Diameter protocol is a corner stone of all-IP telco networks. There are two main signalling protocols in IMS. Session Initiation Protocol (SIP) and right, the Diameter.
The main purpose of the SIP protocol is to establish a real-time multimedia session. This session goes end-to-end between the phones (or other devices) of the originator and recipient.
Internally – within the operator’s network – during the session establishment, we have to trigger many network functions. That can be because of the authentication of users, charging, allocation of resources, application of services, etc. 3GPP mandates for this purpose the Diameter protocol. The Diameter is used by both ePC and IMS for requesting/triggering additional information. These requests are sent in a form of transactions. During one IMS session setup or one IMS registration procedure we have to perform several Diameter transactions.
Diameter triggering during VoLTE session setup
VoLTE is a communication standard defined by GSMA and 3GPP organizations. They created plenty of documents, but these documents are not good when one is a beginner. Still it’s no rocket science. Perhaps it is because the documents don’t contain more good pictures explaining the basic ideas. I believe if the standards would be written as comic books, they’d have much broader audience 🙂
What is VoLTE?
- VoLTE stands for Voice over LTE. LTE is a new standard for wireless communication of high-speed data for mobile phones.
- Sometimes we can see also ViLTE, which means Video over LTE.
T-Mobile USA is on the cutting edge. It was the the first operator who came with HD Voice back in 2013. This month they announced a new upgrade of their network – to Enhanced Voice Services (EVS). From the customer perspective it means a better audio quality – even better than HD. EVS does this with a broader audio frequency range, which translates to richer, more realistic-sounding voice audio. The EVS is supported for both VoLTE and VoWifi. Additionally for the LTE technology it also brings a higher reliability in areas of weaker signal.
T-Mobile has a pending patent for their solution where their customers with EVS compatible phones will benefit even if the B-party doesn’t have an EVS-capable device. Currently the technology is available for LG G5, Samsung Galaxy S7 and S7 edge. T-Mobile plans to support four more smartphones by the end of 2016. I wonder what will be the response of Alliance for Open Media for webRTC.
In IT and particularly in Telco we are obsessed with abbreviations. My wife always loughs and tries to mimic me when she listens to my calls. Today we should be very careful as many of them start on ‘P’ – PCC, PCRF, PCEF, P-CSCF, PGW, PDN, PDG, PDB, PHB. But no worries, there will be abbreviations starting on other letters as well 🙂
In the IMS we have separated signalling and media data. However a full independence of control and user plane is not desirable. We want to control when the media starts and stops, we want to be sure about media routing, we want to ensure Quality of Service (QoS). And, of course, we want to accordingly charge the users.
In order to achieve these requirements we use two techniques in the VoLTE architecture:
- Policy and Charging Control (PCC)
- Differentiated Services (DiffServ)
Policy and Charging Control
PCC functionality comprises of Policy Control (e.g. QoS, media gating, ..) and Flow Based Charging. The ETSI TS 29.212, 29.213, 29.214 and 29.203 define Policy and Charging Control Architecture. There are many PCC functions defined. For us the main 3 PCC elements are:
- Application Function (AF)
- Policy Charging and Rules Function (PCRF)
- Policy Control Enforcement Function (PCEF)
Policy and Charging Control (PCC) Architecture
In VoLTE is the AF incorporated within the Proxy-CSCF. The P-CSCF provides the information related to the control plane signaling. The information is taken from SIP/SDP session setup and it is forwarded to the PCRF via the Rx reference point. Each new SIP message that includes an SDP payload or session events (e.g. session termination, modification) can trigger a new request sent towards the PCRF. This ensures that the PCRF gets the proper information in order to perform reliable PCC.
It’s called convergence. Two different approaches end-up with the same solution. In the nature it can be a bird and a bat (mammal). The both fly using wings although their ways to the result were quite independent.
In technology we can also use the synergism of the convergence. Simply we can reuse a technology which is already in place instead of creating a new one. A good example is the activity of NASA and Verizon to use the LTE eNodeB towers for air traffic control of drones (UAVs). Amazing – just a few years ago there was no LTE and no Drones.
Yesterday Deutsche Telecom (T-Mobile Germany) along with Inmarsat and Lufthansa announced their plans to create a new LTE network which would enable high-speed Internet for aviation passengers in Europe.
The network is called European Aviation Network (EAN). It will consist of the Inmarsat S-band satellite with multi-beam pan-European coverage and approximately 300 LTE sites. Each of the LTE sites will have a range of more than 80 km (comparing to traditional range of 10 km or less) and will be able to transmit data to the planes’ operating altitude. Moreover they should be flexible enough to deal with the speed of a plane. Lufthansa should start with a flight trial program in 2017.
One wonders (with the coming IoT), how far we’re from unpiloted flights now?
The biggest advantage of mobile operators – comparing to OTT applications – is that they are interconnected. No matter what country we are in we can connect into the network, we can place the calls, receive the calls, all under our own identity. We also don’t care in which network is our counterpart if he/she is now present in his/her home network or not. To achieve this operators need to support roaming and interworking. The IMS Roaming and Interworking Guidelines can be found in the GSMA IR.65 and LTE Roaming Guidelines in GSMA IR.88.
GSMA Statistics – Interconnections
We’ve mentioned the roaming when we discussed the SBC. In this post we’ll take a look on what options we have. So what can be the flows when one of the participants is not physically present in the home network and needs to be connected via some other infrastructure – the visited network.
Just recently the GSMA announced the first commercial interconnected VoLTE service in South Korea. This just illustrates how far we’re still from the real IMS-based roaming.
Update: GSMA Next-generation Interconnection and Roaming Analysis for Mobile Services
In general there are 3 options how we can implement roaming nowadays:
Roaming opt. 1
Firstly the roaming can be done in the packet core. The P-CSCF is in the home network. But that requires a mechanism which obtains the IP address of the A-SBC. This solution called S8 (reference point) Home Routing (S8HR) is very easy and maybe a bit preferred by operators these days because that’s a standard way for data roaming. However there are many open questions here related to Lawful Intercept, Emergency Calls, SRVCC, etc. GSMA works on a new variant of S8HR which is a part of the GSMA Network 2020 Programme and was recently as agreed as VoLTE Roaming candidate.
There are two big enemies of mobile communication. Weak signal and battery drain. With the LTE technology in place we need to deal with the situation when our signal gets too weak. The higher frequencies have a problem to penetrate walls, currently the radio network is not covering whole country yet, etc.. these problems seems to be only temporary (as the LTE is moving forward) but at this point of time we can’t rely on the LTE network only. When we are out of 4G coverage while we are not calling we’ll simply fallback to CS network and VoLTE supports CS breakout or CS retry so our voice service is still working.
But what if it happens during an ongoing call? Can we handover and still maintain the session? It is possible but not easy as because of the battery drain we can access only one radio network at the time. What we are looking for is called enhanced Single Radio Voice Call Continuity (eSRVCC) and it is described in the GSMA IR.64 and ETSI TS 124 237. (SRVCC is applied during PS to CS access transfer in a single radio system from LTE to 3G/2G. For devices that support active WiFi, and 3G/2G and LTE dual radio, the enhanced Dual Radio Voice Call Continuity (eDRVCC) is applied.)
eSRVCC call flow is probably one of the most complex flows you can encounter in VoLTE. Read firstly about the basic flows VoLTE in IMS. In case of doubts or when more scenarios/details needed please refer to the spec.
To allow this functionality we need to add some more IMS elements in the network:
Access Transfer Control Function (ATCF)
ATCF acts as SIP signalling anchor and is located in the SIP signalling path between P-SCSF and S-CSCF. Very often it is part of the SBC. ATCF controls the ATGW, where the media plane is anchored. During the session transfer, the ATCF establishes a new session with the SCC AS. This new session substitutes the old session between the ATCF and the SCC AS.
Access Transfer Gateway (ATGW)
The ATGW anchors the media session.
Service Centralization and Continuity Application Server (SCC AS)
The SCC AS anchors originated and terminated sessions when using the PS or CS access. It is also responsible for the Terminating Access Domain Selection (T-ADS).