X2 Application Protocol (X2AP)

The X2AP protocol is used to handle the UE mobility within E-UTRAN and provides the following functions:

·         Mobility Management

·         Load Management

·         Reporting of General Error Situations

·         Resetting the X2

·         Setting up the X2

·         eNB Configuration Update

Protocol specification

3GPP TS 36.423 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP)

X2 layer 1

 The main functions of X2 interface layer 1 are as following:

·         Interface to physical medium;

·         Frame delineation;

·         Line clock extraction capability;

·         Layer 1 alarms extraction and generation;

·         Transmission quality control.

Protocol specification

3GPP TS 36.421 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 layer 1

S1 Signalling Transport

S1 signalling bearer provides the following functions:

·         Provision of reliable transfer of S1-AP message over S1-MME interface.

·         Provision of networking and routeing function

·         Provision of redundancy in the signalling network

·         Support for flow control and congestion control

L2 - Data link layer

Support of any suitable data link layer protocol, e.g. PPP, Ethernet

IP layer

·         The eNB and MME support IPv6 and/or IPv4

·         The IP layer of S1-MME only supports point-to-point transmission for delivering S1-AP message.

·         The eNB and MME support the Diffserv Code Point marking

Transport layer

SCTP is supported as the transport layer of S1-MME signalling bearer.

·         SCTP refers to the Stream Control Transmission Protocol developed by the Sigtran working group of the IETF for the purpose of transporting various signalling protocols over IP network.

·         There is only one SCTP association established between one MME and eNB pair.

·         The eNB establishes the SCTP association. The SCTP Destination Port number value assigned by IANA to be used for S1AP is 36412.

Protocol specification

3GPP TS 36.410 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 layer 1 general aspects and principles               

3GPP TS 36.411 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 layer 1

3GPP TS 36.412 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 signalling transport

IETF RFC 2460: "Internet Protocol, Version 6 (IPv6) Specification"

IETF RFC 791 (September,1981): "Internet Protocol"

IETF RFC 2474 (December 1998): "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers"

S1 layer 1

the main functions of S1 interface layer 1 are as following:

·         Interface to physical medium;

·         Frame delineation;

·         Line clock extraction capability;

·         Layer 1 alarms extraction and generation;

·         Transmission quality control.

Protocol specification

3GPP TS 36.411 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 layer 1

Reason for SDCCH DROP in GSM Network

·         Low Rx level DL& UL, Bad Qual (Interference ) DL & UL, and High TA

·         Drop on LAC boundary.

·         SW congestion

·         Due to TCH Congestion.

·         Hardware issue, Timeslot Faulty, High VSWR, Alarm or Antenna Faulty, mismatch combining.

·         Interference or error on E1.

·         Drop due to radio failure.

·          SD drop due to MS faulty.

·         SD drop due to mismatch Rxlev_access_min value & RACH busy threshold

·         SD drop due to mismatch BA2 list, due to drop occur during HO.

Reason for TCH DROP in GSM Network

·         The parameters SACCH Multi-Frames and Radio Link Timeout are set to too small values.

·         The parameter RXLEV_ACCESS_MIN is set to a too small value.

·         The parameter RACH Min.Access Level is set to a too small value.

·         The parameters Min DL Power on HO Candidate Cell and Min Access Level Offset are inappropriately set.

·         The length of timer T3103 (this timer is set to wait for a Handover Complete message) is set to a too small value.

·         The length of timer T3109 (this timer is set to wait for a Release Indication message) is set to a too small value.

·         The length of timer T3111 (this timer specifies the connection release delay) is set to a too small value.

·         The length of timer T305/T308 is set to an invalid or too great value.

·         The parameter TCH Traffic Busy Threshold is set to a too small value.

·         The parameter Call Reestablishment Forbidden is set to Yes.

·         The parameters related to edge handover are inappropriately set.

·         The parameters related to BQ handover are inappropriately set.

·         The parameters related to interference handover are inappropriately set.

·         The parameters related to concentric cell handover are inappropriately set.

·         The parameters related to power control are inappropriately set.

·         T200 and N200 are set to too small values.

·         Some neighboring cell relations are not configured.

·         The parameter MAIO is inappropriately set.

·         The parameter Disconnect Handover Protect Timer is set to a too small value.

·         The parameter TR1N is set to a too small value.

·         The parameters Software Parameter 13 and MAX TA are set to too small values.

·         If a repeater is used, the parameter Directly Magnifier Site Flag is set GSM BSS Network KPI (TCH Call Drop Rate) Optimization Manual INTERNAL to No

·         Drop due to hardware issue (Faulty, VSWR, ICM, Mismatch combining & Alarm)

·         Drop due abis link error or high abis link UTL.

·         Drop due to Co-Channel interference or due to poor Qual UL & DL.

·         Drop due to poor level UL & DL or due to High TA.

·         Drop due to HO Delay.

·         Drop due to Poor Antenna installation.

·         Drop due to poor path Imbalance ( UL & DL)

Cell Selection and Re-selection in GSM

Cell Selection Procedure

• First MS powers-on

• MS starts measuring received power level from all cells in range

• MS calculates average power level received from each cell:

• Stored in RXLEV(n) parameter

• MS calculates C1 parameter for each cell based on RXLEV(n)

• Mobile compares cells which give a positive value of C1 and ‘camps-on’ to the cell with the highest C1 value.

On switch-on, an MS periodically measures the received power level on each of the BCCH frequencies of all cells within range. From these periodic measurements the MS calculates the mean received level value from each cell, stored in the parameter RXLEV (n) where n=neighboring cell number.

Cell Re-selection – GSM Phase 1 Mobiles

For GSM Phase 1 mobiles, cell reselection is achieved by comparing current cell C1 with neighboring C1 cell measurements:

• Between cells within a Location Area:

C1 (new) > C1 (old)         (for more than 5 seconds)

• Between cells on a Location Area boundary:

C1 (new) > C1 (old) + OFFSET      (for more than 5 seconds)

Cell Re-selection – GSM Phase 2 Mobiles

• GSM Phase 2 introduced a separate cell re-selection parameter, C2

• Intended to:

• Prevent multiple handovers for fast-moving mobiles

• Ensure MS camps on to cell with greatest chance of successful communications

• The C2 calculated is:

C2 = C1 + OFFSET – (TEMPORARY_OFFSET x H(PENALTY_TIME –T)