What is the UMTS? List important features of the UMTS air interface.

This question appears in Mumbai University > Wireless Networks

Marks: 10 M

Year: Dec 2013, May 2014, Dec 2015


The UMTS Network architecture has three main entities:

1) UMTS Core Network (CN)

2) UMTS Terrestrial Radio Access Network (UTRAN)

3) User Equipment (UE)

1) Core Network (CN)

The UMTS network architecture is partly based on existing 2G network components and some new 3G network components. It inherits the basic functional elements from the GSM architecture on the core network (CN) side. The CN provides circuit switched (CS) functions as well as packet switched (PS) functions.

The core network can be split into the following different functional areas:

  • Functional entities needed to support PS services (e.g. 3G-SGSN, 3G-GGSN)
  • Functional entities needed to support CS services (e.g. 3G-MSC/VLR)
  • Functional entities common to both types of services (e.g. 3G-HLR)
  • Other areas that can be considered part of the core network include:
  • Network management systems (billing and provisioning, service management, element management, etc)
  • IN system (service control point (SCP), service signaling point (SSP), etc.)
  • ATM/SDH/IP switch/transport infrastructure

i. 3G-MSC

The 3G-MSC is the main CN element to provide CS services. The 3G-MSC also provides the necessary control and corresponding signaling interfaces including SS7, MAP, ISUP (ISDN user part), etc. The 3G MSC provides the interconnection to external networks like PSTN and ISDN.

ii. 3G-SGSN

The 3G-SGSN is the main CN element for PS services. The 3G-SGSN provides the necessary control functionality both toward the UE and the 3G-GGSN. It also provides the appropriate signaling and data interfaces including connection to an IP-based network toward the 3G-GGSN, SS7 toward the HLR/EIR/AUC and TCP/IP or SS7 toward the UTRAN.

iii. 3G-GGSN

The GGSN provides interworking with the external PS network. It is connected with SGSN via an IP-based network. The GGSN may optionally support an SS7 interface with the HLR to handle mobile terminated packet sessions.


The overall requirement for these two nodes is to handle the SMS from point to point. The functionality required can be split into two parts.

The SMS-IWMSC is an MSC capable of receiving an originating short message from within the PLMN and submitting it to the recipient service center.

The SMS-GMSC is an MSC capable of receiving a terminated short message from a service center, interrogating an HLR for routing information and SMS information, and delivering the short message to the SGSN of the recipient UE.

i. Firewall

This entity is used to protect the service providers’ backbone data networks from attack from external packet data networks. The security of the backbone data network can be ensured by applying packet filtering mechanisms based on access control lists or any other methods deemed suitable.


The DNS server is used, as in any IP network, to translate host names into IP addresses, i.e., logical names are handled instead of raw IP addresses. Also, the DNS server is used to translate the access point name (APN) into the GGSN IP address.

It may optionally be used to allow the UE to use logical names instead of physical IP addresses.

A dynamic host configuration protocol server is used to manage the allocation of IP configuration information by automatically assigning IP addresses to systems configured to use DHCP.

Fig12. UMTS Architecture


UMTS terrestrial RAN (UTRAN) i. UTRAN consist of Radio Network Subsystems (RNSs). The RNS has two main elements: Node B and a Radio Network Controllers (RNC).

ii. Radio network controller (RNC):

  • The RNC is responsible for control of the radio resources in its area. One RNC controls multiple nodes B.
  • The RNC in UMTS provides functions equivalent to the Base Station Controller (BSC) functions in GSM/GPRS networks.
  • The major difference is that RNCs have more intelligence built-in than their GSM/GPRS counterparts. For example, RNCs can autonomously manage handovers without involving MSCs and SGSNs.

iii. Node B:

  • The Node B is responsible for air-interface processing and some radio-resource management functions.
  • The Node B in UMTS networks provides functions equivalent to the base transceiver station (BTS) in GSM/GPRS networks. UMTS operates at higher frequencies than GSM/GPRS and therefore the signal coverage range is less.

3) UE:

The MS of GSM is referred as user equipment (UE) in UMTS. It is enabled with an UMTS SIM (USIM).

Features of UMTS interfaces:

i. The UMTS interfaces can be categorized as follows:

a. Uu :

  • This is the interface between the user equipment and the network. That is, it is the UMTS air interface.
  • The equivalent interface in GSM/GPRS networks is the um interface.

b. The Iuis split functionally into two logical interfaces, Iupsconnecting the packet switched domain to the access network and the Iucsconnecting the circuit switched domain to the access network. The standards do not dictate that these are physically separate, but the user plane for each is different and the control plane may be different.

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c. Iu –CS :

  • This is the circuit-switched connection for carrying (typically) voice traffic and signaling between the UTRAN and the core voice network.
  • The main signaling protocol used is Radio Access Network Application Part (RANAP).
  • The equivalent interface in GSM/GPRS networks is the A-interface.

d. Iub :

  • This is the interface used by an RNC to control multiple Node Bs.
  • The main signaling protocol used is Node B Application Part (NBAP).
  • The equivalent interface in GSM/GPRS networks is the A-bis interface.
  • The Iubinterface is the main standardized and open, unlike the A-bis interface.

e. Iu –PS :

  • This is the packet-switched connection for carrying (typically) data traffic and signaling between the UTRAN and the core data GPRS network.
  • The main signaling protocol used is RANAP.
  • The equivalent interface in GSM/GPRS networks is the Gb-interface.

f. Iur :

  • The primary purpose of the Iur interface is to support inter-MSC mobility. When a mobile subscriber moves between areas served by different RNCs, the mobile subscriber’s data is now transferred to the new RNC via Iur.
  • The original RNC is known as the serving RNC and the new RNC is known as the drift RNC.
  • The main signaling protocol used is Radio Network Subsystem Application Part (RNSAP).
  • There is no equivalent interface in GSM/GPRS networks.

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