Communication Towards Bts.docx

Communication Towards BTS The feature Packet Abis over IP uses IP networks for transmission between BSC and BTS. It requires entity called Site Transport Node (STN). It is is network equipment used by the different types of RBSs for IP RAN transport as well as management of hardware at a BTS site. The STN provides a north bound LAN interface supporting IP over Ethernet communication. This interface carries both Packet Abis data and O&M data for the STN itself. Depending on the type of RBS different hardware is used to implement the STN:   

In the Micro and Macro RBSes SIU (SIU-01, SIU-02) is used for Abis provisioning For the Pico RBS (RBS 2409) the STN is fully integrated with the BTS and it has in-built support for IPsec VPN For RBS 6000 the SIU-02, Transport Connectivity Unit (TCU) or Baseband T605 may be used

STN equipment divides Abis link into two sections: Abis Upper and Abis Lower part. Abis upper is the BSC-to-STN part and is switched on the Service Access Point Identifier (SAPI) value giving the opportunity to differentiate on traffic type. The transport function uses L2TP over IP/Ethernet. On Abis Upper the Link Access Procedure on the D-Channel (LAPD) frames are carried through L2TP tunnels (or transport sessions) over IP. On Abis Lower the LAPD frames are always carried through an overlaid structure called Super Channel from STN to RBS switched on TEI value, see Figure 4.

Figure 4

Abis Overview

Security Gateways (SEGW) could be installed on both ends of Abis Upper link if the security level in RAN transport network is not good enough. This functionality is included in some STN variants. The network components like site router and SEGW is not part of the GSM RAN, except that the RBS 2409 and STN of type SIU which has built-in IPsec and Firewall functionality. Baseband T605 provides IPsec functionality for O&M traffic only.Establishment of a secure IPsec tunnel between the RBS 2409 and other nodes (BSC, OSS-RC, Time Server) requires that a security gateway (SEGW) is placed between the node and the RBS 2409.