The existing global telecommunications infrastructure was originally built around SS7, a widely used signaling protocol that controls call set-up, tear-down, number portability, caller-ID and much more. Despite efforts to move on, today much of the existing wireline and wireless telecommunications infrastructure continues to depend on SS7.
One significant ongoing issue is that SS7 relies on leased line DS1 circuits to interconnect service providers and their peers. These DS1 circuits are becoming increasingly expensive to maintain, and in many cases (Verizon and AT&T), the circuits are no longer offered by interconnect carriers.
How then can service providers migrate SS7 to IP-based networks?
One solution that many service providers are implementing is a group of IP-based protocols defined by the IETF Signaling Transport (SIGTRAN) working group, designed to relay SS7 signaling over low-cost IP infrastructure.
As shown below, SIGTRAN is largely equivalent to traditional SS7 and outlines two protocol variants used to transport SS7 messages over IP networks. One variant is based on M2PA, while the other uses M3UA – we’ll discuss both here.
Comparison of the OSI network model, SS7, and the two SIGTRAN variants
Following the OSI model, let’s begin at the Physical layer and work our way up:
Ethernet and IP – providing both Physical and Data Link layers, as does virtually all other IP-based networks.
SCTP – Stream Control Transmission Protocol provides Transport layer functionality for SS7 messages, making sure SS7 messages are delivered and in the correct order.
M2PA – Message Transfer Part 2 User Peer-to-Peer Adaptation Layer is defined by RFC 4165, facilitating transmission of SS7 MTP Level 3 messages via SCTP in a peer-to-peer arrangement.
M3UA – Message Transfer Part 3 User Adaption Layer is defined by RFC 4666, facilitating transmission of SS7 user messages (ISUP) via SCTP.
ISUP – ISDN User Part, used to set up and tear-down telephone calls
Given the two protocol options, which should you choose? The decision usually falls to your SS7 service provider or the equipment you plan to interconnect. As an example, Metaswitch CFS supports both M3UA and M2PA, making the decision pretty simple.
The Role of SS7 to SIGTRAN Gateways
There are many situations where traditional SS7 over TDM circuits would best be converted to one of the SIGTRAN variants, allowing a mix of new and old equipment and software to interoperate.
TelcoBridges offers the Tsig family of SS7/SIGTRAN signaling gateways, facilitating interoperability between the SIGTRAN variants and SS7. Tsig supports SS7, SIGTRAN, ISDN PRI, CAS R2, and T1 CAS protocol conversion as well as network diagnostics tools.
TSG800 Hardware Platform
Based on the TSG800 or TSG3200 hardware platforms, Tsig scales from a single SS7 signaling link to 64 links in a single 1U chassis, powered by either AC or -48V DC power. With proven reliability, Tsig devices can be deployed fully redundant (1+1) providing high availability (HA) of over 99.999% availability.
SIGTRAN Use Cases
There are many use cases for SS7 and SIGTRAN in modern telecommunications networks – the most common are as follows:
Traditional SS7 TDM Connection
This diagram shows the typical SS7 implementation without SIGTRAN, showing redundant A-Links from an SS7 provider to a service provider switch via DS1 leased lines.
SIGTRAN M3UA over IP
An example of an SS7 service provider that supports M3UA, connecting over an IP network to a switch that also supports M3UA.
SIGTRAN M3UA to non-SIGTRAN Switch
A cost-saving architecture for older switches that do not support SIGTRAN, using signaling gateways to eliminate the expensive leased lines. In this configuration, the SS7 provider uses an IP connection to deliver either SIGTRAN M3UA or M2PA signaling, while redundant TSG signaling gateways convert the signaling to traditional TDM SS7 A-links, compatible with older equipment.
SS7 A-Links to SIGTRAN-capable Switch
In situations where an existing SS7 provider needs to interconnect A-Links with a SIGTRAN-capable switch. This architecture is common in situations where a new switch is deployed in a local, remote, or cloud platforms environments.
Hybrid SS7/SIGTRAN Links
A cost-saving approach for service providers is a hybrid SS7/SIGTRAN deployment, using the existing SS7 A-Link for the primary signaling path, while using SIGTRAN to carry the second/redundant signaling path. With the rising cost of leased lines, this will significantly reduce costs while maintaining network diversity and reliability.
Benefits of Migrating to SIGTRAN
The benefits of integrating SIGTRAN into service provider networks includes:
- Cost savings
- Eliminate expensive DS1 leased lines
- Reduce SS7 port costs
- Reliability – IP Networks have greater reliability than dedicated leases lines, leveraging built-in redundancy and auto-healing
- IP Networks offer virtually unlimited bandwidth, allowing multiple SS7 links to be carried on a single lower-cost IP network
- Greater Flexibility
- Connections and re-configurations are software-defined allowing for network reconfigurations on-the-fly
- Greater options for SIGTRAN service providers, providing connectivity compatibility to older equipment or switches
- Facilitates connectivity between legacy TDM SS7 circuits and virtualized or cloud-based switches
You can learn more about TelcoBridges Tsig line of SIGTRAN signaling gateways at: https://telcobridges.com/products/signaling-gateways-tsig/
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