With the change from 2G to 3G, the emphasis for the systems changed from a focus on mobile voice communications to mobile data and general connectivity.
The foundations for the UMTS network had been set in place when GSM was launched. This provided the basic access elements as well as circuit switched voice.
The additional of packet data with GPRS required additional network entities to be added. It was the combination of these two network elements that provided the basis for the 3G UMTS network architecture.
The radio access network changed considerably as a completely new radio interface was used based around the use of CDMA. Also the handset name was changed to user equipment indicating a change in its use from just a voice phone to a data set which could have been a phone, PDA or laptop, with many laptops requiring a 3G dongle to plug into a USB port.
3G UMTS network constituents
Although for the 3G UMTS wireless communications system, there had been many changes when compared to the previous generation, the same basic top level system overview was the same.
However within the three top level constituents of this wireless communications system there were many changes.
The top level network architecture for the 3G UMTS wireless communications system could be divided into three main elements:
- User Equipment (UE): The User Equipment or UE is the name given to what was previous termed the mobile, or cellphone. The new name was chosen because the considerably greater functionality that the UE could have. It could also be anything between a mobile phone used for talking to a data terminal attached to a computer with no voice capability.
- Radio Network Subsystem (RNS): The RNS also known as the UMTS Radio Access Network, UTRAN, was the equivalent of the previous Base Station Subsystem or BSS in GSM. It provided and manages the air interface for the overall network.
- Core Network: The core network provided all the central processing and management for the system. It was the equivalent of the GSM Network Switching Subsystem or NSS.
The core network was then the overall entity that interfaced to external networks including the public phone network and other cellular telecommunications networks.
User Equipment, UE
The USER Equipment or UE was a major element of the overall 3G UMTS network architecture. It formed the final interface with the user.
In view of the far greater number of applications and facilities that it could perform, the decision was made to call it a user equipment rather than a mobile.
However it was essentially the handset (in the broadest terminology), although having access to much higher speed data communications, it could be much more versatile, containing many more applications.
It consists of a variety of different elements including RF circuitry, processing, antenna, battery, etc.
There were a number of elements within the UE that can be described separately:
UE RF circuitry: The RF areas handled all elements of the signal, both for the receiver and for the transmitter. One of the major challenges for the RF power amplifier was to reduce the power consumption.
The form of modulation used for W-CDMA required the use of an RF linear amplifier. These inherently take more current than non linear amplifiers which could be used for the form of modulation used on GSM.
Accordingly to maintain battery life, measures were introduced into many of the designs to ensure the optimum efficiency.
Baseband processing: The base-band signal processing consisted mainly of digital circuitry. This was considerably more complicated than that used in phones for previous generations.
Again this had been optimised to reduce the current consumption as far as possible.
Battery: While current consumption has been minimised as far as possible within the circuitry of the phone, there had been an increase in current drain on the battery.
With users expecting the same lifetime between charging batteries as experienced on the previous generation phones, this had necessitated the use of new and improved battery technology. Lithium Ion (Li-ion) batteries started to be more widely used to address this issue.
These phones needed to remain small and relatively light while still retaining or even improving the overall life between charges.
Universal Subscriber Identity Module, USIM: The UE also contained a SIM card, although in the case of UMTS it was termed a USIM (Universal Subscriber Identity Module).
This was a more advanced version of the SIM card used in GSM and other systems, but embodied the same types of information. It contained the International Mobile Subscriber Identity number (IMSI) as well as the Mobile Station International ISDN Number (MSISDN).
Other information that the USIM held included the preferred language to enable the correct language information to be displayed, especially when roaming, and a list of preferred and prohibited Public Land Mobile Networks (PLMN).
The USIM also contained a short message storage area that allowed messages to stay with the user even when the phone was changed. Similarly "phone book" numbers and call information of the numbers of incoming and outgoing calls were stored.
The UE could take a variety of forms, although the most common format was still a version of a "mobile phone" although having many data capabilities. Other broadband dongles started to be used as well - these could be used with laptops or even desktop computers on occasions.
3G UMTS Radio Network Subsystem
This was the section of the 3G UMTS / WCDMA network that interfaced to both the UE and the core network - it handled the wireless communications elements of the network.
The overall radio access network, i.e. collectively all the Radio Network Subsystem was known as the UTRAN or UMTS Radio Access Network.
3G UMTS Core Network
The 3G UMTS core network architecture was a migration of that used for GSM with further elements overlaid to enable the additional functionality demanded by UMTS.
In view of the different ways in which data could be carried, the UMTS core network was split into two different areas:
- Circuit switched elements: These elements were primarily based on the GSM network entities and carry data in a circuit switched manner, i.e. a permanent channel for the duration of the call.
- Packet switched elements: These network entities were designed to carry packet data. This enabled much higher network usage as the capacity could be shared and data was carried as packets which were routed according to their destination.
Some network elements, particularly those that were associated with registration were shared by both domains and operated in the same way that they did with GSM.
Circuit switched elements
The circuit switched elements of the UMTS core network architecture included the following network entities:
- Mobile switching centre (MSC): This was essentially the same as that within GSM, and it managed the circuit switched calls under way.
- Gateway MSC (GMSC): This was effectively the interface to the external networks.
Packet switched elements
The packet switched elements of the 3G UMTS core network architecture included the following network entities:
- Serving GPRS Support Node (SGSN): As the name implies, this entity was first developed when GPRS was introduced, and its use has been carried over into the UMTS network architecture. The SGSN provided a number of functions within the UMTS network architecture.
- Mobility management When a UE attached to the Packet Switched domain of the UMTS Core Network, the SGSN generates MM information based on the mobile's current location.
- Session management: The SGSN managed the data sessions providing the required quality of service and it also managed what were termed the PDP (Packet data Protocol) contexts, i.e. the pipes over which the data was sent.
- Interaction with other areas of the network: The SGSN was able to manage its elements within the network only by communicating with other areas of the network, e.g. MSC and other circuit switched areas.
- Billing: The SGSN was also responsible billing. It achieved this by monitoring the flow of user data across the GPRS network. CDRs (Call Detail Records) were generated by the SGSN before being transferred to the charging entities (Charging Gateway Function, CGF).
- Gateway GPRS Support Node (GGSN): Like the SGSN, this entity was also first introduced into the GPRS network. The Gateway GPRS Support Node (GGSN) was the central element within the UMTS packet switched network. It handled inter-working between the UMTS packet switched network and external packet switched networks, and could be considered as a very sophisticated router. In operation, when the GGSN received data addressed to a specific user, it checked if the user was active and then forwarded the data to the SGSN serving the particular UE.
The shared elements of the 3G UMTS core network architecture included the following network entities:
- >Home location register (HLR): This database contained all the administrative information about each subscriber along with their last known location. In this way, the UMTS network was able to route calls to the relevant RNC / Node B. When a user switched on their UE, it registered with the network and from this it was possible to determine which Node B it communicated with so that incoming calls could be routed appropriately.
Even when the UE was not active (but switched on) it re-registered periodically to ensure that the network (HLR) was aware of its latest position with their current or last known location on the network.
- Equipment identity register (EIR): The EIR was the entity that decided whether a given UE equipment could be allowed onto the network. Each UE equipment had a number known as the International Mobile Equipment Identity. This number, as mentioned above, was installed in the equipment and was checked by the network during registration.
- Authentication centre (AuC) : The AuC was a protected database that contained the secret key also contained in the user's USIM card.
The 3G UMTS wireless communications system provided the first step in the transition from a mobile voice network thatw as provided by the 1G and 2G services, and this meant that far more data capability was needed. This was reflected in the network architecture.
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