This page addresses M-Switch capabilities for constrained and low-bandwidth networks. Other pages give a more general overview of M-Switch SMTP and M-Switch X.400.

M-Switch provides capabilities for messaging over constrained networks, including:

  • Multicast and Point to Point operation.
  • EMCON (Emission Control) – Radio Silence.
  • Operation over Satcom and other IP Networks.
  • Optimization for HF Radio and other slow radio links using STANAG 5066.
  • SMTP based messaging.
  • STANAG 4406 (Military Formal Messaging).
  • Control based on six level military message priority.
  • Multi-protocol and multi-network operation.
  • File transfer by email.

Constrained Networks

Constrained networks can exhibit one or more of the following characteristics:

  • Low bandwidth: generally less than 28 kbits/second and down to 50 bits/second.
  • High latency (half a second or more).
  • Long turnaround time (seconds or tens of seconds).
  • High error rate.
  • Extended outages.
  • One way transmission.
  • Broadcast or multicast.

M-Switch provides optimized protocol support to handle constrained networks. Further information on constrained networks, Isode's general approach and support for applications other than messaging are described on our solutions page on Applications for Military Radio & Satellite.

Deployment Scenarios

Constrained networks are of particular importance for military scenarios, including naval, air, and land deployments. They are also important for civil applications, such as coast guard and emergency services. Two Isode whitepapers contain information on a number of deployment scenarios:

Deployment scenarios need a number of supporting capabilities provided by M-Switch, including:

  • Point to Point networks.
  • Multicast/Broadcast networks.
  • Use of alternate networks,
  • Single hop and multi-hop message routing.
  • EMCON (Emission Control) or Radio Silence.

Server to Server Approach

Isode's approach to constrained bandwidth networks is to use server/server communication over the constrained link, rather than client/server. This means that a server will always be used, even to support a single user. The advantage of this approach is that the client (and user) is isolated from performance problems of the underlying network, and that slow network performance does not slow down the user interface. This approach also means that special low-bandwidth clients are not needed; any standard familiar client can be used.

A consequence of this is that Isode's optimized messaging protocols in M-Switch, and other messaging server products (in particular M-Box and M-Store X.400), will always communicate through M-Switch and so do not need optimized protocol support. Examples of how the various servers and clients are deployed are shown in the whitepaper [Packaging Military Messaging for HF Radio and other Low Bandwidth Links].

ACP 142

The central protocol to Isode's constrained communication solution is ACP 142, sometimes known as 'P_Mul'. ACP 142 provides reliable multicast, which is an essential base service. ACP 142 is implemented as an M-Switch channel. M-Switch uses channels for each of the various protocols that it supports, such as X.400 P1 and SMTP. The ACP 142 channel runs as a server, to process incoming requests. A more detailed description of the architecture is given in whitepaper [The Architecture of Isode's STANAG 4406 Annex E Solution].

ACP 142 can carry both STANAG 4406 messages and SMTP messages. The core function is protocol independent message transport. Details of M-Switch support for these protocols are described below. ACP 142 Parameters for Radio and Satellite Networks are described on a separate page.

STANAG 4406 (Military Formal Messaging)

STANAG 4406 specifies military formal messaging, and is based on the X.400 messaging protocols. STANAG 4406 Annex E defines support over constrained networks using ACP 142, giving simple protocol mapping and extensible compression mechanism. Default compression uses ZLIB, which in turn uses DEFLATE. This combination is illustrated below.

M-Switch conforms to the TMN-1 profiles of STANAG 4406 Annex E, and can be configured to operate as an LMTA (Lightweight MTA) or TIA (Tactical Interface Agent).

SMTP (Informal Messaging)

SMTP messaging is also supported by M-Switch over ACP 142, using the protocol stack shown above. Compression is achieved using the STANAG 4406 extensible encoding.

The SMTP message is encoded with a BSMTP (Batch SMTP) format. The format used is based on that defined in RFC 2442, optimized for use with ACP 142. A more detailed description of this protocol stack is given in the whitepaper [Messaging Protocols for HF Radio].

Network Mappings & Services

ACP 142 provides a reliable multicast service used for message transfer. Capabilities provided by ACP 142 include:

  • Efficiently sending a block of data (message) to multiple recipients over a broadcast network.
  • Ability to use underlying multicast and broadcast network capabilities, and enable setup of static and dynamic multicast groups.
  • Deal with extended network outages.
  • Handle EMCON recipients. EMCON (Emission Control) is where a recipient can receive messages but not send. It is also referred to as radio silence. EMCON support is important for many military deployments.

As well as supporting the base ACP 142 standard, M-Switch provides two extensions.

  1. Connection Oriented ACP 142 (CO ACP 142), shown in the diagram above. Although a multicast protocol can be used for a single recipient, it is often not optimal, particularly where the underlying network can offer improved performance for point to point connectivity. This is particularly important for radio networks, where ARQ operation can be offered for point to point, but not for multicast. CO ACP 142 can be configured for pairs of nodes that support it, and will offer improved performance for unicast communication. The ACP 142 channel can mix ACP 142 and CO ACP 142 communication.
  2. EMCON extensions to support broadcast only networks (e.g. NATO BRASS (Broadcast as Ship to Shore)) and extended outages. The two mode model of ACP 142 EMCON is extended to four modes, and the option is added to treat a message as delivered after a configurable number of transmissions. Further information is provided here in the Isode blog post on EMCON extensions to ACP 142 to support BRASS and Data Diode.

M-Switch provides two network mapping for its ACP 142 protocols. The first is to use IP, which is widely supported and ideal for some underlying networks and in particular for Satcom. ACP 142 uses UDP to map onto IP, and CO ACP 142 uses TCP. ACP 142 utilizes IP multicast addressing.

The second mapping is to use the STANAG 5066 protocol, as shown above. The M-Switch ACP 142 channel access as STANAG 5066 server using the STANG 5066 SIS (Subnet Interface Service) protocol. Note that Isode does not supply STANAG 5066 servers. These will typically be obtained from a modem supplier, such as Isode's partner RapidM.

STANAG 5066 is a standard interface for HF Radio, and is often supported for higher frequencies (in particular VHF and UHF). Where available, STANAG 5066 will give significantly better performance than use of IP and it is always recommended.

Multi-Protocol and Multi-Network Operation

A key feature of M-Switch is its ability to support and map between multiple protocols using the channel architecture, with different channels for different protocols. M-Switch can be configured to use multiple networks, and associate a channel with each network. For example you could have two (IP addressable) Satcom nets, and configure an ACP 142 channel with each one. Similarly you could have two radio nets, each with its STANAG 5066 modem, and an ACP 142 channel configured to access each modem.

M-Switch can then route messages between the various networks. Where a destination can be reached over multiple networks, route weighting can be used to select the preferred route. This can use M-Switch’s flexible authorization to take into account factors such as message size and priority in the choice. When requirements for network choice change, the route weighting can be modified to reflect this. This will change routing for new messages. Queued messages can be 're-processed' if desired to change to the new routing.

File Transfer by Email

Messaging provides reliable multicast of bulk data, and this is a useful building block for other applications. M-Switch makes this available as a general service, using a simple file interface to a special channel. This is described in the whitepaper [File Transfer by Email]. Isode makes use of this capability for supporting directory replication, which is described in the whitepaper [Directory Replication by Email and over 'Air Gap'].


Configuration of the ACP 142 channel and use of constrained networks is handled by Isode’s general configuration in MConsole, which also provides advanced monitoring. This monitors the M-Switch queues, giving additional information on ACP 142 traffic, including partially received and transmitted messages.

MConsole provides a “Message Transfer View” that is optimized for ACP 142 operation. It shows inbound and outbound ACP 142 messages, with progress bars for messages in transit, and estimated times for message and full queue delivery. Operators have control to delete queued messages, which may be desirable when queues build up.

Management of M-Switch for constrained networks is discussed further in the whitepaper [The Architecture of Isode's STANAG 4406 Annex E Solution].


  • STANAG 4406, Edition 2. "Military Message Handling System", March 2005: Annexe E: "Tactical MMHS Protocol and Profile Solution"
  • ACP 142, Version 1.0, "P_MUL - A Protocol for Reliable Multicast Messaging in Bandwidth Constrained and Delayed Acknowledgement (EMCON) Environments". December 2001.
  • Proposed Changes to ACP 142 version 1.0 (December 2001). NATO MMHS WG 642 (September 2004).
  • STANAG 5506 Edition 1 Amendment 1- "Profile for High Frequency (HF) Radio Data Communications", October 2005.
  • STANAG 5506 Edition 2. "Profile for High Frequency (HF) Radio Data Communications", December 2008.


Performance over constrained networks is critical and complex. M-Switch has been developed to optimize performance. Information is provided in a number of whitepapers listed here.

  • Performance Measurements of Messaging Protocols over HF Radio: This whitepaper sets out and analyses the results of a measurements of various messaging protocols over HF Radio.
  • STANAG 5066 Performance Measurements over HF Radio: This white paper sets out the results of measurements done by Isode of STANAG 5066 over military HF Modems and emulated HF Radio.
  • Measuring MMHS Performance over HF Radio and Satellite: STANAG 4406 Annex E Encoding and Compression: This whitepaper is the first of a set of papers reporting on measurements made of MMHS (Military Message Handling Systems) operating over HF Radio and Satellite. This paper looks at the encoding and compression of STANAG 4406 Annex E messages, which is common to both HF Radio and Satellite transmission. 
  • Measuring MMHS Performance over Satellite: This whitepaper is the second of a set of papers reporting on measurements made of MMHS operating over HF Radio and Satellite. This paper looks at operation over Satellite networks, and compares the performance of STANAG 4406 Annex E which is designed for constrained bandwidth networks with STANAG 4406 Annex A, which is intended for high speed networks.
  • Performance Measurements of Applications using IP over HF Radio: This paper sets out the results of measurements made when running applications and layer protocols to support applications over IP via HF Radio using STANAG 5066. The goal of this work was to get a quantitative measure of the performance impact of using applications running over IP over HF Radio in comparison with applications running directly over specialized HF Radio protocols.

Further Reading

Apart from the whitepapers mentioned in the preceding sections, there are a number of Isode whitepapers relevant to communications over constrained networks: