Beginning with GPRS in 1999, cellular data speeds have increased over the last decade by a factor of 10 every three to five years. Increased consumer demand for wireless data bandwidth has driven this growth (Fig. 1). Reporterlink has estimated that wireless data traffic will increase tenfold between 2009 and 2017 for a 59% compound annual growth rate (CAGR).1 Data traffic is expected to hit 1.8 exabytes a month, fueled by a rapid increase in interactive data and multi-play applications.2 Video is the largest bandwidth consumer today, a fact that will continue for the foreseeable future.

Long-Term Evolution (LTE), as defined by the Third Generation Partnership Project (3GPP), is widely acknowledged as the next-generation technology for both voice and data wireless transmission. LTE was first specified in the 3GPP Release 8 specification in December 2008. With the exception of the air interface, LTE is an all-IP (Internet Protocol) network, taking advantage of and converging with IP network technology.

LTE has some impressive capabilities. For example, it supports multiple-input, multiple-output (MIMO) antenna technology, including 2x2 and 4x4 configurations. It also provides 300-Mbit/s downlink and 150-Mbit/s uplink bandwidth when using 4x4 MIMO. It boasts latencies of less than 5 ms. And, it can support hundreds of users per cell.

Most major telecom equipment manufacturers (TEMs) and carriers have announced their intention to develop and provide LTE products and services. As of early 2010, 51 providers in 24 countries have made commitments.3 Early carrier deployments are expected in Asia and North America in 2010, with significant expansions in all major markets in 2012. 2013 will see some 85 million LTE subscribers,4 and nearly half a billion people will use LTE by 2015.5

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The strategic LTE components consist of a new radio standard and the eNode B, which supports the LTE air interface and performs radio resource management. The eNode B integrates the functions of the 3G node B and radio network controller (RNC), making the eNode B a complex element and flattening the network architecture. Alongside LTE development is the evolution of the core architecture, called the evolved packet core (EPC), which maximizes data throughput while minimizing latency and network complexity. The key functional elements of the EPC include:

• Mobility management entity (MME): The MME has a key role in the EPC in the handling of mobile users. It performs the signaling and control functions that manage the mobile users’ access to LTE, assigns network resources, and manages mobility states that support roaming, paging, and handovers. The MME oversees all control plane functions related to subscriber and session management.

• Serving gateway (SGW): The SGW is a node that provides data paths between eNode Bs and the PDN-GW. The essential capabilities of the SGW, aside from routing and forwarding packets, is that it acts as a local mobility anchor point for inter-eNode B handovers as well as managing mobility between LTE and 2G/GSM and 3G/UMTS networks. The SGW also provides charging services for user equipment, the packet data network (PDN), and service classes.

• Packet data network gateway (PDN-GW): The PDN-GW is the termination point of the packet data interface connecting to PDNs, providing the anchoring function for sessions with external networks. A critical function of the PDN-GW is the ability to enforce per-user packet filtering, allowing gating and rate enforcement policies as well as service level charging.

The EPC is the all IP-mobile core network for LTE, allowing the convergence of packet-based real-time and non-real-time services.

Other Wireless Technologies

As with existing deployments, LTE networks need to interoperate with a variety of existing wireless edge and core technologies. Principal among these are Universal Mobile Telecommunications System (UMTS) and IP Multimedia Subsystem (IMS).

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UMTS, which uses W-CDMA as the underlying air interface, is one of the 3G mobile telecommunications technologies. UMTS also covers the core radio access network (UTRAN). Two common technologies associated with UMTS are high-speed downlink packet access (HSDPA) and high-speed uplink packet access (HSUPA), together referred to as HSPA and sometimes called 3.5G. HSPA+ is a further evolution of HSPA that uses the same equipment. HSPA+ networks can expect a downlink transfer rate of up to 44 Mbits/s when using HSDPA+ handsets. As of 2010, more than 300 networks in 142 countries offer HSPA, and 66 operators have committed to HSPA+.6

IMS is an architectural framework for delivering IP multimedia to fixed and mobile users. It facilitates the blending of triple-play in interactive, personalized ways. Delivery of a triple-play bundle is a highly desirable objective for service providers of all types. IMS creates a network where new applications can be plugged in like Lego blocks. More competition is fostered, allowing operators to choose the most cost-effective equipment for each function.

Wireless Core And Internet Core Networks

Increased usage of HSPA+ and LTE networks will place increased bandwidth requirements on wireless core and Internet core networks. As more people simultaneously use multiple wireless technologies, the complexity of the core networking control plane signaling and the volume of bearer traffic will rise. The data requirements associated with multi-play applications will have two dramatic effects on the core networks: a need for increased capacity and a need for more intelligence. Wireless frequency bandwidth is an inherently limited resource. That bandwidth must be carefully balanced to provide all users with an acceptable quality of experience (QoE). Techniques such as deep packet inspection (DPI) must be used to properly identify information flows so as to provide them with the proper quality of service (QoS).

Wireless Testing Requirements

As the LTE revolution develops and progresses, wireless network testing not only must encompass the latest technologies, it also must incorporate the interactions between existing systems and emerging solutions. UMTS and IMS must function and interoperate with emerging 4G wireless solutions. Figure 2 shows the major components of a current end-to-end wireless network solution.

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The complete testing of a wireless service must separately test the components in each radio access network, wireless core network, and Internet core network; test each of the three subsystems independently; and test the entire system end to end, from the wireless edge through the Internet core. Proper testing occurs at multiple levels, usually in a sequential manner:

• Compliance testing is an essential first step in ensuring correct operation and interoperability. Compliance tests are built from RFC and other standards, and they are designed to perform positive and negative tests that ensure devices properly implement the standards.

• Functional testing further exercises device capabilities with combinations of options, multiple connections, differing types of traffic, and many sequences of operations. Full protocol stacks are tested at this level, along with further negative tests.

• Performance testing measures raw capacity, such as the maximum number of connections, maximum rate of connection establishment, and maximum uplink and downlink throughput.

• Scalability testing measures real-world effectiveness and the ability to handle a complete user community. This type of testing requires realistic traffic loads that meet and exceed network capacity, coupled with quality of experience measurements.

The testing of LTE components and networks is especially challenging due to their complexity and operational scale.

Radio Access Network (eNode B)

The eNode B is the key element of the LTE radio access network. It is the most complex part of the LTE access network. Also, it is more complex than the UMTS node B or GSM base transceiver station (BTS) since it operates without a controller (an RNC or a basestation controller, or BSC). The eNode B performs the functions of the central controller, making it a critical component of the new LTE network architecture.

The complexity of the eNode B is manifest in its complete stack implementations. The Uu interface embodies a number of previously distributed protocols. It is essential for testing to explore all stack layers, not merely the top layers.

Radio access testing for earlier wireless generations was often accomplished through the use of banks of modified handsets. The scale of modern eNode Bs makes this approach no longer viable. Similarly, low to moderate bandwidth testing used in earlier technologies cannot be used to stress a network that will transport 300 Gbits/s of download traffic.

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Both network and air interfaces must be simulated to fully test the eNode B. Coordinated testing using Uu, X2, and S1 interfaces is required, emulating the operation of other eNode Bs, MMEs, and SGWs. Many eNode B functions and procedures can only be verified through the emulation of all surrounding components.

For example, signaling between the user equipment (UE) and eNode B on the Uu interface is tightly coupled with intra-E-UTRAN signaling on the X2 interface and signaling between the eNode B and the EPC on the S1 interface. This tight coupling makes testing any one interface in isolation difficult, if not impossible. Testing through all of the eNode B’s interfaces verifies all eNode B user plane connections by:

  • Applying a variety of realistic user plane traffic flows
  • Coordinating each user plane flow according to the signaling exchanged with the eNode B
  • Verifying the content of user plane traffic flows transmitted by the eNode B
  • Exercising control of user plane frames at both the source and sink
  • Measuring the QoE delivered for each traffic flow

Evolved Packet Core

The EPC components—MME, SGW, and PDN-GW—together with the eNode B handle user mobility, unique bandwidth, and quality requirements of multi-play applications and error situations. The testing of these components must use multiple connection scenarios and real-world combinations of voice, video, and data traffic to ensure proper, scaled operation. EPC-specific testing challenges include:

• Capacity and performance: With thousands of eNode Bs, potentially carrying from 1 to 300 Gbits/s of traffic each, the capacity in the EPC could quickly become an issue in terms of high availability, validation, and network configuration.

• Media: A typical LTE mobile user will concurrently run many types of applications, including voice, texting, video viewing, and e-mail updates. Real-world traffic is required to fully validate QoE enforcement and the effect of gating and policy control on the network.

• Quality of service: Jitter, latency, packets dropped, and other measurements are key performance indicators in an all-IP network.

• Interoperability with 2G/3G networks: LTE will likely be deployed in small islands in a sea of existing 2G/3G networks.

• Multiple architectures: MMEs, SGWs, and PDN-GWs may be combined into single hardware units. Different combinations and network structures are possible.

• End-to-end data and control plane security: IPSec, TLS, and other security is required in some EPC components and optional in others.

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As with eNode B testing, coordination of control and user plane traffic from all interfaces is essential, utilizing the S11, S1, S8, and S5 interfaces that emulate eNode B, MME, SGW, and PDN-GW operation. With different combinations of these interfaces, it’s possible to test the functionality, performance, and scale of the MME, SGW, and PDN-GW.

UMTS

Wireless providers have invested heavily in UMTS/HSPA/HSPA+ technology, and they will push their deployment in favor of LTE for several years. The testing of these components and subsystems requires the same types of techniques used in LTE testing: coordinated use of all individual device and subsystem interfaces. The essential HSPA components are the node B, RNC, serving GPRS support node (SGSN), and mobile service switching center (MSC), using lub, lu-CS, and lu-PS interfaces.

Though not as much as with LTE, HSPA+ still requires a significant amount of multi-play data traffic to measure capacity—up to 44 Mbits/s on the download link and 11 Mbits/s on the upload link.

IMS

IMS promises to be a key component of LTE deployments, serving as the unifying mechanism for delivering voice and other services. It is a complex system encompassing many possible components that are linked by multiple protocols. Several base protocols, especially session initiation protocol (SIP), are being extended to support IMS functionality. Testing can be performed for the major categories:

• Core network: using protocols for testing call session control functions in the P-CSCF, interrogating CSCF (I-CSCF), and serving CSCF (S-CSCF)

• Interworking elements: using protocols for media and signaling gateways in the IP multimedia media gateway (IM-MGW), signaling gateway (SGW), media gateway controller function (MGCF), and breakout gateway control function (BGCF)

• Application servers: using protocols, including SIP, for application servers, home subscriber service (HSS), and subscriber location function (SLF).

Conclusion

The transition from 3G to 4G wireless networks and the required interoperability with legacy technologies will unleash a level of unprecedented complexity. Legacy technologies need to seamlessly interact with newer technologies for service providers to deploy networks that not only attract subscribers, but also limit maintenance and upkeep costs.

Service providers will continue to support multiple wireless technologies. It is essential that the wireless core network be tested under conditions in which UEs from different sources share the network and that their relative quality and bandwidth requirements be met.

References

1. Reporterlink, September 2009
2. An exabyte equals 1018 bytes, or a billion billion bytes
3. Global Mobile Suppliers Association (GSA)
4. Forward Concepts
5. Analysys Mason
6. Global Mobile Suppliers Association (GSA)