Introduction

Here is the PDF for the content Vance presents. If you find an acronym, check out the abbreviations in this PDF.

Slava summarizes the session best, “if I had received the kind of information years ago when I was just starting to explore the telecom world, it would have saved me hundreds of hours and countless sleepless nights and weekends trying to understand the very concepts Vance so clearly explained.”

My perspective, is Vance provides the breadth of coverage across telecoms, only a master of their craft can deliver. It’s well worth your time.

If you’re an old-hand in mobile telecoms, this is great revision, to remind you in one session of its history, where we are today, and how it relates to modern web technologies. It simply can not be beat.

For the TADHack community, this one hour session is worth your time if you want to learn about mobile telecom networks. BUT take it in small bites. Vance covers much. Use your preferred AI tool to explore the acronyms used and concepts presented. Telecoms is the backbone of all our public communications infrastructure.

Though the web now dominates on services, see the dominance of WhatsApp, Snapchat, Instagram, Zoom, Teams, YouTube, Netflix, etc. And Hyperscalers (Amazon Web Services (AWS), Microsoft Azure, and Google Cloud Platform (GCP), along with others like IBM Cloud, Oracle Cloud, and Meta) control a growing portion of core network, by 2030, they are projected to command 61% of total data center capacity.

While telecoms is often written off as a legacy footnote, given the web bias of many. This footnote supports all emergency communications (e911), and will remain in use for multiple decades. And as Vance describes is built from the ground up for scalability and resilience. As we have seen with SS7, reports of its demise for almost 2 decades have been greatly exaggerated.

The History of Telecommunication Networks

Vance kicks thing off with an introduction to himself and his company SigScale. We did an in depth interview last month with Vance.

Vance produces the proforma for any telecom presentation, a list of acronyms. Standard are built on acronyms, and form a core of telecoms networks as they work globally, and are supported by multiple vendors and multiple service providers.

ITU-T (International Telecommunication Union Telecommunication Standardization Sector) is the main global body, which makes recommendations. The naming is a political thing, it’s a standard as far as you or I are concerned. And the body most relevant to Vance’s presentation, mobile networks, is 3GPP (3rd Generation Partnership Project).

History is important for understanding today. Telecoms is a vital national utility, without which society would not run, just like electricity and water. The government built telecoms networks, through an agency call PTT (Postal, Telegraph, and Telephone service).

In the 1990s there were a couple of interesting transitions. The technology of the PTTs was transitioning from analog to digital. With SS7 and ISDN, delivering crystal clear voice services with no echo or delay, an array of value added services like callerID, 3-way calling, etc. A peak in PSTN experience. Both on fixed and mobile networks.

But telcos tried to go their own way on the core transport protocol with ATM (Asynchronous Transfer Mode), given the importance of voice communications. The rest of the world was following the internet protocol (IP) with datagrams for computer connectivity.

Dial-up internet access was becoming a thing, the web was taking off, and the telecom industry could not afford to build an industry specific transport protocol. Hence ATM was replaced by IP (IETF). As Vance stated, the telecom industry pivoted, to IP, SIP, and the IETF.

For the mobile network the core control was IMS (IP Multimedia Subsystem), which Vance positioned as a dog’s breakfast. SIP was point to point, and did not support centralized control. So IMS was a compromise that enabled telcos to continue with their centralized business model.

With 4G there was no circuit switched core, it uses the IETF diameter protocol. And in the move to 5G, they adopted REST APIs. Incrementally moving towards IETF standards.

Mobile Networks

Radio access networks are a distinguishing feature of mobile networks. And then have traditionally been generational, 1G was analog, 2G digital narrowband (voice), 3G included mobile data. Vance focuses on the 4G and 5G which is the current transition as 3G has been end-of-lifed by many telcos.

The 4G core is called EPC (Evolved Packet Core), and 5G uses the 5G Core (5CC). In the core all the network control happens: authentication, authorization, mobility, roaming, and control of the RAN, etc. IMS is a separate network for communication services based on SIP, e.g. voice, messaging, video. Though video communications is not commonly used in mobile network enabled service today.

Packet Data Network can be the internet, it can be an enterprise VPN (Virtual Private Network), or a hyperscalers network like Meta / Facebook.

Mobility can be service continuity between different cell towers on the same mobile network, voice calls continue as you move between cell towers. But it can also mean service continuity between different mobile networks, also called roaming. EPC and 5GC also supports roaming. The logical entities are the Visited Public Land Mobile Network and the Home Public Land Mobile Network.

The Home Network remains in control on the visited network, as roaming may not be permitted for the phone’s service, or continuously monitored for available credit with prepaid.

IPX, IP exchange or (IPX) is a telecommunications interconnection model for the exchange of IP based traffic between customers of separate mobile and fixed operators as well as other types of service provider (such as ISP), via IP based Network-to-Network Interface.

Core Principles

We’ve covered the main entities and how they connect and control services. Vance then explains the approach of the SDOs (Standards Development Organization) a 3 stage approach:

• Specify requirements from a user perspective.
• Develop a logical model to meet those requirements.
• Develop detailed specifications of protocols.

This enables technology is evolve quickly. And Vance sees this as an important abstraction in 3GPP’s success.

In the logical model, data or information model is used. The benefit of a diligently independent Stage 2 information model becomes
apparent as changes in technology require new Stage 3 specifications.

All data models in Stage 3 are derived from the common Stage 2 information model, simplifying protocol specification choices and allowing interoperability between protocols.

Vance describes 3 planes:

• Management: Operations, administration and maintenance, e.g. observability, billing
• Control: Signaling for control of services, e.g. routing
• User: raw service data, e.g. voice data

Architecture

EPC

Vance runs through the 4G core. The Evolved Packet Core (EPC) provides the packet switching (PS) network for 4G (LTE) as well as for 5G (NR) non-standalone.

The Mobility Management Entity (MME) plays a key role in authentication, authorization and location management.
Subscriber data is stored in the Home Subscriber Server (HSS). The serving and PDN gateways (S-GW, P-GW) provide user plane functions.

The Policy and Charging Rules Function (PCRF) and the Online Charging Function (OCS) ensure that a UE receives (only) subscribed access and quality, within available credit.

5GC

The 5G Core (5GC) provides the packet switching (PS) network for 5G (NR) standalone. The Access and Mobility Management Function (AMF) handles access, authorization and location management. The Authentication Server Function (AUSF) handles authentication. Subscriber data is provided by the Unified Data Management (UDM) function.

The Session Management Function (SMF) handles sessions, including tunnel from access network to the User Plane Function (UPF). The
Policy Control Function (PCF) and Charging Function (CHF) play the same roles as PCRF and OCS, respectively, in EPC.

IMS

The IP Multimedia Subsystem (IMS) provides a VoIP based replacement for the legacy circuit switched (CS) network. Call Session Control Functions (CSCF) handle session control.

A UE’s first contact is the Proxy (P-CSCF), the Serving (S-CSCF) handles session state and the Interrogating (I-CSCF) is the inbound contact point for external entities. A Multimedia Resource Function (MRF) provides bearer terminating endpoints for media processing (i.e. IVR). The HSS and OCS are shared with EPC or provided by the 5GC UDM and CHF respectively.

Protocols

SCTP (Stream Control Transmission Protocol)

The IETF SIGTRAN working group was chartered to address the transport of PSTN protocols (SS7) over IP while taking into account the functional and performance requirements of the PSTN.

The Stream Control Transmission Protocol (SCTP) (RFC9260) was specified as an IP transport alternative to TCP or UDP. It provides
message oriented and reliable in-sequence delivery, with congestion control, over multiple streams to prevent head-of-line blocking. It supports multihoming with redundant paths for resilience and reliability. Many 3GPP protocols support SCTP transport.

DIAMETER

The DIAMETER base protocol (RFC6733) enables clients to exchange messages with servers while providing capability negotiation and application aware routing. Peers connect over TCP or SCTP while clients maintain sessions with servers.

SIP

In the IMS the Session Initiation Protocol (SIP) (RFC3261) is used in the control plane. The UE plays the role of SIP User Agent (UA), CSCFs play a SIP Proxy role and the S- CSCF also plays a SIP Registrar role with the HSS.

Unlike the original peer-to-peer use intent for SIP the IMS is centralized with the S-CSCF performing session routing and control. The HSS provides a set of initial filter criteria (iFC) at registration which define ASs with service triggers the S-CSCF shall match against SIP headers. In the example below two application servers (AS) are matched by iFCs, one for the main Multimedia Telephony (MMTel) AS and another AS
for Value Added Services (VAS) is also added to the path.

SBI (Services Based Interface)

In 5GC Service Based Interfaces (SBI) use the OpenAPI Specification (OAS) as Interface Definition Language (IDL). An NF service producer supports HTTP operations on REST resources for authorized NF consumers.

An NF Service discovery service (Nnrf_NFDiscovery) is produced by the NF Repository Function (NRF). An NF may consume this service to locate NF instances of a certain NF type (CHF in the example below) and optionally other qualifying parameters.

HTTP

The same principle of end-to-end distribution may be accomplished with HTTP transport. The client is aware of multiple REST API endpoints, through DNS lookup of a service name (nf.example.net). The HTTP client should fail early on connection timeout and advance to the next endpoint on connection failures.

Follow-up Questions

Q How can Erlang help in learning telecom?

A  Erlang is a programming language from Ericsson developed for the unique demands of telecom. I’ve been using it as my primary language for decades and highly recommend it.

Q For developers with no telecom background but who are eager to learn, where should they start?

A There are numerous open source projects implementing network functions including Open5GS and the Telecom Infrastructure Project (TIP).

Q What are the most in-demand telecom components today?

A Network functions of the EPC and the 5GC as well as operations and business support functions (OSS/BSS) which commercialize those.

Q How did you personally begin studying telecom?

A I started studying in the ‘80s so it was a trip to my local university’s (UoW) engineering library where I abused their photocopier.

Q How did Erlang help you achieve your goals in the telecom world?

A It provides high level abstractions which make it easy to handle massive concurrency and achieve fault tolerance.

Q Why did you choose Erlang for telecom so many years ago, and why do you still use it today?

A I recognized that communicating finite state machines (FSM) was the core concept to address and Erlang/OTP was designed to handle exactly that.

Q If you could send a message back to your younger self, what learning advice would you give? Or to put it differently: if you were taken back decades ago, where would you start and which area would you dive deeper into?

A I eventually learned that solutions for a profit center are much easier to monetize than for a cost center. As an engineer it’s often more interesting to make the magic happen with features and services, however making the money flow can help get you paid better.

Q What are the essential telecom components that every newcomer should study – the ones no telecom project can do without?

A AAA (authentication, authorization, accounting).

Question:

How is all this tested? Are there any predefined test suites to ensure correct inter-functionality between components? It seems like a lot of moving parts that for some reason seem to work together, it can’t just be trial error. The comparison to the software development world was interesting with the the reference to load balancers, so perhaps there is a different approach to testing that the telco community takes.

A: Protocol specification suites often include test specifications describing test cases against a “system under test” (SUT). The SDOs developed the Testing and Test Control Notation (TTCN-3) language and Ericsson donated an open source framework now available as Eclipse IoT-Testware.

Although it’s not often done lately, protocol specifications often included additional stages beyond 1-3 like in this ITU-T example for SCCP:

• Q.711 SCCP – Functional description of the signalling connection control part.pdf
• Q.712 SCCP – Definition and function of signalling connection control part messages.pdf
• Q.713 SCCP – Signalling connection control part formats and codes.pdf
• Q.714 SCCP – Signalling connection control part procedures.pdf
• Q.715 SCCP – Signalling connection control part user guide.pdf
• Q.716 SCCP – Signalling connection control part performance.pdf
• Q.786 SS7 – SCCP Test Specification.pdf

The Q.786 document contains a description of the test environment, the “system under test” (SUT) and many test cases:

Testing was seen as so important that it was part of the specifications process and even became the subject of standardization with the development of the Testing and Test Control Notation (TTCN), a test specification language.  Test specifications written with TTCN-3 and provided by 3GPP for UMTS, LTE, 5G and IMS are listed here. It could certainly be more extensive but at least we have a language and framework for doing it!

The architecture of a TTCN-3 test system considers the SUT and the control and runtime interfaces:

Ericsson had developed an implementation of TTCN-3 which they eventually donated to the Eclipse Foundation and is available as the Eclipse IoT-Testware.

More recently I’ve become involved in the work at TM Forum (tmforum.org) on Open APIs for test management.  The big picture is provided in document IG1137A .