GTP'

Key takeaways

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

Reference excerpt

GTP' (GTP prime) is an IP based protocol used within GSM and UMTS networks. It can be used with UDP or TCP. GTP' uses the same message structure as GTP (GTP-C, GTP-U), but it is largely a separate protocol. GTP' uses registered UDP/TCP port 3386. GTP' can be used for carrying charging data from the "Charging Data Function" (CDF) of the GSM or UMTS network to the "Charging Gateway Function" (CGF). In most cases, this should mean from many individual network elements such as the GGSNs to a centralised computer which then delivers the charging data more conveniently to the network operator's billing center. GTP' is used on the Ga interface within the 3GPP GPRS Core Network definition. GTP' reuses aspects of GTP, although to quote 3GPP TS 32.295, "only the signalling plane of GTP is partly reused". GTP' defines a different header, additional messages, field values, as well as a synchronisation protocol to avoid losing or duplicating CDRs on CGF or SGSN/GGSN failure. Transferred CDRs, if following 3GPP standards, are encoded in ASN.1.

Header GTP' v1 and v2 headers contain the following fields

Version The first header field in a GTP' packet is the 3-bit version field. For GTP' v2, this has a value of 2 (hence the name GTP' v2). Protocol Type (PT) a 1-bit value that differentiates GTP' (value 0) from GTP (value 1). Reserved a 3-bit reserved field (must be 1's). Header Length (Hdr len) a 1-bit value that for GTP' version 0 indicates if using a 20 byte header (value 0) (as per GTP) or this 6 byte header. This bit must be unset (value 0) for subsequent GTP' versions and in these does not indicate the header length as this must always be 6 bytes. Message Type An 8-bit field that states the message type. Possible values:

Length A 16-bit field that states the length of the packet being encapsulated by GTP' (not including the GTP' header itself). Sequence Number A 16-bit field that uniquely identifies this packet and allows detection of loss or duplication

Message Types GTP' uses the GTP Version Not Supported, Echo Request and Echo Response messages unchanged, but adds the following messages

Node Alive Request Node Alive Response Redirection Request Redirection Response Data Record Transfer Request Data Record Transfer Response

Node Alive Request / Response The Node Alive messages are used to advise other network components that a node has started service. The request is sent from the node starting and so provides a faster method to re-enable service than polling using Echo Request/Response does. This message can also be used to advise of other nodes coming back into service, and (in GTP' version 2) to advise of the IPv6 address of the CGF.

Redirection Request/Response The Redirection messages are used to:

divert the flow of CDRs from the CDFs (SGSN/GGSN) to another CGF when the sender is being removed from service (for maintenance/failure). advise that the CGF has lost its connection to a downstream system In either case the CDFs are given more information about an impending or immediate failure than would be the case if the CDF was polling using Echo Request messages. This message contains details about the cause, and optionally address(es) of an alternate CGF.

Data Record Transfer Request/Response The Data Record Transfer messages are used to reliably transport CDRs from the point of generation (SGSN/GGSN) to non-volatile storage in the CGF.

Data Record Transfer Request Each Data Record Transfer Request message can contain a message of one of four types:

Send Data Record Packet - This message contains zero or more CDRs. CDRs may be encoded in ASN.1 using BER or, less commonly, PER. Send possibly duplicated Data Record Packet - This message contains one or more CDRs, and this message has previously been sent to another CGF. Cancel Data Record Packet - This message orders the CGF to remove one or more Data Record Packet from the CGF "possibly duplicated" pending queue. Release Data Record Packet - This message orders the CGF to write the contents of one or more Data Record Packets from the CGF "possibly duplicated" pending queue. There is a mechanism to attempt to avoid losing or writing any duplicate CDRs. This is described in some detail in 3GPP TS 32.295. The basic premise is that every packet is sequenced and if not individually acknowledged then it will be resent until it is acknowledged by any CGF. Normal Data Record packets are immediately written to non-volatile storage (e.g. disk), but resent packets are marked as "possibly duplicated" and enter a special queue that is not immediately written to non-volatile storage—a second confirmation from the CDF is required. The ability to send a Data Record Transfer Request containing zero CDRs is used as a test to detect the success or failure of the CGF to have already written records assigned to that sequence number and is an important part of the above mechanism.

Data Record Transfer Response The Data Record Transfer Response acknowledges receipt of one or more Data Record Transfer messages; responses can be grouped for reasons of efficiency but must be sent more frequently than the sending CDFs timeout. The acknowledgement includes a cause and can be a rejection of the contained records.

References

External links Direct access to the numbered 3GPP specifications Open source Charging Gateway Function (CGF)

Worked examples

Example 1 — a first encounter with GTP'

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

In research: GTP' appears in engineering 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 GTP' 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: GTP' is common in secondary-school and first-year university syllabi. It links to neighbouring topics 3GPP standards, Mobile telecommunications standards, Telecommunications infrastructure, so understanding it makes those chapters shorter.
In everyday life: Look for GTP' 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.

How to study GTP' in 20 minutes

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

Frequently asked questions

What is GTP' in simple terms?

GTP' (GTP prime) is an IP based protocol used within GSM and UMTS networks. It can be used with UDP or TCP.

Why does GTP' matter?

Because it connects several engineering 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 GTP'?

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 GTP'.