computer science

LAPDm

LAPDm is a computer 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 LAPDm rather than just read about it. In short: LAPDm in telecommunications is a data link layer protocol used in GSM cellular networks. LAPDm forms Layer 2 of the Um interface between the Base Transceiver Station and Mobile station, which is to say that it is used in the radio link between the cellular network and the subscriber handset.

Key takeaways

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

Reference excerpt

LAPDm in telecommunications is a data link layer protocol used in GSM cellular networks. LAPDm forms Layer 2 of the Um interface between the Base Transceiver Station and Mobile station, which is to say that it is used in the radio link between the cellular network and the subscriber handset. LAPDm is derived from a much older link layer protocol called HDLC and specified in 3GPP specifications TS 04.05 and TS 04.06. LAPDm is similar to the ISDN Layer 2, LAPD, but with these simplifications:

LAPDm frames are always 184 bits, with segmentation for larger messages. LAPDm allows no more than one outstanding unacknowledged I-frame (GSM 04.06 Sections 5.8.4 and 6). LAPDm does not support extended header formats (GSM 04.06 Section 3). LAPDm supports only the SABM, DISC, DM, UI and UA U-Frames (GSM 04.06 Sections 3.4, 3.8.1). LAPDm supports the RR and REJ S-Frames (GSM 04.06 3.4, 3.8.1), but not RNR (GSM 04.06 Sections 3.8.7 and 6). LAPDm has just one internal timer, T200 (GSM 04.06 5.8). LAPDm supports only one terminal endpoint, whose TEI is implied. The BTS is always able to enter asynchronous balanced mode when requested. LAPDm can never be in a receiver-not-ready condition (GSM 04.06 Section 3.8.7). LAPDm supports only two SAPs: SAP3 for SMS and SAP0 for everything else (GSM 04.06 Sections 3.3.3 and 6). In SAP0, asynchronous balanced mode is always initiated by the MS (GSM 04.06 Sections 5.4.1.1 and 6). Another important difference between LAPDm and LAPD is the establishment contention resolution procedure of GSM 04.06 Section 5.4.1.4, wherein the MS sends an L3 message in the information field of the SABM frame which is then echoed back by the BTS in the corresponding UA frame. This procedure is required in LAPDm because of the possibility of a handset accidentally attempting to use the wrong channel. LAPD does not require contention resolution, since hard-wired ISDN devices cannot accidentally use a wrong channel. The Associated Control Procedure used in Layer 2 of the iDEN RF interface is very similar to LAPDm.

References

External links http://www.3gpp.org/ftp/Specs/archive/04_series/04.06/0406-840.zip http://www.3gpp.org/ftp/Specs/archive/04_series/04.05/0405-802.zip

Worked examples

Example 1 — a first encounter with LAPDm

Start with the simplest possible case. Write down what LAPDm claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer 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 LAPDm 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 LAPDm 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 LAPDm

In research: LAPDm appears in computer 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 LAPDm 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: LAPDm is common in secondary-school and first-year university syllabi. It links to neighbouring topics Integrated Services Digital Network, Link access protocols, so understanding it makes those chapters shorter.
In everyday life: Look for LAPDm 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 LAPDm in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what LAPDm 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 LAPDm out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is LAPDm in simple terms?

Why does LAPDm matter?

Because it connects several computer 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 LAPDm?

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 LAPDm.