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EWSD switching system Upgrade and Migrate

Selection Universal VoIP Gateway to Affordably upgrade and Migrate EWSD Public Switched Telephone Exchange in to IP Network


The rapid development of VoIP technologies has forced Fixed-line Operators to integrate existing TDM networks with IMS/NGN networks.

The SIP phone revolutionized the fixed-line telephony world and became an alternative to analog telephone or its complement.

The EWSD switching systems that are reliable in maintenance and operation deserve to be further used in modern conditions.

This requires upgrading the EWSD Telephone Exchange, task which is the migration of subscribers to the IP network.

Solution Overview

Universal VoIP Gateway from Terratel company is a budgeting device designed to upgrade an outdated digital Telephone switch and integrate it into the softswitch infrastructure.

Network diagram for upgrade DLU EWSD to IP network, NGN multiservice networks

Pic 1. Universal VoIP Gateway Network diagram

Universal VoIP gateway connects directly to the DLU (Digital Line Unit) of the EWSD system via the E1 interface and works towards the IP network using the SIP protocol (Session Initiation Protocol).

Switching EWSD platform from SIEMENS

Siemens’ EWSD digital switching system is one of the most popular telephone switches in the world.

More than 200 million subscriber telephone ports installed in the world.

The manufacturer of the EWSD platform was Siemens, as well as a division of Nokia Siemens Networks (after re-branding).

Currently, the production of EWSD switching system equipment is discontinued.

The EWSD switching platform consists of an EWSD core and an EWSD peripheral.

EWSD digital switching system photo

Pic 2. Appearance of EWSD digital switching system

EWSD Technical Information

Core EWSD consists of a digital switching field SN (Switching Network), a central control device CP (Coordination Processor) and a control device with an SS7 SSNC (Signaling System Network Control).

Functionally, these modules create connections between telephone subscribers, handle call processing and routing, and also manage SS7 signaling traffic.

The EWSD periphery consists of the RSU (Remote Switching Unit), LTG (Line Trunk Group) and DLU (Digital Line Unit).

These modules and units are designed to connect subscribers and external connecting lines.

DLU reuse

DLU (Digital Line Unit) is an access equipment and is responsible for the direct connection of POTS and ISDN telephone subscribers.

DLU – this is a digital subscriber unit, which is used as a subscriber equipment PBX or as a remote subscriber hub.

The DLU unit performs the following main functions: support of the subscriber set, scanning telephone subscriber line, outputting information on changing the state of telephone subscriber line, converting dialing pulses into digital form.

This feature makes DLU the optimal place to connect and integrate into the IMS/NGN network using a universal VoIP gateway from TERRATEL.

Network diagram for upgrade DLU EWSD to IP network, NGN multiservice networks

Pic 3. Network scheme for integrating DLU into NGN multiservice communication networks.

The optional solution is to reuse the DLU as a terminal PBX with the inclusion in the PSTN network via the SS7 signaling system.

Strukture diagram of the integration DLU and universal VoIP gateway

The hardware units of the DLU represent several functional groups:

  • Central Functional Units
  • Peripheral Functional Units
  • Functional Units for Remote Operation

The central functional units in the DLU are reserved and together form DLU systems 0 and 1.

The peripheral functional units are:

  • Subscriber Line Modules (SLMA for POTS, SLMD for ISDN, SLMX for V5.x interfaces)
  • Test Equipment (TU, MTA)
  • Ringing and Metering Voltage Distribution (RGMG)

The functional units for remote operation include:

  • Stand-Alone Service Control (SASC)
  • External Alarm Set (ALEX)
DLU functional units and their integration with VoIP gateway

Pic 4. View of the DLU functional units and their integration with VoIP gateway.

Universal VoIP Gateway can directly provide V5.1 and V5.2 interfaces for various subscriber access networks (AN).

DLU connections to the E1 interface of the Universal VoIP Gateway are made through one, two or four primary digital carriers (PDC). Universal VoIP Gateway supports connection of communication links of 2048 kbps and 4096 kbps.

The mechanical design of the DLU consists of racks, frame, modules.

Universal VoIP Gateway can be installed in a free rack space.

Existing DLU Types

During the evolution of the EWSD system, various types of DLUs were produced:

  • DLU (G): SLMA: ITM, SLMA: ITF, SLMD: QFB, SLMI: xxx, etc.

Subscriber line module analog (SLMA)

The Subscriber Line Module Analog (SLMA) is used for the connection of analog subscribers POTS to the DLU.

Depending on the production version, the SLMA module contains 4, 6, 8, 16, 24 or 32 analog subscriber lines (SLCA) and besides a shared control section with the processor (SLMCP).

There are many variants for different types of line, including analog subscriber line modules for DLU.

The subscriber line module analog for ordinary subscribers (SLMA:COS) is used as the standard module in the DLU.

Photo of Subscriber line module analog (SLMA: ITM)

Pic 4. Appearance of Subscriber line module analog (SLMA: ITM)

Access equipment (PRI, V5.2)

Universal VoIP Gateway also has the support of the full SS7, PRI, V5.2 signaling protocols.

Thanks to this, when upgrading the EWSD Telephone Exchange, the existing access equipment will be switched to the Universal VoIP Gateway.

Requirements for a Universal VoIP Gateway

To complete the tasks of integrating DLUs into an IP network, the universal VoIP gateway supports the following functions:

  • PSTN signaling protocols: SS7, PRI, V5.2
  • IP signaling protocols: SIP, SIP-T, SIP-I, RTP, RTCP, TCP, UDP
  • Voice codecs: G.711, G.723.1, G.726, G.729a / b, GSM
  • Registration and authentication of SIP subscribers
  • Fax Transmission: G.711 Fax, T.38 Fax Relay
  • Hardware echo cancellation
  • Control and management: Web, Telnet, SNMPv2 monitoring, RADIUS login

Advantages of the solution

In the course of modernization and migration, the fixed-line operator ceases to use the switching core of the EWSD and does not bother its customers, who are still on the POTS lines, that is making a smooth transition from TDM to VOIP.

At the same time, CAPEX is optimal due to partial use of previously paid equipment.