understanding sigtran.pdf

More Efficient Network Architectures

© Copyright 2013 Performance Technologies, Inc. All Rights Reserved. 2

Introduction In today’s Signaling architecture, SIGTRAN capabilities are being used to overcome

bandwidth constraints, reduce signaling transport cost, and to position the network

for migration to the Next-Generation Network (NGN). A full understanding of

the utilization and implementation of SIGTRAN is required to efficiently design a

network suitable for the transport of SS7 Signaling over IP. The major stumbling

block to fully understanding SIGTRAN is that all available explanations relate to

next-generation architecture, including Media Gateway Controllers (MGC), Media

Gateways (MG), and Signaling Gateways (SG), with very little information regarding

the traditional network elements or network topology.

This paper will focus on the SIGTRAN Adaptation Layers. A cursory overview of

SCTP is provided in this paper; however detailed SCTP information will be covered

in a subsequent white paper. Included in this paper will be detailed discussions on

network topology, impact on existing infrastructure, and the benefits of each adap-

tation layer.

Market Drivers Impacting the Deployment of SIGTRANIncreased signaling traffic, cost limitations, and next-generation network migration

are factors driving the deployment of SIGTRAN. The explosive growth in SS7 trans-

actions traffic is stretching the current network architecture to its breaking point.

According to The Insight Research Corporation, the volume of transaction services

involving SS7 message data will increase from 10 billion signaling bytes in 2009 to

over 20 billion bytes in 20141.

The premier mobile application, SMS, continues to outstrip all others in revenues

and messages generated. IRC states that “SMS has generated revenues of USD 89

billion in 2008, and the world has seen traffic of almost 3.5 trillion SMS messages

in 2008. Our forecasts predict that SMS will become a USD 100 billion business by

2010, and worldwide total traffic will reach almost 5 trillion messages in FY 2011,

and growth will continue from there.”2

1 The Insight Research Corporation (January 2009) Transaction Service Processing and Telecommunications 2009 - 20142 Portio Research Corporation, www.portioresearch.com, Mobile Messaging Futures 2009 – 2013



As traffic, messaging, and services are increasing, carriers are being tasked with

reducing cost. The Insight Research Corporation study finds that “Traditional carriers

are seeing increased operating expenses cutting into margins and revenue growth.

With network and network development costs typically accounting for more than

35 percent of fixed-line operating costs, telecom service providers are under pres-

sure to reduce their network-related expenses to sustain margins.” 3

A recent study conducted by Venture Development Corporation, SS7 Market

Opportunities and Requirements: Global Market Demand Analysis, shows shipments

of SIGTRAN stacks to rise 22% between 2006 and 2012.4 This increase is from

USD15.9 Million in 2006 to USD 53.1 Million in 2012. This represents a shift away

from legacy SS7 protocol stack shipments leading to the conclusion that SS7 net-

works are rapidly migrating to SIGTRAN.

HistoryThe SIGTRAN working group of the Internet Engineering Task Force (IETF) was

formed in 1999 and tasked with defining the architecture for transporting real-time

signaling information over an Internet Protocol (IP) network. The working group’s

effort yielded three key results:

1) A new network architecture.

2) New transport protocol.

3) Numerous adaptation layers.

This architecture framework centered on a restructuring of the circuit switch into

distinct parts: Media Gateway Controllers (MGCs), Media Gateways (MGs), and

Signaling Gateways (SGs).

New Network ArchitectureThe segmenting of the legacy switch functionality into MGC, MG, and SG

presented several benefits. First, it is a more distributed switching architecture,

allowing a single MGC to control one or more geographically dispersed MGs.

3 The Insight Research Corporation (January 2009) Transaction Service Processing and Telecommunications 2009 - 20144 Venture Development Corp. (VDC) SIGTRAN Stacks Displacing SS7 in the Market May 8, 2008



The second characteristic of the new switching architecture is a total separation of

signaling from the media and the media control plane. This three-layer split in net-

work switching functionality was originally defined in IETF RFC 2719 “Framework

Architecture for Signaling Transport.”

RFC 2719 provided the following definitions for the MGC, MG and SG.

Media Gateway (MG) – A MG terminates switched circuit network (SCN)

media streams, packetizes the media data, if it is not already packetized, and

delivers packetized traffic to the packet network.5

Media Gateway Controller (MGC) – A MGC handles the registration and

management of resources at the MG. The MGC may have the ability to autho-

rize resource usage based on local policy. For signaling transport purposes, the

MGC serves as a possible termination and origination point for SCN application

protocols.6

Signaling Gateway (SG) – An SG is a signaling agent that receives/sends SCN

native signaling at the edge of the IP network. The SG function may relay,

translate, or terminate SS7 signaling in an SS7-Internet Gateway. The SG function

may also be co-resident with the MG function to process SCN signaling associ-

ated with line or trunk terminations controlled by the MG.7

RFC 2719 also states that each of these functions can be deployed as separate

entities or the MG and MGC, or the SG and MG may be combined. The IETF

provided a high degree of network flexibility with these definitions. However, there

is no mention in RFC2719 about: legacy networks, legacy network elements, or

hybrid networks where both new and old network elements coexist. This concept

will be clarified as we increase our understanding of the SIGTRAN protocol includ-

ing transport and adaptation layers.

5 Internet Engineering Task Force (IETF) RFC 2719 October 19996 Internet Engineering Task Force (IETF) RFC 2719 October 1999 7 Internet Engineering Task Force (IETF) RFC 2719 October 1999



Stream Control Transmission Protocol (SCTP) The next task addressed by the IETF was the selection of a transmission protocol to

be used on top of Internet Protocol (IP). This transport protocol must meet the rigid

constraints of a real-time protocol such as SS7. The transmission protocol had

to parallel SS7’s stringent requirements for guaranteed delivery, sequence delivery,

and had to include Multi-Homing for reliability. User Datagram Protocol was

immediately ruled out due to its inherent lack of reliability and its inability to provide

sequence delivery capabilities. The next existing transmission protocol examined

was Transmission Control Protocol (TCP). TCP had error checking, sequence

delivery, and acknowledgment capabilities – it seemed like a perfect fit. However,

TCP’s stringent sequence delivery concept, which allows a single packet loss to

delay all other subsequent packets, would not meet the real-time requirements of

SS7. Also, without modification, TCP’s lack of support for Multi-Homing would

seriously limit the reliability of the network.

The absence of acceptable, existing transmission protocols required the IETF to

define a new transmission protocol to carry SS7 related protocol levels over an IP

backbone network. Stream Control Transmission Protocol (SCTP) was born.

SCTP provides the following functions as defined by RFC 4166 “Telephony Signalling

over Stream Control Transmission Protocol (SCTP) Applicability Statement”:

u Reliable Data Transfer.

u Multiple streams to help avoid head-of-line blocking.

u Ordered and unordered data delivery on a per-stream basis.

u Bundling and fragmentation of user data.

u Congestion and flow control.

u Support for continuous monitoring of reachability.

u Graceful termination of association.

u Support of multi-homing for added reliability.

u Protection against blind denial-of-service attacks.

u Protection against blind masquerade attacks.



These requirements and more were addressed (or resolved) by the SIGTRAN work-

ing group in RFC 2960, “Stream Control Transmission Protocol,” and further

updated by RFC 3309, “Stream Control Transmission Protocol (SCTP) Checksum

Change.” SCTP can be used for the transmission of any real-time sensitive, se-

quenced delivery protocol, and as such is not limited to SS7 related data. The rela-

tionship of SS7 levels and IP/SIGTRAN layers is shown in Figure 1.

Figure 1. SS7 Levels and IP/SIGTRAN Layers

Adaptation LayersThe general function of adaptation layers is to encapsulate upper levels of the SS7

protocol and transport them over IP utilizing the services of SCTP.8 Because each

adaptation layer is based on the SS7 level being transported or replaced, there are

common capabilities across all adaptation layers.

Each adaptation layer must provide:

u A seamless operation of SS7 level peers over an IP network.

u A primitive interface boundary that the corresponding SS7 level had with its

underlying SS7 level.

u Management of SCTP transport associations and traffic between Signaling

Gateways (SGs) and IP Signaling Endpoints (ISEPs) or two ISEPs.

u Asynchronous reporting of status changes to management functions.

The goal of each of the adaptation layers is to replace an existing SS7 level with

an adaptation layer which utilizes the services of SCTP/IP. This replacement should

be accomplished without any modification to the SS7 level being transported by

8 Internet Engineering Task Force (IETF) RFC 4166 February 2006

IPMTP 1

SCTPMTP 2

M2UAM3UA

SUA

M2PA

MTP 3 MTP 3

ISUP SCCP

SIGTRAN SS7 TCAP



the adaptation layer. To accomplish this function, the adaptation layer must com-

municate with the carried SS7 level in exactly the same manner as SS7 would, i.e.

through using the same primitives.

MTP 2 Peer to Peer Adaptation (M2PA) LayerAs defined by RFC 4166 – “M2PA protocol is used between SS7 Signalling Points

employing the MTP Level 3 protocol. The SS7 Signalling Points may also use

standard SS7 links using the SS7 MTP Level 2 to provide transport of MTP Level3

signalling messages.”9

M2PA is a SIGTRAN protocol for transporting SS7 MTP Level 2 user part signaling

messages (i.e. MTP Level 3) over IP using the Stream Control Transmission Protocol

(SCTP). Unlike M2UA, M2PA is used to support full MTP Level 3 message handling

and network management between any two SS7 nodes communicating over an IP

network. IP signaling points function as traditional SS7 nodes using the IP network

instead of the SS7 network. Each switched circuit or IP signaling point has an SS7

point code. The M2PA protocol layer provides the same set of services that MTP

Level 2 provides to MTP Level 3. M2PA operates in a point to point manner and

transports MTP3.

M2PA functionality includes:

u Data retrieval to support the MTP3 changeover procedure.

u Reporting of link status changes to MTP3.

u Processor outage procedure.

u Link alignment procedure.

As indicated by the functions of M2PA, the SCTP associations in an M2PA environ-

ment are treated as SS7 links over IP.

MTP 2 User Adaptation (M2UA) LayerRFC 4166 states that the “M2UA protocol is typically used between a Signalling

Gateway (SG) and Media Gateway Controller (MGC). The SG will terminate up to




MTP Level 2, and the MGC will terminate MTP Level 3 and above. In other words,

the SG will transport MTP Level 3 messages over an IP network to an MGC.”10

M2UA employs a client server concept. The client side of M2UA has the resident

MTP3 with its SS7 Point Code. The server functionality provides the SS7 physical

termination and communicates with the client side using SCTP over IP. There are

two main functions of M2UA. First, it provides a mechanism for the transport of

SS7 MTP2 user signaling (e.g., MTP3 messages) over IP using SCTP. Second, it

allows remote termination of SS7 links for the backhaul of traffic to a centralized

node in the network.

Functions provided by M2UA are:

u Flow Control.

u SCTP Stream Management.

u Seamless SS7 Network Management Interworking.

u Active Association Control.

Comparison of M2PA and M2UAM2PA M2UA

Point Codes SG is an SS7 node and has point code.

SG is not an SS7 node and has no point code.

Types of Links SG to IP signaling point is an SS7 link over IP.

SG to IP signaling point is not an SS7 link.

SS7 Upper Levels SG can have upper SS7 levels.

SG does not have upper SS7 levels – does not have MTP3.

Primitives IP signaling point processes MTP3 to MTP2 primitives.

IP signaling point transports MTP3 to MTP2 primitives to SG's MTP2 for processing.

Interface with MTP3 Presents an MTP2 upper interface to MTP3.

Presents an MTP2 upper interface to MTP3.

MTP3 Data Messages Transports MTP3 data messages.

Transports MTP3 data messages.

Management Relies on MTP3 for management procedures.

Uses M2UA management pro-cedures.

Table 1. Comparison of M2UA and M2PA




MTP 3 User Adaptation (M3UA) LayerM3UA protocol supports the transport of any SS7 MTP3-User, such as TUP, ISUP, and

SCCP over IP using the services of SCTP. TCAP and any other SS7 levels above SCCP

are carried in the Data Payload portion of SCCP due to the fact that they are SCCP

users, and not MTP users. M3UA provides a mechanism whereby MTP3 services are

provided to an IP-based node, thus extending the reach of SS7 into the IP realm.

The typical uses of M3UA are to communicate between the Signaling Gateway (SG)

and Media Gateway Controller (MGC) or between the SG and an IP resident data-

base (IPSCP). See Figure 4 for deployment of M3UA in hybrid network architectures.

M3UA is extremely important in communication with high traffic databases where

the 16 link linkset limitation of traditional SS7 is causing bandwidth issues. M3UA

and its associated SCTP/IP solve these issues while reducing the network complexity

in terms of linksets, combined linksets, and routes.

In a Signaling Gateway (SG) configuration of M3UA, the SG has an SS7 point code

because it has a resident SS7 MTP3 presence. This also means that the SG can be

used to route to other SGs or nodes.

SCCP User Adaptation (SUA) LayerSUA protocol supports the transport of any SS7 SCCP-User signalling such as TCAP.

The inherent capabilities of TCAP(MAP, INAP, SMS, BSSAP, or RANAP)are trans-

ported over IP using the services of SCTP.

SUA supports the following SCCP capabilities:

u Transfer of SCCP user part messages (TCAP, MAP, RANAP, etc.). u SCCP Connectionless Services. u SCCP Connection Oriented Services. u SCCP management services – Remote Destinations – Subsystems

u Distributed IP-based Signaling Nodes.



Comparison of M3UA and SUAWhen comparing the protocol stacks of M3UA versus SUA; the SUA protocol stack

is much simpler, therefore it is more efficient and easier to implement. The differ-

ences between the implementation of M3UA and SUA can be seen in Table 2.

M3UA SUA

SCCP Variants Signaling Point is required to support different variants of SCCP to interface with different countries.

Only the one node has to have SCCP therefore the point code issue is eliminated.

ISUP Services Supported Not Supported

Addressing Each node required to have both IP address and SS7 point codes.

Using SUA does not consume point codes since there is no MTP3.

Routing M3UA messages are routed from point code to point code.

SUA allows IP network to route messages based on Global Title Information.

Implementation Complexity

M3UA requires services from SCCP.

One less protocol layer. The elimination of SCCP reduces the complexity of the network node therefore reducing cost.

Table 2. Comparison of M3UA and SUA

Network Deployment Using M2PAThe deployment of SIGTRAN M2PA in conjunction with SCTP and IP provides each

of the interconnected devices (Core STP, SCP, and Edge STP) with a resident SS7

MTP3. Each of the associations is treated as a conventional SS7 link transported

over IP.

The Edge STP in this scenario can have “Local routing” to switch signaling traffic

between the interconnected SSPs and MSC. The Edge STP would route traffic to

the Core STP when it is destined for the SCP or another interconnected network.

See Figure 2 for the Network Interconnection using M2PA.

More Effi cient Network Architectures


Figure 2. Network Interconnection

Network Deployment Using M2UAAs shown in Figure 3, M2UA is being used to communicate between the “Edge

Device” and the “Core STP.” In this type of deployment, the Edge Device does

not have an instance of MTP3 and is acting as a signaling gateway converting

from TDM SS7 to M2UA SIGTRAN. Since there is no MTP3 in the Edge Device, it

is transparent to the SS7 network, i.e. it has no Point Code. All MTP3 routing for

messages generated by the subtending nodes (SSPs and MSC) is handled by the

Core STP. SS7 views this interconnection methodology as a remote termination of

the SS7 links.

The advantage of this confi guration is twofold. First, the network operator realizes

the cost reduction in transporting the SS7 connectivity from the Edge Device to the

Core STP over IP. Second, the implementation is simplifi ed since the Edge Device is

transparent to SS7. No SS7 routing changes need to be made in either the subtend-

ing offi ces or the Core STP.

SSP

SSP

MSC

M2PA

M2PA

M2PA

TDMSS7 Links

Edge STP Core STP

IP Network

More Effi cient Network Architectures


Figure 3. Network Deployment Using M2UA

Network Deployment using Multiple SIGTRAN Adaptation LayersIn a typical network, the implementation of SIGTRAN multiple adaptation layers

are used to connect to different network elements. The selection of the adaptation

layer type is dependent upon three things: the network element type, the type of

traffi c generated by the network element, and the architectural requirements of

the service provider. See Figure 4 for examples of Network Interconnectivity using

SIGTRAN.

Core STP to Core STP

M2PA was selected as the connection methodology between the Core STP

pairs. M2PA was selected because it treats the associations as SS7 links carried

over IP. This concept uses MTP concepts such:

u Link alignment (normal and Emergency).

u Changeover and Changeback.

u Processor outage.

u Congestion, etc.

Core STP to Soft Switch

M3UA was chosen for communication between the Core STP and the Soft

Switch based on the effi ciency of the protocol stack and the types of traffi c

generated by the switch. ISUP and TCAP over SCCP are generated by switching

elements whether they are MSCs, Tandem switches, wireline offi ces, or Soft

SSP

SSP

MSC

M2UA M2UA

TDMSS7 Links

Edge Device Core STP

IP Network



Switches. These traffic patterns limit the choice of SIGTRAN connectivity to

M2PA, M2UA, or M3UA. M3UA was chosen because it transports both ISUP

and SCCP with the added benefit of being more efficient than M2PA due to the

absence of a protocol stack level.

Core STP to SCP

SUA was the selected adaptation layer going from the Core STP to the SCP for

the following reasons:

u It will transport TCAP traffic.

u It has lower overhead than M3UA.

u It requires less configuration than M3UA.

Core STP to Edge Device

When aggregating network element connectivity based on topology or

geography, M2UA is the adaptation layer of choice based on the following

benefits:

u No valuable Point Code resources required.

u Easy to implement, i.e. few switch or STP translations to change.

u Transparent to the network.

u SS7 routing centralized in Core STP.

The driving factor in this network implementation is to achieve signaling trans-

port with significant cost savings, through aggregation and transport over IP.

Core STP to Edge STP

M2PA is implemented between the Core STP and the Edge STP because local

routing is desired at the Edge STP. Local routing is desired because it adds an

extra level of network survivability. If the Core STP or the connectivity between

the Edge STP and the Core STP fails, intra Edge STP calls can then be completed

based on local routing within the Edge STP.



Figure 4. Network Interconnectivity

What to look for in SIGTRAN-enabled devices (Signaling Transfer Point, Signaling

Gateway, and IP Edge)

When looking for a source to provide SIGTRAN-enabled equipment for network

deployment, consider the following requirements:

Designed and architected for IP deployment: The revolution in the Signaling Network

demands that more and more signaling traffic is placed on the IP network. It is

critical to select equipment that is designed from the ground up as IP-centric. These

capabilities should include, but not be limited to, the following: embedded IP

capabilities (Back plane, Ethernet switches, high bandwidth, etc.), ease of IP

connectivity, and ease of IP configuration.

SSP

SSP

MSC

M2UA

SUA

Multiple Association TypesM2UA, M2PA, M3UA & SUA

TDMSS7 Links

SSP

SSP

MSC

TDMSS7 Links

Edge Device Core STP

IP Network

Edge STP

M2PA

SCPHLR ... LNP

Core STPSoftswitch

M3UA



High processing capabilities: With the ever increasing traffic on the Signaling Network

due to Number Portability, SMS, Location Updates, etc., it is extremely important to

select equipment that will process today’s traffic loads and be extensible for future

traffic requirements.

SIGTRAN’s ability to efficiently relieve bandwidth bottle necks in the network

makes it the ideal choice.

Most cost-effective (initial purchase and life cycle): When selecting network equip-

ment, it is extremely important to consider not only the initial purchase price, but

also the ongoing support and maintenance of the equipment. The most economical

combination of these two should be selected, assuming that all of the other require-

ments are met.

Standards-based: Ensure that selected systems are designed to the latest proto-

col standards and have the widest breadth of protocols available. These protocols

should include, but not be limited to the following:

u SS7 both ITU and ANSI

u SIGTRAN (M2PA, M2UA, M3UA & SUA)

u SIP

Most environmentally friendly: With today’s focus on cost and environmental

concerns, another important consideration is whether or not new equipment is the

environmentally friendly. Two factors that should be taken into consideration are:

1. Least amount of power consumed.

2. Least amount of heat generated.

Extensive Support Services: Explore companies that will partner with you for the

long term. The company should provide network planning, network engineering,

installation, training, and support services. These services should be on an a la carte

basis, so only the required services need be selected.



The SEGway™ Solutions AdvantagePT’s SEGway™ portfolio includes IP-centric network elements and applications de-

signed for high availability, scalability, and long life cycle deployments. These offer

carriers and service providers extensive IP networking options, unrivaled in the

industry with features such as high density signaling, advanced routing, IP migra-

tion, gateway capabilities, SIP bridge, and core-to-edge distributed intelligence. In

addition, these carrier grade solutions provide lower cost of ownership from initial

purchase through their entire product life-cycle deployment. The SEGway product

portfolio provides the following unique advantages:

Designed and architected for IP deployment: SEGway products are designed to be

a mere extension of the IP network. The internal architecture of SEGway platforms

include intelligent IP backplanes for both internal and external communications.

Also included in the design is an integrated, five-nines reliable, gigabit Ethernet

switch. The inclusion of the carrier grade Ethernet switch reduces the requirement

for an expensive, external Ethernet switch or IP router ports.

High processing capabilities: PT announced in 2009 a threefold increase in the

number of links supported by its’ SEGway X401 platform to over 1500+ low speed

link equivalencies. Within the same year, a doubling of processing capabilities was

announced. In 2011 PT introduced the X401e – a 7.5 foot telco rack including three

X401s supporting 4,536 links. These upgrades can occur without a massive hard-

ware change out. Simply add processors as required for increased traffic demands.

Most environmentally friendly: The SEGway product portfolio has the lowest

power consumption and heat generation of any signaling product available, thus

reducing its carbon footprint.

World Class Support: PT provides a vast array of support services including: network

planning, engineering, installation, and training. These services are offered on an a

la carte basis and can be tailored to meet individual customer requirements. Trans-

port and Protocol support: The SEGway signaling solutions have been deployed for



international and domestic applications in wireless and wireline configurations all

over the world, including the United States, Canada, France, United Kingdom,

Netherlands, Brazil, Mexico, Japan, China, Africa and many others. A vast array of

standards-based protocols are supported including: SCTP, M2PA, M2UA, M3UA

and SUA. Also supported are traditional TDM, ATM and Annex “A.”

For more information on PT and the SEGway signaling solutions or to schedule a

demonstration, please contact [email protected].



About PT (www.pt.com) PT (NASDAQ: PTIX) is a global supplier of advanced, high availability network

communications solutions. Its SEGway™ Diameter and SS7 Signaling Systems

provide tightly integrated signaling and advanced routing capabilities and

applications that uniquely span the mission critical demands of both existing

and next-generation 4G LTE and IMS telecommunications networks. The

Company’s IPnexus® Multi-Protocol Gateways and Servers enable a broad range

of IP-interworking in data acquisition, sensor, radar and control applications for

aviation, weather and other infrastructure networks. Established in 1981, PT is

headquartered in Rochester, NY and markets and sells its products worldwide

through its direct sales organization as well as through channel partners that include

major telecommunications equipment vendors, government prime contractors and

value-added resellers.

About the Author Tom Jenkins has over 42 years of experience in telecommunications. During his

career, he has held positions related to SS7 Signaling including: Technical Support

Manager, Manager of Product Management for STPs, International Sales Director

for SS7 Test Equipment, and Vice President Sales and Marketing for SS7 Test

Equipment. In 1997 Tom started Center Point Consulting, Inc., providing SS7,

SIGTRAN, and SIP training to over 2500 students worldwide. Tom has been actively

involved with telecommunications signaling including SS7, SIGTRAN, SIP and

Diameter for 26 years. Tom has been working directly with the Diameter Protocol

since 2008. Today, Tom is currently the Senior Director of Marketing at PT. You can

contact Tom at [email protected].

understanding sigtran.pdf

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