ACP/SGN3 WP/1-10

12/11/03

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

Working Group N - NETWORKING

SUBGROUP N3 – GROUND-GROUND Applications

Bangkok, November 2003 (SGN3 first meeting, WGN second meeting)

Agenda Item 3 : ATS Message Handling Services

AMHS Address Scheme concerns

Presented by Brian Cardwell

Summary

This working paper reviews use of the CAAS and XF AMHS Addressing Schemes and raises concerns about the impact of scheme selection on global transition from AFTN to AMHS.

1Introduction

A recent ICAO State Letter requested States to declare an AMHS PRMD and either the CAAS, XF or another AMHS addressing scheme. The State Letter recommended, as Doc 9705 does, that States declare a CAAS scheme.

This working paper looks at the impact of widespread declaration of CAAS schemes and questions whether that will be beneficial in transition to AMHS.

2Background

2.1Summary of The addressing schemas

Example use of CAAS and XF schemas.

Parameter / CAAS / XF
Country Code - C / XX / XX
Administrative Domain - A / ICAO / ICAO
Private Domain - P / EG / EG
Organisation - O / EGBB / AFTN
Organisational Unit - OU / EGBC
Common Name - CN / EGBCABCD / EGBCABCD

The values shown in bold are variable and must be derived when an AMHS message is created, the values in plain text are fixed and the underlined values are known a priori when the address resolution is commenced.

2.2Address resolution using CAAS

With the CAAS scheme, to produce an AMHS OR-address from a known 8-letter AFTN address it is necessary to derive the PRMD and O fields. The following process is necessary:

  1. Using Nationality Letters in the 8-letter address (EG in the example), consult ICAO AMHS Management Domain Identifiers and Addressing Scheme Document "Table of PRMDs and Addressing Schemes" and derive the PRMD. In the example above, this is also EG. There will be approx. 188 entries in this table, one for each ICAO State. It should change infrequently.
  2. In the same Table, determine that CAAS is in use.
  3. Consult the second table in the ICAO AMHS Address Doc, "Detailed Addressing Information for AMHS Management Domains (MDS) implementing the Common AMHS Addressing Scheme (CAAS)". Using the LI of the 8-letter address (EGBC in the example above), determine the Geographic Identifier listed for the O parameter, which returns EGBB in the example above.
  4. This produces a CAAS OR-Address of XX/ICAO/EG/EGBB/EGBC/EGBCABCD.

2.3Address resolution using XF

With the XF scheme, to produce an AMHS OR-address from a known 8-letter AFTN address it is necessary to derive the PRMD name. The following process in necessary:

  1. Using Nationality Letters in the 8-letter address (EG in the example), consult ICAO AMHS Management Domain Identifiers and Addressing Scheme Document "Table of PRMDs and Addressing Schemes" and derive the PRMD. In the example above, this is also EG. There will be approx. 188 entries in this table, one for each ICAO State. It should change infrequently.
  2. In the same Table, determine that XF is in use. End process.
  3. This produces an XF OR-Address of XX/ICAO/EG/AFTN/EGBCABCD.

3Observations

The CAAS scheme is a good scheme and entirely appropriate for all AMHS deployments with multiple MTAs within a State or Organisation. It is also possible to use this scheme when a single MTA is deployed, but there is limited advantage over the XF scheme.

The XF scheme is also good and entirely appropriate for AMHS deployments where only a single MTA is in use. It is also possible to use this scheme when multiple MTAs are deployed, but CAAS provides advantages in this situation.

There will be a protracted period of time where AFTN addresses and AMHS OR-address will need to operate together and be translated from one to the other.

Conversion from an AHMS OR-address to an AFTN 8-letter address is simple. The final field of the AMHS OR-address provides the 8-letter AFTN address.

A conversion from an AFTN 8-letter address to an AMHS address can be complicated dependent upon the the AMHS scheme in use and the level of automated support available to the conversion process.

  • With the XF scheme, only the PRMD name needs to be derived and this requires knowledge of ~188 State PRMD declarations.
  • With the CAAS scheme, in addition to the PRMD name, it is necessary to derive the value of the O parameter. This is derived via the Location Indicator. There are currently >12,000 LIs in use.

4Concerns

4.1Use of gateway functions

Where the AMHS address resolution is in a State or Regional Gateway, the conversion will be automated using either look-up tables or a Directory Service. The look-up tables or Directory Service will require maintenance because State address schemes will be updated and new LIs will be declared. N.B. Doc 7910 is updated quarterly (?) at the moment.

Without DIR support the maintenance process is likely to be onerous and, if undertaken manually, may lead to the introduction of errors. DIR support is an Extended AMHS function, although it is recommended as part of the SPACE Initial Profile. It should be assumed that DIR support will not be widely available in initial implementations.

4.2Address Resolution by Users

As States transition to national use of AMHS UAs, those Users will also need to use the AMHS OR-address in order to address each message. A user will not be able to successfully send a message without knowing the full OR-address of the destination.

This knowledge is easy to derive when the destination address is defined using the XF scheme - only the PRMD name is required and there are only ~188 of those. Some PRMDs may even be the same as the Nationality Letters. However, when the CAAS scheme is in use at the destination, each message originator needs to go through the process explained in 2.2 above. This requires use of two tables, the first with ~188 entries and the second with a current worse case of ~12,000 entries. Even CAAS address derivation is fairly simple with DIR support or even electronic look-up tables available to each UA, but again we should assume that DIR suport will not be widely available initially, especially to UAs. As before maintenance of look-up tables or a Directory will be necessary, this time perhaps by each UA in an AMHS State.

4.3Changes to Declared addressing schemes

It is important that changes to a State's declared scheme are always possible and relatively painless. It is unlikely that all States have managed to define a long term AMHS addressing plan with all future MTAs identified and all LIs permanently allocated to those MTAs. As the AMHS must expect regular changes to the AMHS addressing details (e.g. new MTAs causing new O definitions or new users causing new LIs to be created and allocated to MTAs), change must be supported and be relatively simple. As such, a future change from an initial XF declaration to a CAAS declaration should require no more effort than adding a new O definition to an existing CAAS scheme. Thus there is no disadvantage in declaring an initial XF scheme where appropriate.

It would be fair to state that changes of this type will always be easier with automated DIR support. Changes from XF to "CAAS with multiple Os" are more likely to occur when many more States have DIR support deployed.

5Conclusion

To encourage early migration from AFTN to AMHS, the transition should be made as simple as possible for users and this requires simple AMHS address resolution at all relevant address resolution points. There are advantages to the whole AFS community if an XF scheme is declared where AMHS deployment is still in the early planning phases or where a single AMHS MTA has been or will be deployed.

6Recommendations to the meeting

If the State Letter process has produced a large number of returns with CAAS schemes with a single Organisational Name definition, consider a new State Letter cycle that make the address resolution benefits of an XF Scheme declaration more clear.

If the State Letter process has produced a large number of returns with a CAAS scheme with many Organisational Name definitions, consider a new State Letter cycle that asks for States to declare a Schema that reflects their initial deployment.

Consider changes to the AMHS Guidance Material to reflect the information included in this paper.

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NATS/sgN3w1-10.DOC19/11/03

WGN02-WP___ - SGN3 WP/1-___