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Sunday, May 15, 2011

UMTS Femtocell Architecture:

Universal Mobile Telecommunications Service (UMTS) is a third generation (3G) wireless mobile technology that evolved from GSM. It is very widely deployed and as such is one of the first wireless technologies to be standardized for femtocells. The 3GPP standards body defined the UMTS Home NodeB (HNB) architecture in 2009.

He illustration below shows a high-level architecture diagram of the UMTS femtocell network. This architecture is consistent with the generic Femto Forum architecture and I will describe it below;

The mobile phone, also known as the user equipment (UE), interfaces with a Home NodeB (HNB) over the air interface. The Home NodeB is a device that resides in the customer’s home that is analogous to the Femto Forum’s FAP. The HNB’s name stems from the fact that it is a miniature UMTS NodeB providing wireless coverage in the customer’s home. The air interface between the UE and the HNB is defined as the Uu reference point. The HNB interfaces to the mobile network operator (MNO) over a broadband network by interfacing with a broadband access gateway.

The mobile operator’s core network obtains access to the HNB via the broadband access network at the Iu-h reference point. At the Iu-h reference point a mobile service provider uses a security gateway to protect the core network against attacks. On the trusted side of the security gateway resides the Home NodeB Gateway (HNB GW). The HNB GW is analogous to the Femto Forum’s FAP GW, which is responsible for interfacing with the mobile operator’s core network’s Iu interface. The Iu interface consists of the Iu-cs for circuitswitched traffic and the Iu-ps for packet-switched traffic.

A look at the UMTS Femtocell Signaling protocols is illustrated below;

The illustration shows the Iuh control protocol stack used by femtocell networking equipment. The user equipment (UE) stack is the same stack it uses to interface with macrocell NodeB, since the UE sees the HNB equivalent to a macrocell NodeB. At the physical layer is the radio frequency (RF) interface, also referred to as the air interface. The higher-layer protocols gain access to the RF interface via the Media Access Control (MAC) layer. Above the MAC layer the UE’s higher-layer control protocols use the Radio Link Control (RLC) layer to gain access to various radio channels to communicate to the mobile network. The RLC provides error-free delivery of messages through mechanisms of error detection, message acknowledgments, message sequence number, and message retransmissions. The Radio Resource Control (RRC), as the name implies, is responsible for managing the radio resources that are allocated for user equipment. The RRC is responsible for establishing and releasing radio channels, including signaling and bearer channels. Radio signal measurements are invoked and reported by the RRC, allowing it to also perform mobility management. The mobility management includes determining if a call should be handed over, cell reselection, and routing updates. The Radio Access Network Application Part (RANAP) protocol is responsible for radio access bearer (RAB) management. RABs are bearer channels over radio interfaces used for transporting media, including voice and data. The RANAP RAB management includes signaling protocol messages for establishing, releasing, and modifying radio bearer channels. The RANAP User Adaptation (RUA) protocol has been specifically designed for carrying RANAP messages between an HNB and an HNB gateway. The RANAP messages are carried transparently without any alteration.

The HNB gateway is responsible for interfacing with the HNB on the broadband access side via a security gateway. On other interfaces the HNG gateway interfaces with mobile service provider’s core network over the Iu interface. Iu interfaces can be based on an IP network or the conventional ATM network.

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