Figure 11.8: MHS Functional Model.
A functional model of the MHS is shown in figure 11.8. The MHS is a collection of MTAs (Message Transfer Agents), MSs (Message Stores), UAs (User Agents), and AUs (Access Units). MTAs perform the store-and-forward message transfer function. MSs provide storage for messages. UAs enable users to access the MHS, and AUs provide links to other communication systems (e.g., the postal system). A more detailed description of each of these entities follows.
MTAs comprise the MTS (Message Transfer System), the principal component of the MHS. A message is submitted to an MTA by an originating UA, MS or AU, transferred to the recipient MTA(s), and delivered to one or more recipient UAs, MSs, or AUs. If the message is addressed to multiple recipients, the appropriate MTAs perform any splitting (i.e., replicating) of the message needed for delivery to each recipient.
Messages are transferred between MTAs on a cooperating store-and-forward basis. Since no end-to-end association is required, the MTA serving the message recipient need not be active when the message leaves the originating MTA. The message may be stored at a relay (i.e., intermediate) MTA until the recipient MTA becomes operational.
MTAs transfer messages whose content may be encoded in any format. MTAs neither examine nor modify the content of messages except when performing conversion. Conversion increases the effectiveness of the MHS by allowing users to submit messages in one encoded format (e.g., telex), and have them delivered in another encoded format (e.g., IA5). A UA can register with the MTA the encoded information types that may be delivered, and request the MTA to perform any required conversions.
The UA is the MHS component that enables a user to access the MHS, for both the origination and reception of messages. When submitting messages, the UA supplies to an MTA, either directly or indirectly via an MS, the message content, the address(es) of the message recipient(s), and the MTS services being requested. The message content is the information that the originator wants transferred to the message recipient(s). The address(es) and service request data are used by the MTS to deliver the message. When receiving messages, the UA may accept delivery of messages directly from an MTA, or it may employ an MS to accept delivery of messages, and retrieve them from the MS at a later time.
The MS is an optional MHS component that acts as an intermediary between a UA and MTA. The MS often co-resides with the MTA serving it. The primary purpose of the MS is to provide a repository for the delivery of messages. The UA can retrieve messages from this repository. By using an MS to accept delivery of messages, a UA is not required to be constantly available. This is especially useful for UA applications implemented on personal computers, which are typically turned off at night. The MS may also submit and forward messages on behalf of the UA, and notify the UA at the time of message delivery.
The AU is the MHS component that provides a gateway between the MHS and another communications system. AUs may, for example, provide intercommunication with telex, teletex, and facsimile systems. Another AU, the PDAU (Physical Delivery Access Unit) enables MHS users to send messages to users residing on a physical delivery system, such as the Postal Service.