GSM Abis Optimization
The interface between the BTS and BSC is a 3GPP reference interface called the Abis interface. The physical trunk connecting a BTS and BSC is typically a T1/E1 circuit, and carries 24 (T1) or 32 (E1) separate 64 kbps DS0 channels.
The IP•Tube G4 Abis has 1 to 4 Telecom interfaces that can be configured as full or fractional T1 or E1. The Abis traffic is optimized and encapsulated into Ethernet packets for transport over LANs, WANs, MANs, IP Satellite, xDSL, Cable and Wireless Ethernet. The Ethernet Switch provides transparent LAN bridging, user data bandwidth and access control through rate limiting and port-based VLAN filtering and tagging.
Engage’s Abis Optimization produces bandwidth efficiency increases of 33% to 50%, depending on the nature of the traffic on the Abis interface whenever a voice user is silent which is typically 40-60% of the time, which corresponds to a GSM voice call capacity gain of 50-100% per T1/E1.
Idle Channel Suppression
When a sub-rate DS0 is not assigned to a call, a repeating idle pattern is transmitted in accordance with 3GPP TS 08.54 v8.0.1. The Abis Optimization only transmits the idle state of the sub-rate DS0.
The voice and data bearer traffic is carried over the sub-rate DS0s in transcoder and rate adaptor (TRAU) frames in accordance with 3GPP TS 08.60 v8.2.1.
TRAU frames are generated when a sub-rate DS0 is assigned to a call. The transcoder and rate adaptation control function that specifies the TRAU frames provides several opportunities to optimize the Abis interface traffic types:
Idle Speech Frames/Idle Data Frames - Compression of repetitive data patterns as detailed in 3GPP TS (Technical Spec) 08.60 for Full and Enhanced Full Rate Traffic (sects 3.4 & 3.5.5).
Idle Data Pattern – Compression of Layer 1 Idle Pattern, as detailed in 3GPP TS 8.54 – Layer 1 Structure.
Silent Speech frames– Monitoring of Discontinuous Transmission (DTX) for Silence Descriptor (SID) indicator. Reference GSM 06.81
Signaling Optimization – Signaling is based on LAP-D and will be transported as HDLC-over-IP. Provides more robust transport and reduced bandwidth..
Other TRAU Frames (O&M, Data,etc.) transported in a transparent manner.
HDLC Idle Suppression
The DS0 channels that are used to carry control and signaling traffic utilize HDLC, a synchronous serial link protocol, with idle flags during the intervals between messages. Engage’s Abis Optimization technology eliminates these idle flags and only transports the HDLC framed messages.
One to Four T1/E1 to IP interfaces supporting ABIS Base Station Controller to Base Station communication
The IP•Tube ABIS is designed to provide highly efficient bandwidth usage for cell backhaul from Base Station to Base Station Controller circuits.
The majority of Cell site backhaul circuits in use today are TDM T1 or E1 circuits. Designed to carry 24 or 32 digital voice/data channels, converting these circuits to IP for broadband backhaul is a major task in the future installations of higher speed GSM cell services. The Engage IP Tube has been an important building block in the conversion of TDM to IP for cellular backhaul applications due to its reliability and economical installations.
ABIS is a 3GPP protocol for communication between the Base Station Controller (BSC) and the Mobile Base Station (BS). ABIS system interfaces include DSO bearer channels that can be used as sub-rate voice or data carriers. The interface includes in-band control signals that are carried as part of the channel traffic. Optimization of the ABIS interface is enabled by minimizing control traffic when it is repetitive and predictable. During silent intervals (no voice or data packets), the bearer channels send a repeated series of bits to the far end. The IP•Tube ABIS recognizes these idle patterns and reduces their transmission over the backhaul network. This intelligent channel control can provide up to 50% reduction in BSC to BS network traffic.
This available bandwidth means existing TDM circuits can carry more traffic, voice and data, with no additional Capital Expense (CAPEX). As areas are upgraded beyond 2G services, increased users can be supported by the backhaul network without adding additional TDM circuits. Additionally by using the IP•Tube ABIS, remote cellular Base Stations can now be economically connected using satellite circuits or other more expensive techniques for backhaul, enabling the extension of mobile services to areas previously restricted due to access considerations.