Abis Interface for RF Engineers

Abis Interface for RF Engineers

Abdullah Saleh

Introduction Abis Interface: a BSS internal interface that links the BSC and a BTS; it has not been standardized. The Abis interface allows control of radio equipment and radio frequency allocation in the BTS. If the BTS uses TDM mode, the requirement of timeslots changes as the BTS traffic increases. In this case, the Abis timeslot assignment mode should be modified. The Abis timeslot assignment modes:
Fix: The Abis timeslots and Um timeslots are fixedly assigned. They have a one-to-one relationship.
Flex: The Abis timeslots can be shared among different services of the same BTS or among different BTSs.

Fix Abis

E1 TDM Abis frame is a Multiplexing of 32 channels. Each channel has 64 Kb/sec. Channel zero is used for E1 signaling and synchronization, then we have 31 effective channels for each E1. In other vendors, maybe channel zero is used for E1 signaling and channel 16 is used for E1 synchronization, so we have 30 effective channels for each E1.

Traffic on E1: Air interface full-rate = 13 kb/sec . & Air interface half-rate = 6.5 kb/sec. ∴ 13 kb/sec + 3 bits for error detection and correction = 16 kb/sec. ∴ One E1 channel can hold 4 TCH-FR or 8 TCH-HR.

One E1 channel =

   

= 4 configured TCH-FR on a TRX.

Signaling on E1: Operation and Maintenance Link (OML): Network management; used to monitor the operating status of the TRXs or BTS; OML messages have priority over other layer 2 messages. One BTS is configured with one OML that is need one E1-Time slot. Radio Signaling Link (RSL): carries all user signaling messages for connection setup and release. Also handles messages for SMS and Supplementary Services (SS) on the Abis Interface (unlike handling on the Um Interface). Each TRX is configured with one RSL that is need one E1-Time slot.

Multiplexing Mode: the number of multiplexed signaling links per E1. BTS multiplexing mode used to improve the utilization of the Abis timeslot resources. Parameter: Multiplexing Mode: indicates the Abis timeslot multiplexing mode of the BTS. It consists of the following seven types:
1:1 multiplexing mode
2:1 multiplexing mode
3:1 multiplexing mode
4:1 multiplexing mode
5:1 multiplexing mode
6:1 multiplexing mode If Abis E1 is in 4:1 mode, “MR Preprocessing” should be set to “BTS_Preprocessing”, otherwise the Abis capacity for speed is not enough.

Example: TRX configuration: 4/4/5 SDCCH configuration: 2/2/2 Multiplexing Mode = 2:1 For the previous givens how much E1s does the BTS need?? Number of signaling links per BTS = 1 (OML) + 13 (RSLs) = 14 At 2:1 multiplexing mode, then.... Number of E1’s channels for signaling =



= 7 channels

Number of TCHs on Air Interface = (13 8) – 6 (SDCCHs) – 3 (BCCHs) = 95 TCHs Number of E1’s channels for traffic =

 

= 23.75

Total number of channels = 7 + 24 = 31 channels BTS will need one E1 for this configuration.

24 channels

Question: Now!! How much E1s do you think a BTS with the following givens will need? TRX configuration: 4/4/5 SDCCH configuration: 2/1/1 Multiplexing Mode = 2:1

Solution: Number of signaling links per BTS = 1 (OML) + 13 (RSLs) = 14 At 2:1 multiplexing mode, then.... Number of E1’s channels for signaling =



= 7 channels

Number of TCHs on Air Interface = (13 8) – 4 (SDCCHs) – 3 (BCCHs) = 97 TCHs Number of E1’s channels for traffic =

 

= 24.25

Total number of channels = 7 + 25 = 32 channels BTS will need Two E1 for this configuration.

25 channels

Now if you need an expansion for a specific site you can know if this will require a new E1 or not. Since some times such as in Celebration Events you need the expansion as fast as possible so you can calculate the needed Abis time slots to know if the expansion will be a fast action or not. You can know how many E1s per site by write the MML command LST BTSCONNECT in the MML window.

This BTS has 3 E1s

Types of E1 time slots (channels) in Fix Abis:
TCH time slots
Signaling time slots
Idle time slots: used for data traffic as in the example if the rest time slots in E1 are configured as idle then all of them used for GPRS & EDGE.
Null time sots: free time slots on E1 and not configured.
Forbidden time slots: free time slots but don’t allowed to be used. In parallel sharing of E1 between BTSs using Digital Cross Connect (DXX) the E1 time slots for the BTS is forbidden for the other BTS.

E1 – 16 TS

BTS

DXX BTS

BSC

E1 – 15 TS E1 – 31 TS

Abis Time Slots

Fix Abis vs. Flex Abis

Difference between Fix & Flex Abis: 1. Flex Abis has new signaling link to transfer the Abis timeslot dynamic connection message. Called Extend Signaling Link (ESL). The ESL can be multiplexed only with the OML of the same BTS 2. NO Null or Idle time slots in Flex Abis. 3. Abis handles the incoming traffic (voice/data) using first come first served methodology. Parameter: Flex Abis Mode If this parameter set to FLEX_ABIS the Abis mode will be flexible. Parameter: PCU Support PREEMPT_ABIS_LINK message If this parameter set to YES and the BTS supports the FLEXABIS function, the CS service can preempt the timeslots on the Abis interface used by the PS service.

Abis Related RF-Parameters

Abis Resource Adjustment TCHH Function Switch This parameter determines whether the BSC preferentially assigns a half-rate TCH to an MS when the Abis resources are insufficient. Fix Abis Prior Choose Abis Load Thred{%} When the static Abis resource load is lower than Fix Abis Prior Choose Abis Load Thred(%), the full-rate channel is preferentially assigned. Otherwise, the full-rate or half-rate channel is preferred according to the dynamic Abis resource load. Flex Abis Prior Choose Abis Load Thred{%} When the static Abis resource load is higher than Fix Abis Prior Choose Abis Load Thred(%) and the dynamic Abis resource load is higher than Flex Abis Prior Choose Abis Load Thred(%), the half-rate channel is preferred. Otherwise, the full-rate channel is preferred. The counter that measured the half-rate channel assignment because of the Abis resource congestion: R3165A:Number of Preferable Selections of TCHH in a Cell (Abis Resource Congestion)

Abis Counters

Abis Performance:
L1151A:Mean number of occupied timeslots on the Abis interface
L1121A:Abis Timeslot Fault Times of the Site Effect of Congested Abis:
H322M:Number of Unsuccessful Incoming Internal Inter-Cell Handovers (No Abis Resources Available)
H302E:Number of Intra-BSC Intra-Cell Handover Failures Due to No Abis Resources Available
A312F:Number of Assignment Failures (No Abis Resource Available)

Counters to calculate Abis Congestion Rate:
R3162B:Number of Failures to Allocate Abis Resources (TCHF)
R3163B:Number of Failures to Allocate Abis Resources (TCHH)
R3164B:Number of Failures to Allocate Abis Resources (Dynamic PDCH)
R3162A:Number of Requests for Allocating Abis Resources (TCHF)
R3163A:Number of Requests for Allocating Abis Resources (TCHH)
R3164A:Number of Requests for Allocating Abis Resources (Dynamic PDCH) The BSC regularly checks the number of Flex timeslots in the site and measures the maximum number of available Flex timeslots in the measurement period.
R2741:Number of Available Flex Timeslots