SIP is a signaling protocol that is enabling unified communications applications by allowing IP-based communications to be extended across different networks, devices, and transport mediums. Key advantages of SIP include supporting mobility, virtual numbers, and business continuity. SIP trunking provides cost benefits like reduced network service costs by converging voice and data on a single IP connection. Considerations for SIP implementation include ensuring interoperability between systems, implementing security measures like encryption, and properly provisioning E911 services for mobile users.

1. xo.com
Energize Your Unified
Communications with SIP
WHITEPAPER | Dr. Steven Shepard, Ph.D

2. Energize Your Unified Communications with SIP XO Communications
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Table of Contents
Abstract 3
Introduction 3
What Makes SIP so Popular? 3
What the SIP Signaling Protocol Does 4
The Way Workers Want to Communicate 4
SIP Advantages and Applications 5
1. Intelligence at the Edge 5
2. SIP Is Part of the Overall IP Suite 5
3. Supports Any Network Transport Medium 5
4. Mobility and Presence Support 6
5. Virtual Numbers 6
6. Business Continuity and Disaster Recovery 6
7. Supply Chain Support 7
8. Unified Communications Applications 7
Cost Benefits of SIP Trunking 8
SIP Trunks 8
Reduced Network Service Costs 9
Eliminating PRI Trunks and IP Conversion Devices 10
Evolutionary Migration Path 10
Considerations for Implementation 10
Interoperability 10
Security 10
E911 Requirements 11
Conclusion: Making the SIP Decision 11
The XO Advantage 11
About Steve Shepard 12
Appendix 13
About XO Communications: 14

3. Energize Your Unified Communications with SIP XO Communications
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Abstract
For IT decision makers in medium to large- sized businesses, this white paper presents an
overview of the Session Initiation Protocol (SIP) standard, business applications for SIP trunking,
and the advantages of SIP as an enabler for unified communications.
Introduction
Imagine having secure access to work-related content anytime, anywhere in the world, on any
network, using any access device over any access technology? Would that not simplify life
dramatically, and make the use of the network more efficient and relevant? Yes. This is becoming
the reality today. The evolution of SIP, SIP’s impact on real-time unified communications, and
its ability to simplify mobility have helped make SIP the choice for modern communications
networking for businesses.
What Makes SIP so Popular?
Three phenomena made the evolution to SIP possible:
1. Inevitable, steady migration of content from the hard drive on the worker’s desktop or
laptop computer to storage located within the network—either managed by the network
provider or hosted by the enterprise.
2. The growing proliferation of Unified Communications (UC) technology in the workplace,
including IP telephony, unified messaging, instant message/chat, texting, presence,
conferencing, video chat, as well as support for Internet Protocol (IP) protocol applications
across different devices.
3. The extension of the Internet Protocol as an application-layer, signaling protocol in the
network infrastructure. SIP enables IP-based Unified Communications applications to be
extended from IP-PBX systems, and/or an IP network across different network transport
services and worker devices. In essence, it allows any worker on any network to have access
to any content on-demand, irrespective of the worker’s location, device, or type of network
access.
SIP enables IP-based
Unified Communications
applications to be extended
from IP-PBX systems, and/
or an IP network across
different network transport
services and worker devices.
In essence, it allows any
worker on any network to
have access to any content
on-demand, irrespective of
the worker’s location, device,
or type of network access.

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The Way Workers Want to Communicate
Today, mobile access reigns as the preferred mode of connectivity as employees demand
to use their own smartphones and laptops for work-related activities. For this reason, many
communications service providers, including XO, have added mobility support for unified
communications through downloadable applications. These types of applications can spur
innovation and productivity among workers and eliminate the need for IT to provide company-
issued mobile phones with separate calling plans.
The three phenomena that made the evolution to SIP possible—migration of content, growing use
of unified communications, and the extension of the Internet Protocol–enable:
• Increased productivity through improved support for distributed workforces
• Timelier and high quality responsiveness to customers, and
• Reduced capital expenses (CAPEX) and operating expenses (OPEX) for networking
infrastructures.
SIP helps reduce the cost of networking, and SIP trunks free businesses from being locked into
a specific network provider, now and in the future. Add the productivity and customer service
benefits of a SIP network with Unified Communications—and it’s easy to see why the use of SIP
technology is so compelling, and quickly rising in popularity among forward-thinking businesses.
What the SIP Signaling Protocol Does
As an application-layer protocol, SIP can establish and manage:
• Two-party, multiparty, or multicast sessions
• Internet telephony
• Distribution of multimedia content
• Management of multimedia conferences
SIP is designed to be completely independent of the transport layer and can operate over
the Transmission Control Protocol (TCP), the User Datagram Protocol (UDP), or the Stream
Control Transmission Protocol (SCTP). SIP-based devices can use TCP or PDP protocols to
connect to SIP servers.

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SIP Advantages and Applications
When deployed in a network, SIP offers distinct advantages that can be used to develop powerful
worker applications. For example:
1. Intelligence at the Edge
SIP-enabled telephony systems offer most of the call processing and feature invocation
procedures offered through traditional voice networks — but in a different way. Traditional
voice networks use a hierarchical, centralized, core-based protocol designed around the limited
requirements of telephone sets — which have no innate intelligence. SIP, on the other hand, is a
peer-to-peer protocol, which requires a different type of network core infrastructure (preferably
MPLS built on an IP platform) with a blend of intelligence located at the edge (i.e., software
or end-user device hardware or PBX), complimented by more scalable, granular, and rapidly
deployed services offered by service providers.
2. SIP Is Part of the Overall IP Suite
As part of the overall IP suite, SIP is flexible and extraordinarily dynamic. Its functionality can
be extended to any number of applications, including enhanced signaling for value-added
services, VoIP, and XML-tagged applications. Because XML is used to structure, store, and
send information across the network, it works well with SIP in environments where data needs
to be retrieved and used, as in a contact center environment where customer records must
be accessed, or in a healthcare environment where access to customer data is critical. As a
“lightweight, text-based protocol,” SIP relies on a text-based command structure that uses the
now universally familiar HTTP syntax and URL addressing, both ideal for delivering telephony over
an IP network where the logical integration of applications (e.g., voice, messaging, conferencing,
and Web access) can create an enhanced user experience.
3. Supports Any Network Transport Medium
Because SIP is an application layer protocol, it can ride seamlessly across any transport scheme
and be transported across any access type — cable, DSL, private line, Ethernet, and wireless.
Thus, SIP can enable a broad range of applications and remote session capabilities (such
as mobile application delivery and supply chain management) without the need to provision
additional transport services. From an enterprise point-of-view, this is critical because SIP offers
seamless connectivity options for service delivery for branch locations, remote workers, or
business partners.

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4. Mobility and Presence Support
SIP is now incorporated into a range of worker devices, including smartphones and laptops.
Using SIP, session establishment requests aren’t sent to a device; they are sent to the network.
The network locates the worker’s “presence” and establishes a session based on the user’s
current location and usage profile. Employees can collaborate with coworkers, and enjoy
integrated, real-time communications, wherever they are.
Figure 1. A customer wants to get back in touch with a specific agent in a contact center, and places a call to
that agent (yellow arrow, lower right). Since the agent is out of the office, SIP routes the call to the agent’s remote
location (blue arrow, lower left), thus avoiding an interruption in customer service. To the customer, it appears as if
the agent has answered from the office, which further projects a consistent experience and professional image.
5. Virtual Numbers
A fifth advantage is the ability to utilize virtual numbers, an assignable telephone number that has
no physical phone line associated with it. In most cases, virtual numbers are forwarded to either a
VoIP account or to an alternate fixed or mobile number. For example, virtual numbers are perfect
for sales forces, business travelers, small businesses, and field service personnel. With virtual
numbers, businesses can also create a local identity in markets that the company serves.
6. Business Continuity and Disaster Recovery
SIP trunks, in concert with VoIP, can play a major role in business continuity and disaster recovery.
With a SIP-supported IP-PBX, many businesses are now able to design disaster recovery plans
using plug and play phones, softphones, and IP-PBX programmability capabilities. Automatic
reroute in IP environments is possible, thus reducing the headache of planning for every
contingency. For businesses of any size, SIP trunks provide connections to the PSTN so that
outbound calls can be rerouted and delivered over an Internet connection when the normal
connection (or location of the connection) is unavailable.

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Unified communications applications exist that are already integrated with SIP and VoIP services.
These applications automatically route calls to workers’ smartphones so that they can continue
business as usual even in the event of a disaster.
In addition, business-class service providers build multiple layers of redundancy into their
networks to provide for business continuity. A carrier should provision network session capacity
to support failover of traffic between designated sites. Service providers also should track
session usage for each of their customer’s sites and implement a primary/backup arrangement
to manage overflow traffic. When sessions in one site are fully used, the network automatically
routes inbound traffic, normally delivered to the first site, to the second site.
7. Supply Chain Support
The overall supply chain is enhanced when executed in a SIP environment. By converging voice,
data, and expanding video/rich media applications, businesses can enhance the end-to-end
supply chain, thus measurably improving their effectiveness and efficiency.
8. Unified Communications Applications
Collaboration and Unified Communications applications that overcome the challenges of
distance are key to today’s businesses. Companies should look for service providers who offer
both mobility and unified communications solutions as part of an overall SIP solution. These
applications should maintain separation between work and personal identities on smartphones
and ensure that employees’ personal phone numbers, applications and information remain
private. UC applications should enable unlimited free calling between company offices, and bring
together business voice, video chat, instant messaging, texting and presence into a consistent,
end-to-end unified communications experience. These types of unified communications
tools create energy and foster engagement among workers, partners and customers to fuel
productivity and success.

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Cost Benefits of SIP Trunking
With these advantages in mind, let’s now take a closer look at SIP trunking and the cost benefits
that SIP trunks make possible.
SIP Trunks
SIP trunks are one of the more remarkable offshoots of the SIP family of capabilities and one of
the more important enablers of SIP-dependent applications. SIP trunks are nothing more than
virtual circuits configured and delivered over an Internet connection, typically via the private IP
backbone of a VoIP-enabled carrier, as shown in Figure 2.
Figure 2. SIP trunks enable convergence to one IP connection over a standards-based connection, eliminating the
need for TDM-IP gateways.
SIP trunks are often used with an IP-PBX as replacements or “evolutionary next stages” from
traditional ISDN-PRI or analog circuits. SIP trunks not only make network deployment more
flexible, but also make possible the seamless assurance of operational continuity in the event
of a network failure. Their popularity, which is growing rapidly, is largely due to a collection of
factors, including cost savings and overall reliability. Some of the more relevant cost benefits are
as follows.

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Reduced Network Service Costs
Convergence implies that a single connection can serve multiple access requirements. With SIP
trunks:
• Voice and data applications ride over one IP connection, instead of separate voice and data
services.
• The connection is highly efficient because unallocated SIP bandwidth is automatically and
dynamically made available for other uses and applications as required (see Figure 3). Added
voice compression is available from some service providers, such as XO Communications,
enabling higher throughput and efficiency as well.
• With centralized SIP trunking, companies can provide high-capacity connections between
their IP-PBX phone systems in one or more primary locations that serve as demarcation
points to the service provider’s core network and the universal Public Switched Telephone
System. This reduces the costs associated with managing separate networks, eliminates
local telephone lines and equipment at many locations, and eliminates long-distance charges
among headquarters and branch offices. [see Appendix A].
Figure 3. Real-time IP dynamic bandwidth allocation gives priority to voice traffic but makes additional data
bandwidth capacity available when phone lines are not in use.
Centralized SIP trunking
reduces the costs associated
with managing separate
networks, eliminates
local telephone lines
and equipment at many
locations, and eliminates
long-distance charges
among headquarters
and branch offices.

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Eliminating PRI Trunks and IP Conversion Devices
SIP trunks eliminate the need for PRI PBX cards and IP conversion devices on the customer
premises, typically referred to as TDM-IP gateways. This device supports the conversion from IP
packets to PSTN traffic, which is normally transmitted over an ISDN PRI. In addition to hardware
savings, better throughput is also achieved by minimizing a protocol conversion step.
Evolutionary Migration Path
Companies that are not ready to replace their legacy TDM PBXs but wish to deploy SIP trunks
can deploy media gateways at branch offices to convert TDM voice communication into IP voice
traffic (and vice versa), instead of installing a brand new IP system. Thus, legacy TDM PBXs
placed in multiple sites are being interconnected via IP networks with the help of gateways that
transform the PSTN traffic into VoIP packets.
Considerations for Implementation
Interoperability
As IP-PBXs have entered the market, problems of interoperability have arisen with regard to SIP.
Almost all of the first-generation IP-PBXs on the market were designed around proprietary IP
signaling stacks because universal agreement on a single protocol had not yet been achieved.
Ultimately, SIP was chosen as that universal protocol, and PBX manufacturers wrote proprietary
interfaces for their legacy TDM interfaces. This created problems for developers looking to write
interfaces for VoIP environments built on media server platforms, as well as complications that
required system-by-system interoperability testing or, in some cases, the creation of software
interfaces to perform a protocol conversion that ensures interoperability beyond very basic
connect-and-disconnect capabilities.
Security
Thanks to new encryption capabilities such as SIP Transport Layer Security (TLS) and Secure
Real-Time Transport Protocol (SRTP), security vulnerabilities are no longer a major issue for SIP
implementations. Just as most enterprise organizations encrypt the data that transverses their
network, they are requesting that VoIP providers encrypt voice and data packets transmitted over
their VoIP networks. Encryption provides customers with an added layer of protection to maintain
privacy of VoIP communications and helps prevent unauthorized access to voice conversations.
Business-grade service providers offer enterprise SIP customers a choice of either or both of
the industry signaling standards for encryption: SIP TLS and SRTP. TLS is based on the earlier
Secure Sockets Layer (SSL) method of encryption and uses cryptography to provide endpoint
authentication and communications privacy over the internet. SRTP defines a profile of RTP,
intended to provide encryption, message authentication and integrity, and replay protection to the
RTP data.

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E911 Requirements
SIP and Unified Communications extend the mobile nature of VoIP by enabling communications
on a range of wireline and wireless devices that are not tethered to a physical location address.
Today’s VoIP-enabled, Internet-based phones offer multiple features for convenience of use that
allow callers to use phones from virtually any Internet connection. While this technology has
many benefits for end users, it has created many challenges for emergency communications call
centers that were designed to receive calls from landline phone services associated with a fixed
address. So, how does a carrier provide E911 services to remote locations; and what are the legal
requirements for providing back up and E911?
Interconnected VoIP service providers are required to comply with enhanced 911 rules adopted
by the FCC that are designed to integrate nomadic interconnected VoIP services with the existing
PSTN emergency 911 system. Careful planning and proper service provisioning help to ensure
phone users in all locations have accurately set up E911 capabilities.
To provide E911 service to distributed users on a centralized, shared SIP trunk, the caller’s
telephone number must be provisioned so that the E911 service is programmed to ring to the
caller assigned local emergency dispatch call center, rather than to a center miles away, across
the region or across the country. Nomadic, or wireless 911 users require a different solution from
a fixed-IP E911 solution because the VoIP service provider normally delivers the service to a
stationary location. Providers that offer enterprises a nomadic E911 capability enable employees
to move their phones and still get the proper address information transmitted to the Public Safety
Answering Point (PSAP), as long as the nomadic VoIP user keeps their information up to date.
Updated information must be provided by the user via location update functionality provisioned
by the service provider.
Conclusion: Making the SIP Decision
Like any large-scale technology shift, the move to SIP should be undertaken only after
considering all options and determining that a SIP migration strategy is the right move for the
enterprise. In most cases it will be, but it makes sense to ask the questions. Furthermore, it is
critical to seek the advice of SIP-seasoned professionals before undertaking the migration.
SIP is a fundamentally important technology that will play an increasingly important role in
networking, SIP, like VoIP, is a when rather than an if question in today’s enterprise. Its time has
come, and businesses of all sizes will be well served if they consider it as part of their evolving
network collaboration and unified communications strategy.
The XO Advantage
Explore your SIP trunking options and savings potential. SIP solutions from XO provide direct
access to XO’s private, award-winning nationwide network to bring your employees and offices
together with the right connectivity, powered by unified communications and collaborations
technology.

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Market research firm, Infonetics Research, ranked XO as a leading business VoIP provider in
North America in 2013, recognizing XO as one of the top two IP connectivity suppliers of business
VoIP services1
. XO Communications provides business VoIP services to more than 1.5 million VoIP
end users and 35,000 businesses and large enterprises. The company’s suite of communications
and collaboration services include award-winning business VoIP, SIP Trunking and Hosted PBX
services as well as its XO WorkTime unified communications applications.
With the cloud-based mobility and Unified Communications application called WorkTime,
businesses can securely integrate and extend the company’s XO VoIP, SIP trunking or Hosted
PBX services to the employees’ personal smartphones and computers. This allows IT and
network managers with the flexibility to implement mobility with a Bring Your Own Device (BYOD)
strategy while reducing costs and eliminating any need to provide and manage company-
issued mobile phones. WorkTime empowers employees to innovate, collaborate, and be more
productive wherever they go, regardless of device. Workers can select their preferred method of
communication from a range of options like IM, text and more—present a consistent work number
and presence to others, and use the company calling plan.
About Steve Shepard
Dr. Steven Shepard, Ph.D. is president of the Shepard Communications Group. A professional
writer, educator and industry analyst with more than 25 years of experience in the technology
industry, Dr. Shepard specializes in international telecommunications issues, the social
implications of technological incursion and the analysis of financial issues related to technology-
dependent businesses. He is the author of 45 books on a wide variety of topics and regularly
speaks to audiences throughout the world, and has clients in more than 50 countries. Dr. Shepard
can be reached at: Steve@ShepardComm.com.
1
Source: Infonetics Research, North America Business VoIP Service Leadership Scorecard, May 7, 2013.

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Appendix
Appendix A:
Before Enterprise SIP
+
+
+
DIA #1
ILEC
#1
PRI
DIA #2
ILEC
#2
PRI
Headquarters
Regional Office
Branch 1
+
++
+
Provider
1
Provider
3
Provider
5
Provider
6
Provider
8
Provider
4
+
Provider
2
Provider
7
+
+
Branch 8
Branch 7
Branch 6
Branch 5
Branch 2
Branch 3
Branch 4
BEFORE: Enterprises had to work with more than one local carrier to provide separate Primary Rate Interface (PRI)
lines or IP connections to each location.
After Enterprise SIP
MPLS IP-VPN Network
Headquarters
Regional Office
Failover
Branch 1
Branch 2
Branch 3
Branch 4
Branch 5
Branch 6Branch 8
Branch 7
XO Communications
AFTER: XO Enterprise SIP provides high capacity SIP trunking to one or more primary locations, and uses your
existing phone systems (such as IP-PBXs) and MPLS network to deliver VoIP across all locations. This streamlines
management, reduce costs by eliminating lines, equipment and many long-distance charges, provides business
continuity, and dynamically shares voice calling capacity across the company.

14. © Copyright 2014. XO Communications, LLC. All rights reserved.
XO, the XO design logo, and all related marks are trademarks of XO Communications, LLC.
Energize-Your-Unified-Communications-with-SIP | O140314 10:10:48
About XO Communications:
XO Communications is a leading nationwide provider of advanced IP communications,
intelligent networking, and network-enabled cloud solutions for business, large enterprise and
wholesale customers. These customers include more than half of the Fortune 500, in addition to
leading cable, mobile wireless and domestic and international telecommunications companies.
XO offers a superior customer experience through its innovative solutions, its employees’ focus
on customers and the proven performance of its advanced network. To learn more about XO
Communications, visit www.xo.com or blog.xo.com.
For XO updates, follow us on: Twitter | Facebook | Linkedin | SlideShare | YouTube | Flickr