The M-Switch ACP127 add-on enables message conversion to/from ACP127, SMTP and STANAG 4406 and provides full support for the ACP127 protocol and for the related ACP126, ACP128, JANAP128 and DOI103S protocols.


Two add-ons, providing ACP127 Core capabilities and ACP127 Broadcast, as set out in BRASS (Broadcast Ship to Shore), are available for the following products:

Deployment Modes

When added to the appropriate M-Switch product, the ACP127 add-on can be configured to address various deployment modes, which can be used in isolation or in combination.

ACP127 End User System

Added to M-Switch SMTP, the add-on can be used to support end users using Isode's Harrier (M-Switch), an SMTP/IMAP military messaging client which includes an ACP127 mode. Harrier provides all of the capabilities needed in ACP127 including user addresses in ACP127 format (Routing Indicator (RI) and Plain Language Address (PLA)), as well as a choice of IA5 and ATA2 character sets.

ACP127 Relay

Added to an M-Switch gateway, the add-on can be configured as an ACP127 relay. Routing between ACP127 peers is configured use Routing Indicator (RI) prefix. This allows for completely flexible routing, with the prefix approach allowing for succinct specification of routing in common relay setups. Protocol conversion (e.g., between ACP127 and ACP128) can be performed.


M-Switch may be used in support of NATO BRASS (Broadcast and Ship to Shore) and BRE1TA (BRASS Enhancement 1 Technical Architecture) deployments. This capability includes RECAP Messages, OTAM (Off The Air Monitoring) and other elements important for these deployments. A detailed description is given in the whitepaper [Isode's Solution for BRASS (Broadcast and Ship to Shore)].

Gateway to SMTP/STANAG 4406

Added to M-Switch SMTP (Gateway) the add-on enables message exchange between ACP127 and M-Switch SMTP, which supports extensions for military messaging as described in the whitepaper [Military Messaging (MMHS) over SMTP]. M-Switch X.400 (Gateway) with the M-Switch ACP127 add-on can be deployed as a gateway between ACP127 and STANAG 4406 (the NATO standard for organizational messaging), following the specifications set out in STANAG 4406 Annex D.


Message Transport

Support is provided for the three primary approaches used for connecting ACP127 systems; Serial Line, HF Radio and TCP.

ACP127 was initially designed for operation over serial lines to communicate with teletypes and operation directly over serial ports is still important for many deployments, including direct operation over HF Radio modems without use of STANAG 5066 ARQ. The following serial interfaces are supported:

  • Windows COM Port
  • Digiport TS Server Asynchronous serial hub

ACP127 is widely deployed over HF Radio using STANAG 5066, particularly in Naval environments. This is done using the Character-Oriented (COSS) protocol defined in Appendix F of STANAG 5066. COSS support is included in the M-Switch ACP127 channel which can be selected by configuring a circuit to use a STANAG 5066 server such as Isode's Icon 5066.

Address Mapping

The preferred approach for address mapping between ACP127 and SMTP and/or STANAG 4406 is to have a directory entry for each user supported at the gateway. The directory entry will include:

  • ACP127 Plain Language Address (PLA).
  • ACP127 Routing Indicator (RI).
  • SMTP address and/or X.400 O/R Address.

Directory search can then be used to identify the entry and map between addresses at the gateway. A Directory entry is essential for any SMTP or X.400 user communicating with ACP127, as there is no mechanism to encode SMTP and X.400 addressing information in ACP127.

For ACP127 users communicating across the gateway, use of a mapping entry is recommended. This enables ACP127 users to have "clean" addresses for use in SMTP or STANAG 4406 systems.

Collation of Large Messages

ACP127 messages are small and do not have attachments. A typical M-Switch ACP127 configuration will use authorization to prevent messages with attachments or very large messages being sent over ACP127 (and an appropriate error message sent to the sender).

Some variants of ACP127 (e.g., DOI-103S) have a mechanism for splitting large messages into small messages, which M-Switch supports. M-Switch will also re-assemble such messages so that message splitting is transparent to SMTP and STANAG 4406 users.

In most situations, the will provide a reliable service for sending large messages, and use of service messages to request retransmission (based on channel sequence number) will ensure no message loss. In the event of a message being received with missing fragments and the missing fragments not arriving in response to a service message request, the message will be sent with clearly marked "gaps" at a timeout interval configurable according to message priority. For very high precedence, only minimal time can be waited for collation, so they will typically be sent as fragments without collation.

Security Labels and Access Control

ACP127 provides two security label mechanisms. A few basic labels are in the core ACP127 protocol and extended labels may be used. This enables security label based access control for inbound ACP127 messages and flexible security label mappings, for example to facilitate integration with ACP145 gateways.

ACP127 Service Messages & Reliability

ACP127 sends messages over a TCP connection or serial line. There is no protocol acknowledgement and there are no end to end delivery reports as in X.400 and SMTP.

ACP127 achieves reliability by sequentially numbering each message sent to a peer, which enables the receiver to detect if any messages are missing. ACP127 can send a special service message to request retransmission of any missing messages. M-Switch will detect missing ACP127 messages and request retransmission. The ACP127 sending channel holds all recently sent messages in a local database in order to facilitate this automatic transmission.

M-Switch also provides a configuration option to use ZIC/ZID service messages at intervals to automatically verify that a link is correctly working in both directions. ACP127 also defines special reliability options for FLASH messages which has special acknowledgement protocol. This can be selected for circuits where this is needed.


Full GUI management for ACP127 is provided in the MConsole management GUI, including configuration of ACP127 peers and gateway mappings. General information on MConsole can be found on separate pages covering M-Switch configuration management and M-Switch operational management.

Specific capabilities relevant to ACP127 deployments of M-Switch, which include Error Handling (Operator Correction & ACP127 Repair) and Circuit Monitoring & Control, can be found on the M-Switch ACP127 Management page.

Open Standards

The M-Switch ACP127 Messaging add-on complies with the following Open Standards

STANAG 4406 Edition 2: Annex D Military Message Handling System, Annex D: MMHS APS/ACP127 Gateway, March 2005
ACP127 Communication Instructions – Tape Relay Procedures, November 1998
ACP126 Communications Instructions Teletypewriter (Teleprinter) Procedures, May 1989
ACP128 Allied Telecommunications Record System (ALTERS) Operating Procedures - ACP128(A), May 2005
DOI 103S Defense Operating Instructions DSSCS Messages
JANAP 128 AUTODIN Operating Procedures JANAP 128(I), March 1983
STANAG 5066 Edition 2 Profile for High Frequency (HF) Radio Data Communications: Annex F.3 "Character-Oriented Serial Stream (COSS) Client", December 2008