science

H.248

H.248 is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand H.248 rather than just read about it. In short: The Gateway Control Protocol (Megaco, H.248) is an implementation of the media gateway control protocol architecture for providing telecommunication services across a converged internetwork consisting of the traditional public switched telephone network (PSTN) and modern packet networks, such as the Internet. H.248 is the designation of the recommendations developed by the ITU Telecommunication Standardization Secto…

Key takeaways

H.248 belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect H.248 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of H.248 from memory before moving on to harder problems.

Reference excerpt

The Gateway Control Protocol (Megaco, H.248) is an implementation of the media gateway control protocol architecture for providing telecommunication services across a converged internetwork consisting of the traditional public switched telephone network (PSTN) and modern packet networks, such as the Internet. H.248 is the designation of the recommendations developed by the ITU Telecommunication Standardization Sector (ITU-T) and Megaco is a contraction of media gateway control protocol used by the earliest specifications by the Internet Engineering Task Force (IETF). The standard published in March 2013 by ITU-T is entitled H.248.1: Gateway control protocol: Version 3. Megaco/H.248 follows the guidelines published in RFC 2805 in April 2000, entitled Media Gateway Control Protocol Architecture and Requirements. The protocol performs the same functions as the Media Gateway Control Protocol (MGCP), is however a formal standard while MGCP has only informational status. Using different syntax and symbolic representation, the two protocols are not directly interoperable. They are both complementary to H.323 and the Session Initiation Protocol (SIP) protocols. H.248 was the result of collaboration of the MEGACO working group of the Internet Engineering Task Force (IETF) and the International Telecommunication Union Telecommunication Study Group 16. The IETF originally published the standard as RFC 3015, which was superseded by RFC 3525. The term Megaco is the IETF designation. Megaco combines concepts from MGCP and the Media Device Control Protocol (MDCP). MGCP originated from a combination of the Simple Gateway Control Protocol (SGCP) with the Internet Protocol Device Control (IPDC). After the ITU took responsibility of the protocol maintenance, the IETF reclassified its publications as historic in RFC 5125. The ITU has published three versions of H.248, the most recent in September 2005. H.248 encompasses not only the base protocol specification in H.248.1, but many extensions defined throughout the H.248 sub-series.

Protocol overview

H.248/Megaco due to its master-slave nature does not describe the establishment of calls across domains or across media gateway controllers. H.248/Megaco is used for communication downward, to the media gateways and does not constitute a complete system. The architecture requires other protocols for communication between multiple MGCs. The device that handles the call control function is referred to as an intelligent media gateway controller and the device that handles the media is referred to as a relatively unintelligent media gateway. H.248 defines the protocol for media gateway controllers to control media gateways for the support of multimedia streams across IP networks and the public switched telephone network (PSTN). It is typically used for providing Voice over Internet Protocol (VoIP) services like voice and fax between IP networks and the PSTN), or entirely within IP networks. Because of the types of devices targeted for control by H.248/Megaco and the low level of its control structure, H.248 is generally viewed as complementary to H.323 and Session Initiation Protocol (SIP). While a media gateway controller (MGC) uses H.248/Megaco to manage media establishment and control with a number of media gateways (MGs), other VoIP protocols, such as SIP and H.323 are used for one communication between controllers. From a SIP perspective, the combination of MGC and MGs are treated together as a SIP Gateway.

The H.248/Megaco model describes a connection model that contains the logical entities, or objects, within the Media Gateways (MGs) that can be controlled by the Media Gateway Controller. The main entities are Contexts and Terminations.

Terminations These source or sink one or more media streams or control streams. Terminations may be physical or ephemeral

Contexts These are star connections created by associating multiple terminations. A logical entity on an MG that is an association between a collection of Terminations. A NULL context contains all non-associated terminations. A Context is a logical entity on an MG that is an association between a collection of Terminations. A ContextID identifies a Context. The normal, "active" context might have a physical termination (say, one DS0 in a DS3) and one ephemeral one (the RTP stream connecting the gateway to the network). Contexts are created and released by the MG under command of the MGC. A context is created by adding the first termination, and it is released by removing (subtracting) the last termination. A termination may have more than one stream, and therefore a context may be a multistream context. Audio, video, and data streams may exist in a context among several terminations. In IP Multimedia Subsystem (IMS), Media Gateway Control Function (MGCF) control Media Gateways (MGW)s to send and receive call to / from the PSTN circuit switched (CS) networks using. H.248. The MGCF uses SIP messages to interact with Call Session Control Function (CSCF) and Breakout Gateway Control Function (BGCF). Although the modeling of the Media Gateway differs in H.248/Megaco when compared to MGCP, there is a similarity between the semantics of the commands in the two specifications. There is almost a one-to-one mapping between the commands of MEGACO and MGCP. For example, the Create connection command in MGCP has an equivalent ADD termination command in MEGACO, the Modify connection command in MGCP equates to the MODIFY termination command of MEGACO and the Delete connection command equates to the SUBTRACT termination command of MEGACO.

Messages and commands The Media Gateway Controller always manages the media channels available on the Media Gateway itself by managing specific contexts and terminations. When the call state changes, the MGC sends a corresponding message to the gateway and waits for an acknowledgment. Each message is a transport mechanism for transmitting commands, and not the command itself, unlike most other telecommunication protocols. Protocol Commands:

Add Move Subtract Notify Modify AuditValue AuditCapabilities ServiceChange

Typical MGC and MG message exchange Media Gateway Media Controller Gateway ISUP IAM (Start Call)

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with H.248

Start with the simplest possible case. Write down what H.248 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to H.248 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about H.248 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of H.248

In research: H.248 appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses H.248 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: H.248 is common in secondary-school and first-year university syllabi. It links to neighbouring topics ITU-T H Series Recommendations, ITU-T recommendations, Internet Standards, so understanding it makes those chapters shorter.
In everyday life: Look for H.248 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study H.248 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what H.248 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain H.248 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is H.248 in simple terms?

Why does H.248 matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study H.248?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on H.248.