Demystifying SS7 & Sigtran Networks (With Labs!) – Part 1 – Intro

This is part of a series of posts looking into SS7 and Sigtran networks. We cover some basic theory and then get into the weeds with GNS3 based labs where we will build real SS7/Sigtran based networks and use them to carry traffic.

If you use a mobile phone, a VoIP system or a copper POTS line, there’s a high chance that somewhere in the background, SS7 based signaling is being used.

The signaling for GSM, UMTS and WCDMA mobile networks all rely on SS7 based signaling, and even today the backbone of most PSTN traffic relies SS7 networks. To many this is mysterious carrier tech, and as such doesn’t get much attention, but throughout this series of posts we’ll take a hands-on approach to putting together an SS7 network using GNS3 based labs and connect devices through SS7 and make some stuff happen.

Overview of SS7

Signaling System No. 7 (SS7/C7) is the name for a family of protocols originally designed for signaling between telephone switches. In plain English, this means it was used to setup and teardown large volumes of calls, between exchanges or carriers.

When carrier A and Carrier B want to send calls between each other, there’s a good chance they’re doing it over an SS7 Network.

But wait! SIP exists and is very popular, why doesn’t everyone just use SIP?
Good question, imaginary asker. The answer is that when SS7 came along, SIP was still almost 25 years away from being defined.
Yes. It’s pretty old.

SS7 isn’t one protocol, but a family of protocols that all work together – A “protocol stack”.
The SS7 specs define the lower layers and a choice of upper layer / application protocols that can be carried by them.

The layered architecture means that the application layer at the top can be changed, while the underlying layers are essentially the same.

This means while SS7’s original use was for setting up and tearing down phone calls, this is only one application for SS7 based networks. Today SS7 is used heavily in 2G/3G mobile networks for connectivity between core network elements in the circuit-switched domain, for international roaming between carriers and services like Local Number Portability and Toll Free numbers.

Here’s the layers of SS7 loosely mapped onto the OSI model (SS7 predates the OSI model as well):

OSI Model (Left) and SS7 Protocol Stack (Right)

We do have a few layers to play with here, and we’ll get into them all in depth as we go along, but a brief introduction to the underlying layers:

MTP 1 – Message Transfer Part 1

This is our physical layer. In this past this was commonly E1/T1 lines.

It’s responsible for getting our 1s and 0s from one place to another.

MTP 2 – Message Transfer Part 2

MTP2 is responsible for the data link layer, handling reliable transfer of data, in sequence.

MTP 3 – Message Transfer Part 3

The MTP3 header contains an Originating and a Destination Point Code.

These point codes can be thought of as like an IP Address; they’re used to address the source and destination of a message. A “Point Code” is the unique address of a SS7 Network element.

MTP3 header showing the Destination Point Code (DPC) and Origin Point Code (OPC) on a National Network, carrying ISUP traffic

Every message sent over an SS7 network will contain an Origin Point Code that identifies the sender, and a Destination Point Code that identifies the intended recipient.

This is where we’ll bash around at the start of this course, setting up Linksets to allow different devices talking to each other and addressing each other via Point Codes.

The MTP3 header also has a Service Indicator flag that indicates what the upper layer protocol it is carrying is, like the Protocol indicator in IPv4/IPv6 headers.

A Signaling Link Selector indicates which link it was transported over (did I mention we can join multiple links together?), and a Network Indicator for determining if this is signaling is at the National or International level.

TUP/MAP/SCCP/ISUP

These are the “higher-layer” protocols. Like FTP sits on top of TCP/IP, a SS7 network can transport these protocols from their source to their destination, as identified by the Origin Point Code (OPC), to the Destination Point Code (DPC), as specified in the MTP3 header.

We’ll touch on these protocols more as we go on. SCCP has it’s own addressing on top of the OPC/DPC (Like IP has IP Addressing, but TCP has port numbers on top to further differentiate).

Why learn SS7 today?

SS7 and SIGTRAN are still widely in use in the telco world, some of it directly, other parts derived / evolved from it.

So stick around, things are about to get interesting!

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