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[mon-uh-sak-uh-rahyd, -er-id]

Any of the simple sugars that serve as building blocks for carbohydrates. They are classified based on their backbone of carbon (C) atoms: Trioses have three carbon atoms, tetroses four, pentoses five, hexoses six, and heptoses seven. The carbon atoms are bonded to hydrogen atoms (singlehorzbondH), hydroxyl groups (singlehorzbondOH; see functional group), and carbonyl groups (singlehorzbondCdoublehorzbondO), whose combinations, order, and configurations allow a large number of stereoisomers (see isomer) to exist. Pentoses include xylose, found in woody materials; arabinose, found in gums from conifers; ribose, a component of RNA and several vitamins; and deoxyribose, a component of DNA. Important hexoses include glucose, galactose, and fructose. Monosaccharides combine with each other and other groups to form a variety of disaccharides, polysaccharides, and other carbohydrates.

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Repetitive back-and-forth movement through a central, or equilibrium, position in which the maximum displacement on one side is equal to the maximum displacement on the other. Each complete vibration takes the same time, the period; the reciprocal of the period is the frequency of vibration. The force that causes the motion is always directed toward the equilibrium position and is directly proportional to the distance from it. A pendulum displays simple harmonic motion; other examples include the electrons in a wire carrying alternating current and the vibrating particles of a medium carrying sound waves.

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SIMPLE, the Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions, is an instant messaging (IM) and presence protocol suite based on Session Initiation Protocol (SIP) managed by the IETF. Like XMPP, and in contrast to the vast majority of IM and presence protocols used by software deployed today, SIMPLE is an open standard.

SIMPLE applies SIP to the problems of:

  • registering for presence information and receiving notifications when such events occur, for example when a user logs-in or comes back from lunch;
  • sending short messages, analogous to SMS or two-way paging;
  • managing a session of real-time messages between two or more participants.

Implementations of the SIMPLE based protocols can be found in SIP Softphones and also in SIP Hardphones.

Technical description


The SIMPLE presence specifications can be broken up into:

The core protocol machinery. It provides the actual SIP extensions for subscriptions, notifications and publications. RFC 3265 defines the SUBSCRIBE and NOTIFY methods. SUBSCRIBE allows to subscribe to an event on a server, the server responds with NOTIFY, whenever the event come up. RFC 3856 defines, how to make use of SUBSCRIBE/NOTIFY for presence. Two models are defined, an end-to-end model, -each User Agent handles presence subscriptions itself- and a centralized model. The latter introduces the concept of a presence server. All subscriptions are handled by this server. The message PUBLISH (RFC 3903) allows User Agents to inform the presence server about their subscription states.

Presence documents. The presence information is coded in XML documents, that are carried in the bodies of the respective SIP messages. RFC 3863 and RFC 4479 describe this procedure, RFC 4480 (RPID), RFC 4481, RFC 4482 (CPID) and various drafts describe contents and formats of the presence documents.

Privacy, policy and provisioning. If the centralized model is used, the User Agents need a way to define who may subscribe to which amount of their presence information. RFC 4745 and RFC 5025 define a framework for authorization policies controlling access to application-specific data. The XCAP protocol (RFC 4825), carried by HTML, allows User Agents to communicate their presence rules to a XCAP server, who rules the information exposition of the presence server. RFC 3857 and RFC 3858 define a subscription event "watcher info". User Agents may subscribe to this event to be informed, who is subscribing their presence information.


SIP defines two modes of instant messaging:

The Page Mode makes use of the SIP method MESSAGE, as defined in RFC 3428. This mode establishes no sessions.

The Session Mode. The Message Session Relay Protocol (RFC 4975, RFC 4976) defines text-based protocol for exchanging arbitrarily sized content of any time between users. An MSRP session is set up by exchanging certain information, such as an MSRP URI, within SIP and SDP signaling.


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