Latency Considerations in LTE: Implications to Security Gateway

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Latency Considerations in LTE
Implications to Security Gateway
September 2014

STOKE®, and the Stoke logo are registered trademarks of Stoke, Inc. Copyright ©2014 Stoke, Inc. All rights reserved. Literature # 130-0029-001. 2
Contents
Executive Summary ......................................................................................... 3
Latency - LTE's New Performance Metric ....................................................... 4
Calculating Total Latency............................................................................................................ 4
Stringent New Requirements with LTE-A ........................................................................... 5
Microsecond Performance for X2 ............................................................................................ 6
X2 Delay and User Throughput ............................................................................................ 7
Latency Impacts the User Experience ................................................................................... 8
Lower Latency Improves Page Load Times .................................................................... 8
Estimated Sales Impact ........................................................................................................... 9
M2M and On-Line Gaming ..................................................................................................... 9
Implications for Security Gateway .................................................................................. 10
Conclusions .................................................................................................................................... 11
Stoke® Security eXchange ............................................................................ 11

Executive Summary
Network delay or latency is critical in today’s mobile broadband where both Internet-based businesses
and user expect network response will be close to instantaneous. Excess latency can have a profound
effect on user experience—from excess delay during a simple phone conversation, reducing throughput
at edge of cell coverage areas by reducing effectiveness of RAN optimization techniques, to slow-loading
webpages and delays with streaming video. Response delays negatively impact revenue. In
financial institutions, low latency networks have become a competitive advantage where even a few
extra microseconds, can enable trades to execute ahead of the competition.
The direct correlation between delay and revenue in the web browsing experience is well documented.
Amazon famously claimed that every 100 millisecond reduction in delay led to a one percent increase in
sales. Google also stated that for every half second delay, it saw a 20 percent reduction in traffic.
For LTE network operators, control of latency is growing in importance as both an operational and
business issue. Low latency is not only critical to maintaining the quality user experience (and
therefore, the operator competitive advantage) of growing social, M2M, and real-time services, but
latency reduction is fundamental to meeting the capacity expectations of LTE-A, where latency budgets
will be cut in half and X2 will need to perform at microsecond speed.
Total network latency is the sum of delay from all the network components, including air interface, the
processing, switching, and queuing of all network elements (core and RAN) along the path, and the
propagation delay in the links. With ever tightening latency expectations, the relative contribution of
any individual network element, such as a security gateway, must be minimized. For example, when
latency budgets were targeting 150ms, a network node providing packet processing at 250μs was only
adding 0.17% to the budget. However, in LTE-A, with latency targets slashed to 10ms, that same
network node will consume almost 15x more of the budget. More important, when placed on the S1
with a target of only 1ms, 250 μs is 25% of the entire S1 latency allocation, and endangers meeting the
microsecond latency needed at the X2. Clearly, operators need to apply stringent latency requirements
for all network nodes, when designing LTE and LTE-A networks.
STOKE® solutions are purpose built from the ground up for today's mobile broadband environment to
solve critical, performance impacting problems for mobile network operators. Stoke Security eXchange
™ is a carrier grade, field proven solution ideal for these requirements since it introduces <30
microseconds of latency, supports creation of multiple VLANs and line rate/10Gbps throughput for small
packet sizes of 96 bytes.

Latency - LTE's New Performance Metric
Latency and throughput are the essential factors in network performance and collectively they define
the speed of a network. Whereas throughput is the quantity of data that can pass from source to
destination in a specific time, round trip time (RTT) latency is the time it takes for a single data
transaction to occur, meaning the time it takes for the packet of data to travel to and from the
destination, back to the source.
Latency is often considered to be even more important than speed in determining quality user
experience:
“Network latency, perhaps more so than average downlink speeds alone,
can affect users' experience, especially for real-time services like video
calling, VoIP and even gaming applications.”
Source: Light Reading1
As consumer and businesses increasingly merge user experience across multiple devices and networks,
mobile networks must also be engineered to minimize delay.
Calculating Total Latency
The total latency budget is measured in milliseconds (ms), but is comprised of individual network
elements and interfaces, which individually may only add microseconds (μs), but must be added
together to calculate total end-to-end latency.
Latency must be carefully managed and measured. Latency includes delay from propagation, buffering
and queuing, transmission, and signal processing that is introduced at every link and network element
through which a packet travels. A primary design objective for any individual network element (such as
a security gateway) should be to minimize its latency contribution in order to stay within the overall
latency budget prescribed for the link.
As mobile technology has evolved, the latency targets have become increasingly stringent. The figure
following shows the progression of roundtrip times from GSM/Edge to LTE, as defined by 3GPP.
1 Light Reading, “LTE, A latent problem”

Figure 1. Evolution of Mobile Network Latency Targets2
As shown in Figure 1, for LTE networks, the 3GPP target latency budget for the user plane latency (radio
and core) is about 20 ms, excluding the backhaul transport network. The backhaul network (RAN edge-
Core edge, including security gateway) would add an additional 10 ms.3
Stringent New Requirements with LTE-A
In order to support the predicted growth, the 3GPP has developed LTE-Advanced Release 10, a major
enhancement of the LTE standard deployed in many mobile networks today. The new technology
targets peak data rates up to 1 Gbps and introduces new RAN concepts with the ultimate goal of
designing a system that is drastically enhanced in both cell capacity and coverage. In LTE Advanced,
latency targets are reduced substantially to 10 ms, allocated as shown following.
Figure 2. Target Latency Budget for LTE-A4
2 Nokia Siemens Networks, “LTE-capable transport: A quality user experience demands an end-to-end approach”
3 Qualcomm, “Latency in HSPA Data Networks”, July 2013.

Microsecond Performance for X2
To maintain performance requirements in LTE-A, the latency on the X2 links are especially critical:
“In order to meet stringent latency requirements of less than 1 millisecond,
the physical and logical path of the X2 interface needs to be as short as
possible.” 5
In LTE-Advanced, new RAN optimization techniques introduced in LTE-A impose critical performance
demands on the X2 interface, requiring very short latencies of <1 millisecond across the backhaul
network. This is further described:
 Inter-cell interference coordination (ICIC) and coordinated multi-point (CoMP) transmission are
two techniques defined in LTE-Advanced specifications that target a better user experience at the
cell edge. ICIC is limiting cross-talk by coordinating spectrum allocation across multiple cells.
CoMP allows multiple base stations to simultaneously serve a user device and increase the receive
power level and, therefore, capacity. Both synchronization techniques are implemented over the
X2 interface and require very short latencies of <1 millisecond across the backhaul network to
achieve real-time coordination between base stations.
 In addition to providing low-latency connectivity, base station clocks need to be in phase to
enable proper operation of ICIC and CoMP. This leads to the requirement for highly accurate
phase or time-of-day synchronization. Most 3GPP base station clocks are currently synchronized
on frequency only, since accurate phase synchronization was not a requirement up to now. The
new LTE-Advanced functions, however, require base stations to be in phase with an accuracy of
500 nanoseconds to efficiently operate ICIC and CoMP. This is nearly impossible to achieve
without active time distribution over the X2 interface.
 Mobile operators will also introduce LTE TDD radio interfaces operating in unpaired spectrum.
Many operators already have acquired unpaired spectrum, since TDD provides more flexible
scaling of the up- and down-link capacity and has additional benefits to the overall architecture of
the radio access network. Like ICIC and CoMP, LTE TDD also requires phase alignment of
neighboring base stations over the X2 interface in addition to the traditional frequency
synchronization used in mobile networks today.
4 LOLA, Presentation of WP2 – Scenarios and Target System Architectures”
5 RCR Wireless Readers Forum, “New Backhaul Challenges are emerging with LTE Advanced”, July 15, 2013.

These techniques mandate tighter coordination between base stations and places specific performance
requirements for the X2 interface with respect to capacity, latency and synchronization. Supporting
these requirements is not trivial since most mobile backhaul architecture on which X2 interface is
transported follows a hub-and-spoke design where traffic distribution and re-direction is performed at
the security gateway in the distribution layer of the backhaul network.
X2 Delay and User Throughput
Latency on the X2 interface reduces the benefit of co-ordination schemes. In the figure following, X2
latency as low as 5ms reduced cell edge and median user throughputs by over 20% in this example of a
Joint Transmission CoMP scheme.6 This loss of throughput impairs the user experience and reduces
network efficiency.
Figure 3. Impact of X2 delay on user throughput with CoMP scheme
7
Simpler schemes may not be impacted as much, but this shows that the sophisticated CoMP schemes
require very low X2 latencies to achieve their full potential and delivering the anticipated enhancements
in speed, coverage and overall quality of experience to the end subscriber.
The table below uses the 5ms and 1ms figures above and calculates the throughput loss for 200 μs.
X2 Delay 5 ms 1 ms 200 μs
Throughput Loss (20%) (5%) (1.0%)
Figure 4. Loss of Spectral Efficiency from X2 Delay.
6 Qualcomm, Backhaul Requirements for Centralized and Distributed Cooperation Techniques”, 8 July 2010.
7 “Centralized Scheduling for Joint-Transmission Coordinated Multi-Point in LTE-Advanced”, S. Brueck, L. Zhao, J.
Giese, M. A. Awais, proc. ITG/IEE Workshop on Smart Antennas, Bremen (WSA'10), Germany, February 2010

Latency Impacts the User Experience
High latency causes noticeable delays in, for example, downloading Web pages or when using latency
sensitive applications such as interactive games, VoLTE, M2M, and streaming media.8
Lower Latency Improves Page Load Times
Analysis by Google shows a relationship between page load time and round trip latency. Their analysis
shows reductions of every 20 milliseconds in round trip latency reduces page load time by 7-15%.9 The
figures following compare the impact on page load time from two different performance characteristics
- bandwidth improvements and round-trip-time (RTT). There are diminishing returns as the bandwidth
gets higher, but not for improvements in round trip time (latency). Google concluded that, unlike
improvements to bandwidth, reducing the round trip time always helps the overall page load time.
Figure 5. Comparison of impact on page load times – RTT and Bandwidth
The linear relationship shown in the charts provided by Google suggest that microsecond latency
improvements would also improve the page load times and therefor have some impact on the user
experience.
Both Amazon and Google have confirmed that in e-commerce applications, milliseconds of higher page
load times, which is not consciously perceivable to the user, can still have an impact on user behavior
and experience and can directly impact revenue.
8 Nokia Siemens, “Latency The impact of latency on application performance”
9 Google Blog, “More Bandwidth Doesn’t Matter (much )” , April 2010

“Any increase in the time it takes Google to return a search result causes the
number of search queries to fall. Even very small differences in results speed
impact query volume.”10
“Experiments at Amazon have revealed similar results: every 100 ms increase
in load time of Amazon.com decreased sales by one percent.11
Estimated Sales Impact12
Lower latency, even in microseconds, improves page load times, which in turn can positively impact
revenue for mobile operators and enterprise customers. To illustrate the potential impact on sales from
less efficient network nodes, the table below shows the potential impact of additional 200 μs latency
(page load times) on an on-line enterprise with $14.8B in annual sales, using the 100 ms/1% sales
decrease provided by Amazon. As shown in Figure 6, the estimated impact on sales for each additional
200 μs of page load delay could be as high as $300k annually.
Example - Sales Lost Per 100 ms Per ms 200 μs
2007 Annual Sales : $14,800 M
Negative Impact on Sales (1.0%) (.01%) (0.002%)
Sales Lost ($148 M) ($1.48 M) ($0.3 M)
Figure 6. Estimated sales lost for 200 microseconds delay.
M2M and On-Line Gaming
Two emerging, massive applications also require low latency:
 High Performance On-line Gaming
 M2M, Sensory applications
Ericsson expects that in 2020 there will be 50 billion devices connected and available to be used in
various existing and new applications. The figures below compare the latency requirements of new and
existing applications and provide latency targets of a few representative applications.
10 CNET News.com/ZDNET.com: “Google´s Marissa Mayer: Speed wins” by Dan Farber, November 9, 2006.
11 Nokia Siemens Networks: “Latency: The impact of latency on application performance”, 2009.
12 In order to estimate the impact of an additional 200μ of latency, a linear relationship with the 100ms-level data
available is assumed.

Figure 7. New High Growth Applications require Low Latency13
M2M and Gaming Application Latency Target
On-Line Gaming 80 ms (U-Plane, 1 way)
Gaming – Sports Events 25 ms (U-Plane)
Sensor-Based Alarms 2-12 ms (1 way, U+C plane)
Figure 8. Example latency targets of M2M and Gaming applications.14
Implications for Security Gateway
The necessity to meet the <1 millisecond budget of the X2 interface in turn places strict requirement to
minimize IPsec and packet routing latency in the security gateway, which is the key nodal element
responsible for security and routing of control and data plane traffic in X2 interface.
The entire S1 interface budget is only 1 ms (1,000 μs) each way). Therefore, the example node
referenced earlier that contributed 250 μs of latency, when placed on the S1 would be consuming 25%
of the entire backhaul latency budget. Each additional 200 μs delay from a security gateway would
cause a loss of 1% throughput.
In contrast, the STOKE® Security eXchange™ is engineered to contribute only 30 μs or 3% of the 1 ms
latency budget.
13 Eurocom, Achieving LOw-Latency in Wireless Communications
14 Eurocom: Presentation of WP2 – Scenarios and Target System Architectures

Conclusions
Network latency, even more than download speeds, directly impacts the user experience and bottom
line revenue for on-line businesses. In high frequency financial market trading, microsecond
improvements are considered a competitive advantage. As low latency applications grow in importance
and e-commerce increasing moves to mobile platforms, operators will need to carefully manage and
measure latency budgets to maintain their own competitive advantage. The latency contribution of all
individual network elements, including the security gateway, must be carefully calculated. In LTE-A and
especially for the X2 interface where the latency targets are drastically reduced, an additional 200 μs
delay is a significant difference.
Stoke® Security eXchange™
Operators are challenged to integrate networks and technologies smoothly, sustain a quality user
experience with high network performance, and still keep service delivery costs low. Stoke solutions are
purpose built from the ground up for today's mobile broadband environment to solve critical,
performance impacting problems for mobile network operators. Stoke innovative design and patent
pending technologies enable cost effective, concurrent operation of critical functions while maintaining
line-rate, high performance throughput.
STOKE Security eXchange is a carrier grade, field proven solution ideal for these requirements since it
introduces < 30 microseconds of latency, supports creation of multiple VLANs and line rate/10Gbps
throughput for small packet sizes of 96 bytes.

Latency Considerations in LTE: Implications to Security Gateway

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