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Rethinking data-plane processing in networks

Posted: 01 May 2007 ?? ?Print Version ?Bookmark and Share

Keywords:data plane processing and 3G technology? mobile infrastructure for voice transmission? 3G technology data rates?

Mobile infrastructure is generally accepted as having progressed through three stages of technology evolution: 2G, 2.5G and 3G. The dominant 2G wireless technologies are GSM (which uses time division multiple access) and CDMA. These technologies have migrated to 3G via the 2.5G stepping-stone technologies of GPRS and Edge. UMTS (W-CDMA) is the 3G evolution of GSM/GPRS/Edge, and cdma2000 is the 3G derivative of cdmaOne (Figure).

First- and second-generation mobile infrastructure was architected for voice transmission. The 2.5G technology overlaid data support on the voice network and offered limited data rates. True 3G technology must support data rates of 2Gbps or above per user. This leap in data rate has forced network vendors to rethink how they implement data-plane processing in their networks.

Processing challenges
The 3G radio network controller (RNC) is the switching and controlling function of the UMTS terrestrial radio access network, and controls the use and reliability of radio resources in the network. RNCs can control the radio-channel setup, frequency hopping and handovers for as many as 100 Node Bs. At the RNC, latency-sensitive voice-type traffic is separated from data traffic. Voice traffic is sent to a mobile switching center (MSC) over the Iu-CS interface for transport to the PSTN, and data traffic is sent to the serving GPRS support node (SGSN) over the Iu-PS.

These interfaces are defined in the 3G specifications such that in theory, network operators can mix and match network elements, such as the RNC from one vendor with node B from other vendors. The RNC can be a serving RNC (SRNC) that has control over the bearer for some UE, or it can be a drift RNC (DRNC), where the UE context is assigned to the DRNC after a request from the SRNC.

RNC functionality is divided into two parts: control and radio resource management (RRM). Control functions are concerned with the setup, maintenance and release of radio bearers. RRM is the process of guaranteeing the stability of the radio path (e.g. handovers and power control) and the QoS of connections once established through resource management (e.g. admission control).

UMTS (W-CDMA) is the 3G evolution of GSM/GPRS/Edge, and cdma2000 is the 3G derivative of cdmaOne.

When analyzing the UMTS protocol stack, it's useful to think in terms of "layers" and "planes." There are three network layers (transport, radio network and system or non-access) and two planes (control and user data). The transport layer is concerned with the movement of data from non-adjacent nodes through an internetwork. The radio network layer controls and manages the radio resources. The transport and radio network layers combine to provide a communications link between the user equipment and the core network. The system network layer then provides the user communications services such as call control (establishing and tearing down calls) and mobility management (including authentication).

In the RNC
Although no two architectures will be the same, there are many common elements across the access and core mobile infrastructure at a high level. A Node B, RNC and SGSN will typically have compute cards (similar to an array of GPPs), DSP farms, a number of line interface cards and a switch fabric. The line interface card will contain a processor, which could be a high-performance GPP, an ASIC or a network processor, to implement the user-plane in the transport network layer.

ATM adaptation layers adapt upper layers to lower layers. ATM Adaptation Layer 2 (AAL2) is used for time-sensitive voice data and can adapt up to 248 channels into an ATM virtual circuit. AAL2 itself consists of two sublayers, the common part sublayer (CPS) and the service-specific convergence sublayer (SSCS). The CPS packet consists of a three-octet header followed by a payload. CPS adapts to the upper layer and defines a link between the upper layers and the SSCS.

The SSCS transports voice calls or circuit switched data. SSCS packets are transported as CPS packets over AAL2 connections. The CPS packet contains an SSCS payload. There are three SSCS packet types, depending on the protection required (unprotected, partially protected or fully protected).

At one end of the spectrum, a network processor or ASIC can serve as a pre-processor, where it transforms transport data into a common format for processing by the remainder of the RNCi.e. managing the transport technology. At the other end of the spectrum, the network processor or ASIC would be considered an integral part of the RNC, doing data analysis and switching.

The data flow for the IMA case involves taking multiple lower-bandwidth links and amalgamating them into a higher-bandwidth logical link, which is presented to the remainder of the RNC for processing and switching. When the network processor or ASIC is a more integral part of the RNC, it must be aware of the data types flowing in the system. There may be instances where data isn't sent into the RNC, but switched back out.

ATCA system
Increasingly, mobile infrastructure vendors are looking at commercial off-the-shelf systems such as AdvancedTCA to form the basis of their infrastructural elements. AdvancedTCA is a standard defined by the PCI Industrial Computer Manufacturers Group (PICMG). PICMG is a standards body comprising over 400 companies that define open standards for the telecommunications sector. It was responsible for developing the CompactPCI standard, and AdvancedTCA is its latest family of standards that defines a new form factor for telecommunications equipment based on switch fabric architectures.

The first ATCA standard was defined in 2002 and is referred to as the base spec, or PICMIG 3.0. Following the base spec, a number of sub-specs have been released, with each defining a different fabric interface. In much the same way that the OSI seven-layer model aims to define each function of a software protocol such that equipment from different vendors can interoperate, ATCA defines a modular approach to next-generation mobile infrastructure such that cards from different vendors can be combined to form a given system node.

- Kevin Brennan
Product Marketing Manager, Applied Micro Circuits Corp.

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