White Paper – Small Cells Backhaul Performance Assurance
Small Cell Backhaul Performance
Assurance
Is Your Network Ready?
Introduction As low-­‐power, short-­‐range small cells emerge as the technology of choice for improving mobile network coverage and capacity; operators will need to ensure that subscribers have the same quality of experience on the new network of access points as they do on the traditional macro cellular network. Backhaul network performance is a significant contributing factor to the end user experience. But with the potential introduction of several thousand new small cells in the network, conventional methods of assuring mobile backhaul performance will not be economically feasible. The challenge that mobile network planners and field operations personnel face is how to monitor small-­‐cell backhaul performance during deployment and after service launch without adding significant capital or operating expenditure. This white paper discusses the challenges of small-­‐cell backhaul performance assurance and proposes optimal solutions that will ensure operators’ small-­‐cell backhaul networks can be reliably verified. Small-­‐Cell Networks Need New Performance Assurance Methods The backhaul in a small-­‐cell environment has significantly different deployment characteristics compared to backhaul networks for large macrocells, yet the requirement for performance monitoring remains the same. The basic difference is that the backhaul transport network needs to reach a much larger number of distributed base stations. Also, small-­‐cell backhaul is constrained by cost, size and location to a far greater extent than traditional macrocell networks. These limitations have forced operators to consider using technologies in addition to traditional fiber or microwave for the last-­‐mile connection between the small cells and the macrocell or other aggregation point. Whether the small base stations support 3G, LTE, Wi-­‐Fi, or some combination, the backhaul options that operators are considering, and in some cases already using, are point-­‐to-­‐multipoint fiber (such as PON), various forms of DSL, DOCSIS, Wi-­‐Fi and a plethora of new wireless technologies operating in a range of licensed and unlicensed frequency bands, such as sub-­‐6GHz, 60MHz, 70GHz and 80GHz. But not one of the many technology options will be able to meet all of the small-­‐cell backhaul requirements related to cost, coverage and capacity1. That means operators are likely to use a mix of different fixed-­‐line and wireless technologies for small-­‐cell backhaul. Figure 1 illustrates the concept of small-­‐cell backhaul networks. September 2012 | Rev 1.0
Figure 1. Small-­‐Cell Backhaul Network Concept Source: NGMN Alliance As operators adapt to using an array of backhaul technologies to reach numerous distributed base stations, they also need to rethink their transport network performance requirements and how they are measured at the point of service launch and during on-­‐going monitoring. Just as with macrocell networks, the backhaul connections to the small cells need to be activated, verified and monitored; they require “birth certificates” to ensure they meet an operator’s specified limits on KPIs, such as throughput, delay (latency), packet delay variation, jitter, packet loss and availability. Whether operators build their own backhaul network or buy transport capacity from a wholesale provider, they need accurate performance assurance for on-­‐going service monitoring or SLA validation. Conventional methods of producing birth certificates for macrocell backhaul involve installing demarcation equipment, such as NIDs, at the macrocell site and sending a qualified technician to run tests on the backhaul links. But while this can be cost-­‐justified in the case of a macrocell serving several hundred subscribers, this kind of OPEX outlay cannot be sustained for a small cell supporting a few tens of subscribers. It is also important to note that when planning the backhaul for small cells, operators should take into account their broader RAN evolution strategies for LTE and LTE-­‐Advanced. These transport networks are going to be the foundation for Heterogeneous Network topologies, in which layers of small cells and macrocells are coordinated. Discussed below are challenges that network planners are encountering in the design of small-­‐cell backhaul networks. The Low-­‐Cost Imperative Whether the access points are intended to fill coverage gaps in so-­‐called “not spots” or ease network congestion in busy hot spots, one of the biggest limiting factors on the small-­‐cell business case is cost. The equipment – including the access points, aggregation gateways and backhaul gear – as well as the installation of these devices must be extremely inexpensive compared to conventional base stations. To provide a rough idea of how tight this constraint is, one European operator is aiming for a total cost of ownership (TCO) of a small-­‐cell site to be 10 percent of the TCO for a macrocell site in a similar geographic location2. Given White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
that backhaul accounts for a significant portion of the TCO for a base station site – anywhere from 10 percent to 30 percent, and possibly even more – the downward cost pressure on backhaul for these access points is intense. With such cost restrictions, vendors are under pressure to bring down the cost of their equipment while operators are looking to minimize the number of truck rolls to each site, streamline installation procedures and automate service activation as much as possible without sacrificing quality of service. For example, an operator’s transport and RAN teams can coordinate so that the small cells and backhaul equipment are set up at the same time in order to reduce the number of site visits. Other ways in which operators can shave costs is to find compromises on certain performance requirements depending on the use case of the small cells. For example, the NGMN Alliance suggests that in the case of small cells deployed to boost capacity in network hot spots, the backhaul availability can be relaxed but backhaul capacity should meet or exceed that of the small cell. In another example from the NGMN, backhaul capacity provisioning can be relaxed for small cells used to fill coverage gaps in not spots, whereas availability cannot be sacrificed3. Realizing these cost savings requires accurate monitoring tools to ensure that the transport links are meeting the quality of service parameters specified by the operator. Location, Location, Location The type of backhaul that an operator decides to deploy for small cells will depend on what the base stations will be used for and where they will be located. But maintaining quality of service across different transport types in diverse outdoor and indoor locations is a challenge. This is particularly true of sites where a wireless technology is the most suitable option for backhaul, because it is more vulnerable to weather conditions. There are multiple use cases for small cells, but generally, they are deployed indoors and outdoors to fill network coverage gaps for voice and data services and to supplement network capacity in congested areas. Typical indoor locations are shopping malls, hotel lobbies, airports, train stations, corporate offices, sports stadiums or cafes. Outdoor locations include bus stops, lampposts, utility poles or the sides of buildings. Ideally, a small cell would be located where an operator already has backhaul infrastructure and a power supply available, such as street furniture (like lampposts) or street cabinets that house FTTx equipment. But that is often not possible. Small cells need to be located close to subscribers, and those places do not always have adequate backhaul in place. This is why operators are looking to alternative technologies to connect sites where they don’t have existing backhaul links. For example, DSL is proving to be a useful backhaul option for indoor deployments, such as operator retail stores in shopping malls or residential apartment buildings with poor signal propagation from the nearby macrocell. White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
Backhaul Moves to Layer 3 While mobile backhaul networks have undergone a shift from dedicated TDM-­‐
based circuits to Layer 2 Carrier Ethernet-­‐based transport in recent years to keep up with demands for more flexible capacity provisioning, small cells introduce Layer 3 protocols into backhaul transport. The small base stations will use IP backhaul almost without exception. And for LTE networks, regardless of base station size, IP backhaul is required. To monitor performance on the backhaul links in a macrocell network, operators rely on Layer 2 OAM standards, such as the ITU-­‐T’s Y.1731 or the IEEE’s 802.1ag. But for small-­‐cell backhaul, it will be critical to support Layer 3 test methods both as a passive reflector and generator in order to get a full view of network performance from the small cells to the core network. As Figure 2 shows, transport networks cross multiple Layer 2 network domains, the number of which depends on factors such as the scale of the deployment, the type of backhaul used and the number of third-­‐party providers involved. In a small-­‐cell transport network, a single Layer 3 domain sits above the Layer 2 domains. End-­‐to-­‐end network performance monitoring can be achieved in the single-­‐domain IP layer because it is not limited by the boundaries of operator domains as is the case at Layer 2. Standards for Layer 3 test methods include two-­‐way active measurement protocol (TWAMP, RFC 5357), one-­‐way active measurement protocol (OWAMP, RFC 4656), or the ITU-­‐T’s Y.1564 standard. Figure 2. View of Layer 2 and Layer 3 Networking •
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Arrow represent ‘Domains’ within a layer Each Layer provides a service to the layer above it Layer Protocols don’t leave their domains Carrier ‘A’ owns the service (and customer) •
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End to end service requires a ‘3 Party carrier to complete (Carrier ‘B’) Carriers do not share Domains rd
White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
Getting the Timing Right Where small cells provide handover in coordination with the macro network they will need to support synchronization. Without it calls would simply drop. But synchronization distribution across small-­‐cell backhaul will be a significant challenge for carriers. The synchronization standards that an operator needs will depend on the type of small cell, where it is located and the type of backhaul technology deployed. For example, 3G and LTE access points deployed in FDD spectrum require frequency synchronization, while TDD-­‐based access points also need phase synchronization. Looking ahead to LTE-­‐Advanced specifications, certain interference and coordination techniques that are relevant to small cells also need phase synchronization – such as, CoMP and eICIC. The standards that support both frequency and phase synchronization are the IEEE’s 1588v2, NTP, and GPS4. GPS is somewhat limited in a small-­‐cell synchronization scenario because while outdoor access points can be covered, indoor access points are difficult for the satellite navigation systems to reach, which could result in signal loss. Also, it adds cost to include a GPS receiver in an access point. The packet-­‐based synchronization is affected by delay and jitter, which is why accurate network performance monitoring is essential. But for small cells, the synchronization distribution is more challenging because some of the radio backhaul types used such as NLOS radios could be less reliable compared to the LOS radio and fiber pipes that feed traditional macrocells. Enter the Evolved Packet Core LTE networks require a new core network architecture, which is the flatter, all-­‐IP Evolved Packet Core. With the EPC comes the new X2 interface that creates a direct link between base stations (that is, eNodeB’s) for the first time in the history of cellular network design. The X2 interface opens new paths for traffic to flow between base stations in order to optimize the use of finite radio resources, increase operating efficiency in the RAN, reduce processing load in the core network and improve cell-­‐edge performance. This is particularly relevant to LTE small cells deployed to offload voice and data traffic from busy macrocells in high usage areas. Since handovers are enabled between access points, traffic can remain on the small-­‐cell layer, transiting from one access point to another without having to traverse the whole of the transport network and back again, which is an efficient way of offloading traffic from busy macrocells. But it also creates challenges for small-­‐cell backhaul due to the requirements for low latency and monitoring end-­‐to-­‐end network performance. For EPC networks utilizing the X2 interface, performance assurance is required both between the small cells themselves, and from the aggregation router to the small cells. This requirement drives the need for an active TWAMP generation capability; again, not just a passive reflector (please see Figure 3). Operators will need this generation capability in their Layer 3 testing mechanisms to be able to verify performance between the access points. And since it is a more dynamic network with the X2 interface, operators need to be aware of more subtle White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
changes in the network so that they can track performance trends and raise alarms before their subscribers notice a service problem. 1. Send test packet (probe) with a timestamp
4. Receive test packet and timestamp it
2. Receive test packet (probe) possibly timestamp it
3. Resend the test packet back possibly with another timestamp
Figure 3. Generator and Reflector Techniques in Layer 3 Testing Based on the timestamps it is possible to calculate: •
Delay (2-­‐way) if only the generator timestamps •
Delay (1-­‐way) if both generator and reflector timestamp •
Delay variation (Jitter) This method can also be used to calculate •
Frame loss (dropped packets) •
Connectivity faults (broken network) Source: Accedian Traversing the Internet Given the variety of different backhaul options available, it is inevitable that some operators will use a regular public Internet path as part of a backhaul transport link for small cells. That means Network Address Translation (NAT) will be critical for successful implementation of Layer 3 performance assurance mechanisms. NAT traversal is particular to small-­‐cell backhaul because it rarely happens in macrocell transport networks. To support NAT traversal, the performance monitoring system needs to have a way to “call home” and to act not just as a passive reflector but have some generation capability. Without support for NAT traversal, operators will not be able to verify network performance all the way to the small cell, which would give them an incomplete view of their transport network operations. White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
The Solution: Inject the Performance Assurance Capability into the Small Cell Given that backhaul for small cells has some fundamentally different requirements compared to transport networks for macrocells, traditional performance assurance methods need to be adapted accordingly. For a deployment of small cells, it will not be possible to install performance monitoring instruments at each cell site or send expensive field personnel to hundreds of sites to check that the backhaul links are operating properly before issuing a “birth certificate” before service launch. So, how can an operator measure and gather critical KPIs – such as delay, delay variation, frame loss and availability – across the small-­‐cell backhaul network? The answer is to have standards-­‐based performance assurance software agents reside in the small cells themselves or other network elements, such as a small cell router or demarcation device. This ensures that the backhaul to the small base station will be ready at launch and can be monitored during commercial operation. Since the base station supplier adds this software, operators do not need to install any additional hardware or software at the cell site or send a technician to turn up a service, which accelerates deployment times and saves equipment and operating costs. This solution also saves valuable space at the small cell site, because the software has zero footprint. If an operator’s small cell vendor does not include the performance monitoring software agents, then those capabilities can be added to the small cell via an SFP device. Just like the software-­‐based solution, the SFP device supports monitoring standards such as Y.1731 or TWAMP so that one-­‐way and two-­‐way measurements can be taken across the transport network. This hardware-­‐based solution is cost effective because the SFP device is compact and easy to install, so an operator need only send someone capable of using a screwdriver to install it. The cost savings of these approaches are clear. Using an SFP device costs a quarter of the price of using a traditional NID, while the built-­‐in responder costs one-­‐eighth of the traditional NID price. These performance assurance mechanisms overcome the key challenges specific to small cells – namely, cost, size and location – while providing operators the ability to ensure quality of service across their expanded transport networks in small-­‐cell deployments. According to Roopashree Honnachari, program manager for business communications services at Frost & Sullivan, a virtualized NID is a good value proposition and is suited to a small-­‐cell application. “Adding another NID at the cell tower to manage the service assurance adds cost and complexity to the network infrastructure. Integrating the virtual NIDs into the base station equipment makes it easier to architect these networks, and it’s faster to deploy because NIDs don’t need to be installed at every site, which should result in cost savings for the customer.” The NGMN Alliance has recognized the need for performance monitoring in the small-­‐cell backhaul network: “Finally, and given the rising importance of end-­‐to-­‐
end QoS, it is also desirable that the adapted transport solution provides the means to verify on-­‐demand and/or pro-­‐actively monitor packet delay, jitter and loss rate over the backhaul network segment. For this, standard protocols (e.g. ITU-­‐T Y.1731 and TWAMP/OWAMP (RFCs 5357, 4656)) are preferred.”5 White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
Accedian has taken its Ethernet OAM-­‐based performance monitoring technology and adapted it to meet the requirements of small-­‐cell backhaul. Here’s an overview of Accedian’s solution. Accedian V-­‐NID The V-­‐NID product suite comprises standards-­‐based software reflector agents, an active performance measurement device called an Actuator, a controller unit called the V-­‐NID Manager, as well as open interfaces towards third-­‐party OSS/BSS systems. The reflector agents, from Accedian or a third party, reside in the access points or other RAN equipment and respond to measurement requests sent from the Actuator, which is located in an operator’s core network. The Actuator can measure thousands of access points without the need for synchronized test hardware at each end of the connection. Supported standard reflectors from third-­‐party vendors include Y.1731 ETH-­‐LB, ETH-­‐DM and RFC 5357 TWAMP with control protocol or in light mode. Figure 4. V-­‐NID Performance Assurance Suite Accedian NanoNID The SFP NanoNID™ is a space-­‐saving, cost-­‐effective product aimed not only at small-­‐cell deployments, but also legacy equipment that doesn’t have the latest service assurance functionality. It’s small (2.8 inches long by 0.8 inches wide) and it’s smart. With Accedian’s Plug & Go provisioning feature, the device has automated zero-­‐touch discovery and configuration, which makes it easy to install. It helps the operator perform service activation tests, trouble shooting, performance assurance and capacity planning by leveraging standards based turn-­‐
up and SLA monitoring tests. Loopback requests can be sent to the NanoNID from third-­‐party Ethernet test sets or the IEEE’s 802.3ah and the ITU-­‐T’s Y.1731 OAM loopback commands. It can also do Layer 3 performance management and troubleshooting with support for Accedian’s TWAMP-­‐lite. White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
Managed from the Cloud But won’t the CAPEX and OPEX savings derived from these unique performance assurance products for small-­‐cell backhaul be washed out by the cost of deploying many new aggregation appliances needed to manage the remote agents? Not necessarily. The NanoNID and V-­‐NID are designed to work with an operator’s existing aggregation appliances, whether from Accedian or another vendor. But more interestingly, it is possible to achieve further cost savings if those aggregation functions resided on virtual machines, whereby the cost comes out of an operator’s data center budget rather than RAN or transport budget. Managing and controlling the remote agents, aggregating the data, generating reports and actionable information can all be virtualized in virtual machines inside an operator’s data center. This is far more cost effective than having to deploy thousands of aggregation appliances. Ready for Anything: Today’s small-­‐cell deployment and tomorrow’s network requirements A reality of complex backhaul networks is that performance will vary depending on many criteria, such as network loading, environmental impacts (especially for wireless systems) and time of day. On-­‐going monitoring on the small-­‐cell backhaul network is critical for ensuring both service performance levels and making traffic routing decisions in order to optimize the deployed network. By embedding performance assurance functionality into the small cells or other access network elements, operators can confidentially and cost-­‐effectively turn up thousands of access points and continually monitor the backhaul service. And by virtualizing this functionality, operators will also be prepared to meet the strict performance requirements of future network implementations, such as self-­‐
organizing network (SON) and voice over LTE (VoLTE). SON on the Horizon The overall aim of SON is to reduce operating cost and complexity of LTE and LTE-­‐
Advanced networks, by minimizing the amount of manual configuration required to deploy a network and automating optimization during network operation. There are many specific use cases for SON, but generally, SON encompasses techniques for self-­‐configuration, self-­‐healing and self-­‐optimization. SON capabilities are particularly attractive for small-­‐cell deployments and heterogeneous networks, given the potential to lower physical deployment effort and operating costs. Service assurance monitoring of transport networks has a role in SON. The performance measurements obtained from the nanoNID or V-­‐NID in routine monitoring can be used to feed SON decision-­‐making. Accedian’s products provide metrics for path loss, latency, jitter and Mean Opinion Score (MOS), which allows the SON system to decide whether to move all or selected high-­‐priority traffic to a better performing path. For a SON to operate efficiently, it needs to have knowledge of the end-­‐to-­‐end performance. That isn’t possible from localized agents installed on devices. Getting the full view on end-­‐to-­‐end metrics requires an active measurement White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
system like the V-­‐NID, which injects sample packets and reports on the traversal through the network. By implementing the virtualized performance assurance system today, operators will be prepared to support future network features such as SON. 4G Gets a Voice with VoLTE IMS-­‐based VoLTE may seem like a distant possibility to most mobile operators, since even the earliest 4G operators are content to continue relying on their 3G networks to carry voice services. But when operators do decide to implement VoLTE, driven by the desire to shift network usage away from legacy 2G and 3G networks, then it is likely that any deployed LTE small cells will also support the IMS-­‐based service. Whether the small base stations are deployed for coverage or capacity reasons, it makes sense that they support both voice and data traffic. The implication for performance assurance is that the small-­‐cell backhaul will have far lower latency requirements for handling the voice and IMS signalling traffic. According to the 3GPP, the recommended one-­‐way delay budget for voice and IMS signalling is 100 milliseconds. With a flexible, virtualized performance assurance system in place, such as the V-­‐NID, operators will be in good position to adapt to the strict performance requirements of these new traffic streams. Conclusion Small-­‐cell backhaul networks have significantly different deployment challenges compared to macrocell networks, yet they require the same level of performance assurance. Since conventional performance assurance methods are too costly in a small-­‐cell environment, operators need new ways to monitor performance. Network planners can overcome these challenges by incorporating service assurance functionality into the small cells themselves either by using hardware features in an SFP device or via software agents embedded in the device, which will ensure that the small cells can be reliably verified at service launch and continuously monitoring during service operation. With such a virtualized solution, operators will be ready not only to launch small-­‐cell networks, but also to pursue the next phase of their RAN evolution strategies. White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0
Sources : 1. “Small Cell Backhaul Requirements,” Version 1.0, NGMN Alliance, June 4, 2012 http://www.ngmn.org/uploads/media/NGMN_Whitepaper_Small_Cell_Backhaul_Requirements.pdf 2. “Small Cells and Backhaul Deployment Strategies,” Maravedis-­‐Rethink webinar, May 14, 2013 3. “Small Cell Backhaul Requirements,” Version 1.0, NGMN Alliance, June 4, 2012 http://www.ngmn.org/uploads/media/NGMN_Whitepaper_Small_Cell_Backhaul_Requirements.pdf 4. “Backhaul Technologies for Small Cells,” The Small Cell Forum, February 2013 5. “Small Cell Backhaul Requirements,” Version 1.0, NGMN Alliance, June 4, 2012 http://www.ngmn.org/uploads/media/NGMN_Whitepaper_Small_Cell_Backhaul_Requirements.pdf List of Abbreviations BSS – Business Support System LTE – Long Term Evolution CoMP -­‐ Coordinated Multipoint NID – Network Interface Device DOCSIS – Data Over Cable Service Interface Specification NLOS – Non-­‐Line of Site DSL – Digital Subscriber Line eICIC -­‐ Enhanced Inter-­‐Cell Interference Coordination ETH-­‐DM – Frame Delay Measurement ETH-­‐LM – Frame Loss Measurement FDD -­‐ Frequency Division Duplex GPS -­‐ Global Positioning System IMS – IP Multimedia Subsystem KPI – Key Performance Indicator NTP – Network Time Protocol OAM -­‐ Operations, Administration and Maintenance OSS – Operational Support System PON – Passive Optical Network RAN – Radio Access Network SFP – Small Form-­‐Factor Pluggable SLA – Service Level Agreement SON – Self-­‐Organizing Network TDD – Time Division Duplex TDM – Time Division Multiplexing
Accedian Networks Inc. 2351 Alfred-­‐Nobel, Suite N-­‐410 St-­‐Laurent (Montreal), Quebec, Canada, H4S 2A9 Toll free: 1-­‐866-­‐685-­‐8181 © 2013 Accedian Networks Inc. All rights reserved. Accedian Networks, the Accedian Networks logo, antMODULE, EtherNID, Fast-­‐PAAs, High Performance Service Assurance, Performance Assurance Agent (PAA), Plug & Go, MetroNID, MetroNID G T, MetroNODE 10GE, MetroNODE LT, VeloCITY FS, Multi-­‐SLA, NanoNID, SkyLIGHT, SLA-­‐Meter, Traffic-­‐Meter, VeloCITY FS, VeloCITY FS 10G, Vision EMS, VisionMETRIX and V-­‐NID are trademarks or registered trademarks of Accedian Networks Inc. All other company and product names may be trademarks of the respective companies. Accedian Networks may, from time to time, make changes to the products or specifications contained herein without notice. Some certifications may be pending final approval, please contact Accedian Networks for current certifications. White Paper – Small Cells Backhaul Performance Assurance | Oct 2013 | Rev 1.0