides for a quality user experience by maintaining voice calls when VoLTE becomes unavailable due to loss of LTE coverage 13 Spirent white paper

17 Figure 8: SRVCC in an LTE + UMTS deployment This is not without complications, however. Implementation of SRVCC must take into account that the network and the UE are trying to accomplish at least three non-trivial tasks in near simultaneous fashion while minimizing any disruption to the real-time voice call that is in progress: The UE must retune to a new frequency (and most likely retune to a new band) as it switches from LTE to the legacy network The UE must acquire and begin transmitting on the legacy network Both the network and the UE must transition from delivering audio packets via a packet-switched solution to a circuit-switched delivery As a result of this complexity, commercial deployment of SRVCC is not expected until 2013 at the earliest. Considerations for LTE UE Developers Every one of the RAN and mobility features mentioned above is not only needed to make carrier-grade VoLTE deployments a reality, it also requires implementation within the UE to complete deployment, presenting a new level of complexity in UE development and testing. UE engineers will need virtually unlimited configurability of IMS procedures and SIP signaling to verify the incorporation of RAN features in the UE and the management of mobility scenarios and handovers that will occur between LTE and 3G technologies. For each of the RAN features described in this paper, some considerations for UE developers are listed below. Although not exhaustive, this list is meant to provide a broad view of the complexity involved in UE development in pursuit of carrier-grade VoLTE. Spirent white paper 14

18 Dedicated Bearers: At the end of the SIP negotiation to start a VoLTE call, the Evolved Packet Core (EPC) of the network will initiate the Dedicated EPS Bearer Context Activation Procedure to establish the bearer for the audio traffic. The UE must be able to complete this procedure and use the Dedicated Bearer. SPS: The UE must be able to support RRC messaging specifying periodicity of recurring RB grant; SPS-Config Information Element is described in detail in the 3GPP s TS document. The UE will also need to manage switching on/off SPS based on Data Quality (QCI) and traffic. SPS behavior is defined in RoHC: The UE must be able to support compression and decompression of header information for different traffic types: UDP, RTP, IP as defined in the IETF s RFC 4995 (for RTP & UDP) and RFC 4996 (for TCP/IP). DRx: The UE must have ability to switch between long and short DRx in response to all the relevant timers (as defined in TS ). TTI Bundling: The UE must be able to transmit over multi TTI and receive, per TS Note that while many discussions of TTI bundling treat the bundle size as an arbitrary even number, TS defines TTI_BUNDLE_SIZE as 4. SRVCC: The UE must be able to complete the LTE to legacy network handover as well as change its audio traffic from packet-switched to circuit-switched. IMS Network Emulation: Support of all necessary functionality for successful VoLTE deployment requires a network emulation test solution that provides complete integration of IMS infrastructure emulation, a fully implemented and configurable EPC and a programmable enodeb implementation. Further, the network emulation solution requires incorporation of multiple radio access technologies (LTE plus WCDMA/GSM) along with the tight coupling of the EPC to generate and coordinate the mobility scenarios necessary to verify the UE s SRVCC implementation. 15 Spirent white paper

19 Summary IMS and SIP are necessary technologies for deployment of VoIP in an LTE environment, but it is ultimately the introduction of LTE RAN features that creates the differentiation between VoLTE and VoIP. Specifically; Dedicated Bearers allow for the prioritization of VoLTE audio packets over all other best-effort traffic Semi-Persistent Scheduling reduces the complexity and overhead of the continuous allocation of downlink and uplink physical layer resource blocks to transport the audio traffic Robust Header Compression reduces the bandwidth associated with the headers used to transport relatively small encoded audio packets Discontinuous Reception helps conserve battery life of the UE during a VoLTE call Transmission Time Interval Bundling overcomes the limitation of using short (1ms) TTIs at cell boundaries SRVCC provides the mechanism to maintain an active voice call as a UE moves from LTE coverage to legacy networks While much focus has been placed in testing a UE s IMS connectivity and SIP signaling conformance, ultimate success of carrier-grade VoLTE deployments will depend on fully integrated testing of a UE s signaling along with the negotiation, establishment and usage of the associated RAN features mentioned above. As discussed in this paper, carrier-grade VoLTE presents unique technical challenges and considerations for the UE engineer. Spirent is a global leader in LTE device testing and is well positioned to assist in addressing the challenges and test requirements early on in the development cycle. Spirent s CS8 Device Tester provides all of the components necessary to support development and testing of a UE s VoLTE capability during the research and development phases of the UE lifecycle. This white paper is the third in a series of tools aimed to educate and support UE developers as they contribute to the deployment of IMS/VoLTE. Please see Spirent website ( for other free white papers, recorded seminars, posters and other resources that may be helpful to the UE developer. Spirent white paper 16

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