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Control Plane/User Plane split

The 3GPP standards for 5G assume separation of the Control and User Planes as a mandatory (and basic) network feature. Looking at the development history of the various 3GPP-defined networks this is not a surprising fact. The split architecture concept has appeared as long ago as 3GPP Release 4, when the good old MSC/VLRs got replaced with the MSC Servers serving the Control Plane (CP) and Media Gateways acting as specialized routers on the User Plane (UP). The trend continued, with the functionalities of the SGSN from the 2G/3G environment ending up in an MME for CP and the User Plane gateways (SGW/PGW) carrying the traffic. As the next step, the PGWs themselves also got divided, into a PGW-C and PGW-U. SIP-based IMS networks use Interconnection Border Control Functions (IBCFs) on CP and Transition Gateways (TrGW) on UP at network borders, for VoLTE an CP Access Transfer Control Function (ATCF) and UP Access Transfer Gateway (ATGW) handle SRVCC.


User plane


The trend is obvious, and so are the reasons behind it. The functionalities – and, what follows, the hardware and software needs of the equipment – are quite different when we focus on the characteristics of exchanging signalling messages between the network entities than when we analyse the needs of a transport network that carries user/application traffic. As a consequence separate network functions are defined for these two types of roles, making it possible to update and dimension them independently of one another. A bigger number of subscribers requires an increase in capacity of the CP entities that store their subscription data or handle their attachment procedures, while a big growth in the throughputs required by some new, bandwidth hungry applications results in demands on the capacity of the UP routers.  Scalability of networks becomes much easier – and the costs of purchasing, deploying and maintaining the network elements fall dramatically. With the current pressing need to optimise network costs, the use of CP/UP splitting concept when standardizing the architecture for any new telecommunication network seems an absolute necessity.


The network deployment and running cost factor, always important, is becoming crucial in the current environment. On the one hand – huge amounts of data that needs to be transported, on the other hand – pricing models that force the operators to put low price tags on the carried bytes (or, rather, gazillions of them) make profitability a big (and growing) challenge. Any potential tools that help reduce network costs are very much in demand. UP/CP splitting falls neatly into that category – and together with virtualizing network functions, network slicing, hosting content closer to the users – provide the mobile network operators with a working chance of making a profit rather than losses while running a telecom network. After all, it is the  profit that is the reason for their existence…


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Until next time,

The Apis IP-Solutions Team

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