The VOCAL Embedded TDM SPRAG (Secure Phone Registrar and Gateway) Rackmount V.150 VoIP to E1/T1 gateway, is a self-contained gateway between Secure Phones (using IETF SIP and ITU-T V.150.1 protocols) and TDM telephone systems (using PRI/T1/E1 trunks). It is well suited for private network deployments in secured environments as a bridge to legacy communications systems. All components are internal and interprocess communications are utilized for passing all V.32/V.34 modem signals as G.711 data to/from the TDM channels. Implemented as a true single box gateway with SIP/V.150.1 on the IP network side and channelized T1/E1 on the telco network side.
Sectéra®, TaIkSECURE™, and vIPer™ are trademarks of General Dynamics.
Common vIPer Phone Problems
Sectéra® vIPer™ phone and other secure phone customers often have issues operating from their on-premises secure location with their local telephone switches and/or their organization’s designated telecom provider. The SIP-based local telephone switches and/or telecom providers typically do not understand V.150.1 and related aspects in SDP negotiations. Hence end-to-end calls are relegated to use of G.711 communications of modem signals which is often un-reliable unless the organization has a premium connection to a telephone service provider connected to the nation’s telecom backbone.
Another common problem faced by secure phone customers is operation over a) potentially lossy packet networks and b) networks with poor sample timing synchronization. Data modem and fax machine operations typically do not work well in most environments unless special handling is provided. In some installations, a telecom provider offers a G.711 service which usually works well for fax operations. In most installations, however, fax has to be handled by a T.38 capable Analog Telephone Adapter (ATA). VOCAL supports data (and fax) modem operations with its Analog Modem Adapter (AMA) which may be useful for some vIPer™ phone and other secure communications installations.
VOCAL SPRAG Cloud Service
VOCAL’s SPRAG cloud service solves these problems (and others) by being located in the cloud (using Amazon Web Services) with an MPLS-like quality connection to a major domestic telecom carrier. Connection with the carrier is a) co-located service in the same data center, b) high reliability, low latency connection (often a SDN with a dedicated VPN) and c) monitored high quality of service (QOS). All the major domestic carriers (AT&T, Verizon, CenturyLink, Level3 and Inteliquent) are well interconnected themselves. G.711 modem calls work well on these networks as VOCAL has demonstrated for years with its SAMS (SIP Analog Modem Server) and FoIP/T.38 gateways it has offered to other customers. Timing synchronization is essentially guaranteed with optimal network connections to a telecom carrier.
SPRAG works with secure phone devices as a secure voice gateway to these public telecom carriers. V.150.1 protocols are used in secure mode with the vIPer™ phone over the potentially lossy/poorly timed network connections from the secure customer to VOCAL’s SPRAG server. SPRAG is capable of dynamic transitioning between audio passthrough, VBD, and MR modes making it ideal for secure voice applications requiring just in time transitioning between clear channel audio and secure voice modes. Once at VOCAL’s server, theG.711 modulated signals are exchanged with the other endpoint without loss (assuming the other endpoint has its own robust high-quality G.711 connection).
VOCAL Embedded Platform
The Embedded TDM SPRAG is available in VOCAL-provided hardware as a 1U rack mount chassis in three configurations. All configurations use Intel Xeon 6 SoC processors with varying number of cores/threads ranging from 8 to 20 cores. The largest configuration uses 20 cores to support up to 10 T1/E1 links. SFP connections provide up to 100G Ethernet (other Ethernet speeds available on alternate platforms). T1/E1 links are supported by a single PCIe card with options for 4, 8 and 16 spans. Additional spans support partial link redundancy in case of link failure. Redundant power supplies are standard for this platform.
Shown: Supermicro® based platform. Customer preferred platforms may be considered for the Embedded TDM SPRAG system.
VOCAL’s well deployed SPRAG V.150.1 service for vIPer phones is used on this platform in an embedded environment. All conversions from/to SIP/V.150.1 protocols, as used by vIPer phones (or similar MUOS equipment), to/from G.711 modem V.32/V.34 modulated signals are handled internally in-the-box. ISDN (typically NI-2) and MFC-R2 CAS signaling are supported on the spans for call control.
Interoperability Testing
VOCAL’s SIP-based V.150.1 Media Gateway software (the core of its commercial SPRAG service) has been JITC tested in a router subsystem that is being deployed by the US Navy. VOCAL’s SIP/V.150.1 software has been tested successfully with Avaya, Cisco, NEC, Nortel, Redcom and Ribbon V.150 switches/SBC’s (most today only support V.150.1 protocol pass-through and not media conversion to legacy modem signals). [Older SIP/V.150.1 media gateways have largely been discontinued as they relied on a chip based solution implementing protocol conversion between V.150.1 and modem modulations for V.32 and/or V.34 which has been end of life-ed Starcore starting in 2006, and ceasing all production by approximately 2023.] VOCAL’s SIP/V.150.1 software is implemented on general purpose Intel/AMD x86 (or ARM) processors running either Windows or Linux. As such obsolescence is not dictated by platform retirement with newer x86 (or ARM) processors and platforms constantly being offered commercially. As a result of VOCAL’s extensive testing of V.150.1, VOCAL’s Embedded SPRAG handles nuanced V.150.1 protocol imperfections as deployed by equipment on government DISN/DRSN networks.
Typical Embedded TDM Installation
The VOCAL Embedded TDM SPRAG may be used in a variety of configurations to bridge the divide between SIP/V.150.1 secure devices and legacy telephone systems. The Embedded TDM SPRAG is used for connection to government a legacy T1/E1 time slotted voice network. The vIPer Radio terminals use SIP/V.150.1 protocols to register (optional) with the Embedded TDM SPRAG and to place and receive secure calls. An outgoing call from a vIPer Radio terminal includes the telephone number of a destination terminal. Call signaling is converted from SIP to legacy ISDN by the Embedded TDM SPRAG and passed through the T1/E1 links. Similarly, incoming calls from TDM are received and passed as SIP signaling to secure terminals on the IP network.
Configuration of VOCAL’s Embedded TDM SPRAG is substantially simpler than many other “general purpose SBC/gateway devices”. The role of this VOCAL device is to function as this bridge between SIP/V.150.1 and legacy TDM telephone domains. Registration of each vIPer Radio terminal is optional (and unnecessary) if the Embedded TDM SPRAG is to function as a trunking service to another SIP switch in the network (shown as an optional SIP Server). Each legacy T1/E1 link needs minimal configuration.
Potential TDM Problems
The tight integration of VOCAL’s SPRAG software with TDM interfaces in a single processing platform reduces and eliminates many potential problems. Modem signals are extremely dependent on high-quality exchange of digital signals (G.711) encoding analog content representing modem signals. Loss (or repetition or insertion) of a single sample causes loss of modem synchronization. Modems (such as V.32 and V.34) will typically detect synchronization loss and attempt to recover by retrain procedures. Activation of retrain does disrupt application data which in this case means certain data, such as secure phone SCIP data frames will cease and likely be damaged. Since SCIP data is transported using SPRT TC 3 unacknowledged data, recovery from such disruptions is anticipated and implemented at the application level between the vIPer secure phones themselves. Most applications using modem technology have similar mechanism.
There is, however, a unique problem with SIP/RTP transported G.711 data. The V.32 modulation uses a symbol rate of 2400 with multiple bits encoded per symbol. RTP packets containing G.711 typically are 10 or 20ms in duration meaning the packets contain 80 or 150 G.711 samples. It turns out that either of these common packet durations will contain precisely an even number of V.32 symbols (24 or 48 respectively). Detection of synchronization loss when an entire symbol is lost is very difficult for the modulations (especially V.32). V.34 has certain advantages by using non-integral symbols rates (such as 3429) and a more robust trellis data encoder/decoder. The effect of a single RTP packet loss for V.32 may not be detectable by the modulation. The demodulation will likely “hiccup” and recover decoding of a bit stream (but perhaps not the correct bit stream). A self-synchronizing data scrambler is applied to the transmitted bit stream and if the receiver either misses bits (due to jitter buffer adaptation) or wrongly decodes bits (due to lost RTP data), the receiver’s descrambler will not be in the same state as the transmitter’s. Normally the receiver scrambler will resynchronize only if an idle data pattern is sent (usually all 1’s for data modems). And this normally occurs with secure phones passing voice frames as the modem modulation bit rate is typically higher than the data rate required to send SCIP data frames with encoded/encrypted voice. It is conceivable that if application data external to the vIPer phone is being transported (possibly encoding fax) does not have such breaks in data transmission, synchronization between the endpoints may be lost for some duration.
VOCAL’s Embedded TDM SPRAG is very robust and nearly immune to these issues. A single platform solution, with internal communications between the core of SPRAG’s V.150.1 engine and the T1/E1 hardware, eliminates many potential hazards. RTP G.711 communications are used between internal software modules totally contained in this platform. RTP packet loss is engineered to never occur as no external networks are used between processing units/boxes segregated by function. PCM sample slips are mitigated by the proper use of slaving T1/E1 timing to the incoming leg. It is expected that all T1/E1 will operate with the same timing but not required as each link is slaved separately. RTP packet flow for each TDM channel drives the associated SPRAG V.150.1 processing. This is unlike many VoIP gateway devices which are natively designed around a common 8kHz TDM bus or similar software structure. In an architecture designed for 8kHz sampling, all channels would operate synchronously potentially missing or replicating some sample data thereby affecting the quality of the modulation and demodulation.
In VOCAL’s platform, a switching layer independent of SPRAG is used to monitor and route calls to/from each T1/E1 link. Link failure is detected and mitigated by this switching layer (DAHDI). Fault indications are available for reporting by SNMP. With additional or redundant links to the telephone network, failed or out-of-service links are bypassed in favor of operating links. Note: calls active as a link fails would likely be abnormally terminated and need to be restarted. With significant SCIP data loss, this may be required under any similar circumstance.
Supported Devices
- Sectéra® vIPer™ Universal Secure Phone
- Sectéra® Wireline Terminal
- IP STE Phone (or STE with VOCAL AMA)
- MUOS SCIP V.150.1 Terminal
- Other V.150.1 capable secure phones
- Non-V.150 capable secure devices can be supported using VOCAL’s AMA device
- Other Non-V.150 modem devices using VOCAL’s AMA device
- Misc. Voice Band Data Modem devices using VOCAL’s AMA device
Sectéra®, TalkSECURE™, and vIPer™ are trademarks of General Dynamics.
Related Info
- vIPer Secure Cloud Phone Service – V150 as a service for vIPer phones
- Analog Modem Adapters – Modem to VoIP adapter or last mile modems
- Modem over IP problems and solutions – Details on why Modem over IP is so difficult and how to address it
- Modem over IP – More modem over IP solutions