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- Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
23
The Message Handling
System (MHS)
The message handling system(MHS) is a concept developed by ITU-T thatis intended to lead to
the interconnectivity of all different types of message conveying systems, e.g. telephone, telex,
facsimile, electronic mail, etc. MHS sets out a simple model of basic interconnection between
systems. As it does so it defines a new dictionary of standard terms and jargon to describe the
various phases of a communication. Inplaces it mayeven seem trivial, but the careful definition is
worthwhile if it willsolve today’s incompatibilities. This chapter seeks to explainthemodel,
unravel the jargon and describe the initiatives that will result.
23.1 THENEED FOR MHS
It is commonplace today for a wide variety of telecommunications technologies to be
provided in the sameoffice. Often a number of technologies provide similar but separate
means for conveying identical information, but many users still employ more than one
of the methods. Telex, facsimile and electronic mail all provide means of transferring
text or documents. One or other of the different devices may be favoured to convey
information to a given destination, depending on the facilities available at the far end,
for despite the commonality of each of the different methodsof document transfer, they
are mutually incompatible and will not interwork. MHS defines a simple model of the
procedureneededfordocumentstobepassedelectronicallyina store-and-retrieve
fashion between a wide range of different types and makes of office business machines
(e.g. word processor, facsimile machine, teletype, maybe even the telephone as well). In
time, the modelwill lead on to thedevelopment of communication standards, interfaces
and equipment needed so that machines of dissimilar types may intercommunicate.
23.2 THE CONCEPT OF MHS
The underlying assumption on which MHS is founded is that during any period of
communication only the information content of a message is important. The corollary
413
- 414 THE MESSAGE HANDLING SYSTEM (MHS)
is thatthemeans of message conveyance is immaterial, andtheformat of the
information (e.g. facsimile, telex, electronic mail, etc.) may be changed between the
originating and destination stations. Thus the originator need not worry about whether
a compatible facsimile or telex machine is available at the destination, because MHS
will provide conversion to the required format.
ITU-T’s recommendation X.400 defines a model of message handling ( M H ) service,
a
provided by means of a message handling system ( M H S ) . The M H service allows the
conveyance of messages across a network on a store-and-forward or store-and-retrieve
basis. Thus information between sender and recipient can be sent without interrupting
the recipient from his current activities: themessage is dealt with when it is convenient.
This is important in computing activities if the receiving machine is already busy. It is
also important for human recipients who might be away from their desks. Much like a
postal system, messages may be posted into the system at any time of day and will be
delivered to the recipient’s mailbox. The recipient is free to sort through the incoming
mail at any time of day, and as soon as an item is accepted by the designated recipient,
automatic confirmation of receipt can be despatched back to the sender. This eliminates
any possible concern about whether the recipient has seen a given item.
23.3 THE
MHS
MODEL
The MHS model provides a designaid to thedevelopmentofnetworksproviding
message handling services. It based upon the principles of the International Organiza-
is
tion for Standardization (ISO) open systems interconnection ( O S I ) model (as described
in Chapter 9). Like OSI, MHS is a layered model with general-purpose application.
Unlike the OS1 model, only higher layer functions are described in the MHS model. In
fact, the MHS model is functionally equivalent to the OS1 layer 7, or application layer.
of
Any realization of a higher layer protocol the OS1 Model relies on implementation
also of the underlying layers. The message handling system is no exception. The MHS
defines the format of messages that may be sent between end points and the procedure
for doing so. It does not state any particularphysical or electrical means of conveyance.
Thus any informationwhich may be represented in binary code may be conveyed on a
MHS. Equally, any network capable of carrying the information and conforming to
OS1 layers 1-6 will do. The network must
0 establishthe transport service (OS1 layers 1-4). Suitable protocols in conjunction
with any of the telephone, packet-switcheddata, LAN or leased line networkswill do
0 establish a session for reliable transfer of messages (OS1 layer 5)
but MHS uses a null presentation (data syntax) layer (OS1 layer 6)
Figure 23.1 shows how MHS operates to provide a service to users within a message
handling environment. All the functions within this environment need to be standard-
ized. For this purpose the model defines a number of terms, as follows.
The two basic functions of MHS are the user agent (UA) and the message transfer
agent ( M T A ) . These are not usually distinct pieces of equipment in their own right,
butanumberof thesefunctionsmay be containedwithina single physical piece
- THE MHS MODEL 415
MH environment
I I
I I
, Messagetransfer
system
UA UA
i
l
Figure 23.1 Functional model of MHS. MH, message handling; UA, user agent; MTA, message
transfer agent. (Courtesy ITU - derived from Figure l / X . 4 0 0 )
of equipment. The user agent ( U A ) function is undertaken by a personal computer
(or similar piece of office equipment) which converts theuser’s message into a standard
form, suitable for transmission. Having done the conversion, the user agent submits the
message to the local message transfer agent ( M T A ) . The message is conveyed via a
number of MTAs. Collectively called the message transfer system to the recipient’s
MTA. This is called the message transfer function. Each MTA is usually a mainframe
computer or other device capable of storing and forwarding information. Interconnec-
tion of the UA to MTA and between the MTAs may use any type of network which
provides the functions of OS1 layers 1-6.
The two services provided by message handling systems are called the inter-personal
message service ( I P M S ) and the message transfer service ( M T S ) . These correspond to
the services provided by the user agent and the message transfer agent, respectively, as
Figure 23.2 demonstrates.
Figure 23.2 shows different
three implementations of useragent ( U A ) . This
underlines the point that the user agent is only a conceptual function, which can be
implemented in a number of ways provided that it interacts with the MTA function in
the standard fashion. The threedifferentimplementations of useragentsshownin
Figure 23.2 are
0 auseragentwithinacomputer or wordprocessor;the IPMS provided by this
machine is utilized by a dumb (asynchronous) computer terminal
0 an intelligent terminal, within which the user agent function is contained
0 an interworking device, interfacing an existing telematic (text based) network (telex
or facsimile, etc.) to one of the MTAs
The end-users of a MHS may be either personsor computer applications. Users maybe
either originators or recipients of messages. Messages are carried between originator
- 416 THE MESSAGE(MHS)
HANDLING SYSTEM
IPMS
MTS
Dumb User User
computer ‘intelligent’
terminal
(Computer or
word processor 1 terminal
other
telematic
service ,
Figure 23.2 Message transfer (MTS) and interpersonal message service (IPMS)
and recipient in a sealed electronic envelope. Just as with the postal equivalent, the
envelope provides a means of indicating the name of the recipient, of making sure it
and
reaches the right address with the contents intact. In addition further information may
be added to the envelope for special delivery needs. For example, a confidential mark-
ing could be added to ensure thatonly the named addressee may read the information.
The basic structure of all messages in X.400 is thus an envelope, with a content of
information. The content is the information sent by an originating user agent ( U A ) to
one or more recipientuseragents (UAs). This simple message structure is shown in
Figure 2 3 . 3 .
MHS defines three different types of envelope (although all three are very similar in
structure).Thesearecalledthe submissionenvelope, the relayingenvelope andthe
(Content ) ( Envelope 1
Dear Joc
BLa BIa
BlaBla .... .
...... +
Yours,
Figure 23.3 Basic message structure in MHS. (Courtesy of ITU ~ derived from Figure 2/X.400)
- SENTATION LAYERED OF MHS 417
delivery envelope. The submission envelope contains the information prepared by the
UA, and is labelled with the informationthat the MTSwill need to convey the message,
including the address and any markings such as conJirmed delivery and priority. This
envelope is submitted by the user agent (UA) to the message transfer agent (MTA).
On reaching the MTA, the content of the message is transferred to a relaying envelope
for relay through the message transfer system to the destination MTA. Finally the
message is delivered to the recipient's user agent in a delivery envelope.
In reality, the envelopes are simply some extra data, sent in a standard format, as a
sort of package around the users real message. All three types of envelope are very
similar in structure, though slightly different information is pertinent in each case.
23.4 LAYERED REPRESENTATION OF MHS
As with the OS1 reference model, it is easy to conceive MHS as consisting of functional
layers for which
0 similar functions are grouped in the same layers
0 interactions between layers are minimal
0 interactions within the layers are supported by one or more distinct peer protocols
The principles of layered protocols were covered in Chapter 9, where the OS1 model
and peer protocols are described. In particular, MHS comprises two functional layers,
as shown in Figure23.4, and as MHSitself resides at layer 7 of the OS1 model so these
two layers are sub-functional layers of OS1 layer 7.
A number of new terminologies are introduced by Figure 23.4 and need explanation.
The two layers of MHS are the user agent layer ( U A L ) and the message transfer layer
( M T L ) . These arethelayersinwhichthe user agents and messagetransferagents
operate.
The user agentlayer
comprises user agententities (UAEs) and interacting
an
protocol, called P,. The protocol P, governs the content, format, syntax and semantics
UA M TA M TA UA
Pc (e.g. P 2 1
User agent
layer UAE I + UAE
( UAL 1
Messagetransferlayer
(MTL)
SDE 5; p3 L MTAE h MTAE * "* SDE
Dellvery
submission Relay
Figure 23.4 Layered representation of MHS. (Courtesy of ITU - derived from Figures 12 and
13/X.400)
- 418 THE MESSAGE HANDLING SYSTEM
(MHS)
to be used when transferring information between the two user agents. ( P , stands for
content protocol). A particular type of content protocol is called P2, and is also known
asthe interpersonal messagingprotocol. It is covered by ITU-T’srecommendation
X.420. The user agent entity is the capability, within the user agentwhich is required to
convert the base information into the form demanded by P, (or P2).
The message transfer
layer ( M T L ) comprises submission delivery
and entities
(SDEs), message transfer agent entities ( M T A E s ) and interacting protocols called mes-
sage transfer protocol (or P I ) , and the submission and delivery protocol (or P3).
The submission and delivery entity ( S D E ) is the part of the user agent thatplaces the
information (or content) provided by the user agent entity ( U A E ) into the submission
envelope. The envelope is then addressed using the protocol P3. The message transfer
agent entity ( M T A E ) is the functional part of the message transfer agent ( M T A ) which
relays the message in the relaying envelope. The message transfer protocol, P I , is used
to perform the message relay. Finally the message is delivered to the destination SDE
in the deliveryenvelope, usingprotocol P 3 . Thefullmessagingsequence is thus as
follows.
1. The user agent entity ( U A E ) , which is a functional part of the useragent ( U A ) ,
assists the user to compose the content of the message in P2 (or some other P,)
format.
2. The submission and delivery entity ( S D E ) , also a functional part of the user agent,
envelopes and addresses the message, and then submits it to the message transfer
agent entity ( M T A E ) using P3 protocol.
3. The message transfer
agent
entity ( M T A E ) , which is afunctionalpart of the
messagetransferagent, re-envelopes the message in a P I relayingenvelope, and
relaysitinastore and forward fashion via the MTAEs of other MTAs, until
reaching the destination MTA. For the relay, protocol PI is used.
4. The destination MTAE returns the message to its P3 envelope and delivers it to the
SDE of the destination UA, which removes the envelope.
5. The UAE part of the destination UA reformats the information to the original style
and conveys it to the user.
As an aside, it is interesting to note the difference between the and 1988 versions of
1984
the P3 protocol, even though thisis now largely history, because exemplifies how initial
it
versions of standards may not be completely reliable, even though published. Whereas
CCITT’s 1984 version contained a version P3 protocol which was a
of store-and-forward
protocol, this had the weakness that it had no way of preventing an MTA from attemp-
ting to forward a message to a UA, such as a personal computer, which might havebeen
switched off at the time. In that case the message would be lost. By 1988, the CCITT
recommendation for the P3 protocol had been advanced to P3+ as a store-and-retrieve
protocol. In the later version, the message is retained by the destination MTA until the
recipient user agent chooses toretrieve it.
- THE STRUCTURE OF MHS MESSAGES AND MHS ADDRESSES 419
23.5 THE STRUCTURE OF MHSMESSAGESAND
MHS ADDRESSES
Let us now consider the structure of MHS messages and the structure of messages
which are used to support the inter-personal message service (IPMS). Figure 23.5 shows
the imaginary nested structure of a message, sent in reality as a long stream of binary
coded data.
This structure is apparent also in Figure 23.5, although some other parts of the
message are also shown. The content may be either user information, such as text,
facsimile diagrams, etc.,formatted in the P2-styleas heading and body, or it can take the
form of a status report. A status report is a special type of message required for the
correct functioning of UAEs. A status report may be used, for instance, to convey
receipt
confirmation messages. The heading of the message characterizes user
the
information with elements such as ‘subject’, ‘to’, and ‘copy’. The body is the text and
diagrams. The content of the message must be formatted using a content protocol, P,.
One type of contentprotocolalreadydiscussed is theP2 interpersonalmessaging
protocol. If required, however, a different type of content protocol can be specially
developed for a given application.
The envelope of a message carries the address of the recipient. ITU-T recommenda-
tion X.400 suggests that the address should be one of the following:
a unique identifier of entirely numeric characters; this is called a primitive name, and
indicates the destination user agent in terms of its country, its management domain
(explained below), and a unique customer number
auniquename-styleidentifier,correctlycalleda descriptive name, comprisinga
personal name, an organization name and the name of an organizational unit
I Envelope
I
I
I
I L------r
---------l
Figure 23.5 Message structure for IPMS. (Courtesy of ITU - derived f r o m Figure 21X.420)
- 420 MESSAGE THE (MHS)
HANDLING SYSTEM
All addresses in MHS must be unique, and denote exactly one user. An example of a
primitive name might be a number such as 12345926, etc. By contrast an example of a
descriptive name is
‘The chairman of the XYZ company’
The scope for using descriptive names is helpful to human users, who no longer have to
remember arbitrary numbers for calling their friends or close associates. However, the
name must be sufficient for the message handling system to recognize the user uniquely.
A name such as
‘the floor cleaner of the XYZ company’
will not do as adescriptive name, unless the XYZ company only has one floor cleaner.
23.6 MHS MANAGEMENT
DOMAINS
A management domain might be a complete MHS network, or it may comprise a small
number of MTAs owned and run by a single organization. A management domain might
be a large company’s private MHS network, or a network run by the public telephone
company. The two types of domainarecalled,respectively,a privatemanagement
domain ( P R M D ) and an administrationmanagementdomain ( A D M D ) . Figure 23.6
illustrates the management domains concept, and shows the interconnection of a num-
ber of different domains. The use of management domains allows the addresses in each
domain to be allocated independently of addresses in other domains. Thus the same
address ‘The chairman’ be used to apply to two different people, being the individual
may
users of separate companies’ MHS networks.
Figure23.6 reflects the differentexamplesencounteredinreal life, wheresome
public telecommunications operators may provide only message transfer (i.e. network)
facilities while others provide customer premises equipment as well. Interconnection
between all the management domains is by standard PI (message transfer) protocol.
I------l r-----1
I
I [ UA
I ---_
I r - - - -1 r---1
I[
I
UA IMTA H 1 II
UA
C R M D I--_I
- I AOMDZ
- - - - U
I I
A---
PRMDZ
_1
Figure 23.6 Administration and private management domains
- MHS AND THE OS1 DIRECTORY
SERVICE 421
23.7 MHS ANDTHE OS1 DIRECTORY SERVICE
So that the MHS may deliver messages to the correct recipients, it needs to
0 validatetheaddressnames of originatorsand recipients (originator/recipient,or
OIR name)
0 determine the necessary call routing, and finally
0 determine whether data format, or code conversion is required
A large quantity of reference information, giving details and addressesof all the users,
is required so that, first, the sender can address the message correctly to the intended
recipient; and, second, so that the UA and MTA can determine where this personis and
so deal with the message accordingly. The information needed is drawn from the OSI
directory service which is prescribed by ITU-T recommendations in the X.500 series.
These recommendations lay out a standard structure in which information may be held
in a database, and they provide for a standard method of enquiry (see Chapter 29).
Besides determining the correct routing paths for MHS messages, the service can also
be used to answer users’ directoryenquiries. A widely held view is that largescale
corporate and international interconnection of electronicmail and other messaging
devices cannot take place using the X.400 standard until theX.500 directory service has
been fully defined and widely realized.
23.8 MESSAGE CONVERSION AND CONVEYANCE USING MHS
MHS is intended to recognize all of the standard binarycodes (e.g. telex, facsimile, etc.)
and provide for translation between them by interaction with the OS1 presentation
layer (layer 6 ) . In the case of a telex machine using the message conversion capabilities
of a message handling system to send a message to a facsimile machine, MHS would
convert from telex characters into a facsimile message format. In time, some people
hope that even speech to text conversion may be possible. So far, however, not all
translation functions are understood and further technical development be required
will
before we have a system which provides for all types of translation.
The standard codes understood by MHS include
0 text in IA5 computer code (international alphabet no. 5)
0 telex (international alphabet no 2, or IA2)
0 facsimile
0 digitalized voice
0 wordprocessorformats
There is no restriction on the number of different types of code which may be used in
a single message. Thus messages of mixed text, facsimile diagrams and even voice can
- 422 (MHS) SYSTEM HANDLING MESSAGE
THE
be sent, although there may of course be a limitation on what the receiving device
will comprehend.
Early application of MHS by companies and other users is to convey documents in
electronic form. A prime user of the standard X.400 protocols will be to interconnect
today’sincompatibleelectronicmailsystems.Inaddition MHShas providedthe
foundation for a new era of paperless trading between companies, called EDI (electronic
data interchange). In this environment it is invaluable for large scale distribution of
documents, for rapid confirmation of receipt, and for delivery of messages in electronic
form, allowing subsequent processing if necessary.
23.9 SETTING UP A MESSAGEHANDLINGSYSTEM
Figure 23.7 illustratesapracticalimplementationofamessagehandlingsystem,
showing the items of equipment that the user may be familiar with.
Figure 23.7 shows a network comprising a personal computer, a mainframe computer
and another mainframe computer. The message handling system is only a small func-
tion, but one which each of the computers must be capable of performing. The MHS
itself is a small amount of software in each computer, and so invisible to the user.
Thepersonalcomputeronthe left of Figure 23.7 has within itcommunication
software which performs a user agent function, asas the functions OS1 layers 1-6.
well of
The physical network connecting it to the message transfer agent (MTA), which resides
in the mainframe computer in the centre of the figure, is a packet-switched network. In
thejargon of MHSthepersonalcomputer is said to be an SI-system, becauseit
comprises only auser agent entity( U A E ) and asubmission and delivery entity ( S D E ) but
no message transfer agent entity.
In contrast, the mainframe computer shown in the centre of Figure 23.7 is the main
‘databank’ of theMHS system. It comprises only a message transfer agent entityM T A E )
(
together with communication software supporting the functions of OS1 layers 1-6.
The MTAE performs the function of relaying message between user agents (UAs), and
when necessary it stores messages for later retrieval recipient UAs. As this mainframe
by
computer contains a MTAE, it is called, in MHS jargon, an S2-system. The computer
S1 system S2 system S3 system
Mainframe
Personal
computer
-0-D Packet
network
Mainframe
computer computer and
Figure 23.7 Physical example of MHS realization
terminals
- MESSAGE UP A
SETTING HANDLING SYSTEM 423
may perform other normal data processing functions in addition to its MHS com-
munication functions, although many companies may choose to devote a mainframe
computerfor use as a messagehandlingsystem(S2-system).Finally,thecomputer
shown on the right-hand side of Figure 23.7, comprises both a user agent entity (UAE)
and a message transfer agent entity (MTAE). This is called an S3-system. Mainframe
computers dedicated to use as MHS S3-systems are also likely to become common.
To sum up, we have distinguished three types of systems as realizations of MHS
0 systems containing only user agent ( U A ) functions (i.e. UAE and SDE); these are
termed S 1-systems
0 systemscontainingonly message
transfer
agent ( M T A ) functions(i.e.MTAE),
termed S2-systems
0 systems containing both UA and MTA functions (i.e. UAE and MTAE), termed
S3-systems
The diagram in Figure 23.8 shows the functional structure of the equipment of Figure
23.7. It illustrates the stack of OS1 protocols and functions required within the actual
equipment, to realizeMHS in practice. The diagram shows how theprotocols at each of
the OS1 layers are communicating between S1, S2 and S3systems on a peer level.
A packet network based MHS is shown, using theITU-T X.25 packet protocolat layers
1-3. Meanwhile layer4 and5protocols being used are thosedefinedin ITU-T
recommendations X.224 and X.225. The presentation layer has a null protocol, and
then PI, P3 and P, are used at layer 7. Layers 1-3 might equally have been based on a
LAN (local area network), or a telephone network, and in these cases the X.25 packet
protocols would have been unsuitable.
S 1 system S 2 system S3 system
OS1 layer OS1 layer name
7
6
S
L
3
2
X 2 5 (layer 1) layer 1 I
~ 2(5
1 Physical
L Actual wires
l
Figure 23.8 Packet based realization of MHS
- 424 THE MESSAGE(MHS)
HANDLING SYSTEM
23.10 FILE TRANSFER ACCESSANDMANAGEMENT (FTAM)
We learned in Chapter 22 how it can be useful to be able to manipulate and access files
onremotecomputers,andbeabletotransferthemquicklyand efficiently across
computer networks. These functions are carried out in the Internet by protocols such as
NFS (network $le server) and FTP (file transfer protocol). The equivalent and more
powerful (though perhaps also more cumbersome) protocol in the open systems world is
called FTAM (file transfer,access and management or file transfer and access method).
23.11 SUMMARY
The Message Handling System X400will provide for users an extremely flexible way of
interconnecting different types of office equipment. It operates in a store-and-retrieve
fashion and will be an important tool in computer communication, particularly for
0 passing electronic mail and documents between proprietary computer systems
0 data file transfer between computers separatedby time zones and geography - using
the public packet network
The usefulness of the X.400 service will dependonthe availability of directory
information so that messages can be delivered to the correct recipient. This will govern
the rate at which today’s different electronic mail networks may be linked together. In
essence this means that many potential users must wait for full development of the new
X.500 directory service.
Ultimately, X.400 will provide a most powerful means of interconnection, allowing
paperless interaction not only between different equipment but also between different
companies. It heralds the first real opportunity for paperless trade based on electronic
data interchange ( E D I ) , and it may eventually lead to a genuinely ‘paperless office’.
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