GSM and Its Mobile Services

It succeeded so completely that its architecture, its vocabulary and its service model are still the shape of mobile networks, and its specifications are still maintained by 3GPP alongside the later generations. What the following chapters describe as GSM is therefore both a historical system and the skeleton of what came after.

Bearer services

A bearer service offers only transmission: bits in at one access point, bits out at another, with stated characteristics and no opinion about what they mean. TS 22.002 defines them by attributes, "which are intended to be independent", grouped into the four categories quoted above: information transfer, access, interworking and general.

That structure is worth understanding for its own sake. Information transfer says what the network does with the bits: the rate, whether the transfer is transparent (a fixed delay and rate, with errors passed through) or non-transparent (a protocol that retransmits, giving fewer errors and a variable delay), and whether it is circuit or packet. Access says how the user reaches it. Interworking says what the far network is, which for GSM was usually the fixed telephone network or an ISDN. General covers the rest.

In practice GSM's circuit bearer services offered 300 bit/s to 9.6 kbit/s on one slot, later 14.4 kbit/s, and the program shows what that meant for anyone trying to move a file.

Teleservices

A teleservice is a complete service, including what the terminal does: not merely a pipe but a telephone call. TS 22.003 lists them, and three matter most.

Telephony is the reason the system exists: speech, digitised and compressed by the codec, protected by channel coding, carried in a traffic channel. The program follows a speech block from the codec to the air.

Emergency calls are a teleservice in their own right, not a variant of telephony, because they must work when telephony would not: dialled by a number the mobile itself recognises, permitted where a normal call would be barred, and routed by the network to the appropriate centre.

The short message service carries up to 140 octets of payload, which is 160 characters in GSM's 7-bit alphabet, and rides on the signalling channels rather than on a traffic channel, so it works while a call is in progress and needs no call setup at all. The standard distinguishes mobile terminated and mobile originated point-to-point messages and the cell broadcast service, which sends the same message to every mobile in an area. SMS was an afterthought in the design and became the system's second business.

The list closes with alternate speech and facsimile, automatic facsimile, and the voice group call and voice broadcast services, which serve dispatch-style users, the ground that [TETRA] occupies properly.

Supplementary services

A supplementary service modifies a basic service and cannot stand alone: forwarding modifies a call, it is not a call. TS 22.004 tabulates them with the operations each supports, and the operations are themselves the vocabulary of the subject:

• Provision and withdrawal, by the operator;
• Registration and erasure, by the subscriber, of a parameter such as a forwarding number;
• Activation and deactivation;
• Invocation, either "automatic invocation by the network as a result of a particular condition" or "user invocation, by means of a control procedure";
• Interrogation, either a "status check" or a "data request".

Reading the standard's table with those columns explains the everyday behaviour of a phone. Call forwarding on busy is registered and erased by the subscriber, activated by registration, and invoked by the network when the condition occurs. Call waiting has nothing to register, is activated by the subscriber, and is invoked by the network. Multi party has no registration, no activation and no interrogation: it is invoked by the user during a call and that is all.

The families are: number identification (CLIP, CLIR, CoLP, CoLR, CNAP, call deflection), call offering (the four forwardings, explicit call transfer), call completion (call waiting, hold, completion of calls to busy subscribers), multi party, community of interest (closed user group), charging (advice of charge, information or charging), call restriction (the barring services) and user-to-user signalling.

GSM's services, computed

The program follows a full-rate speech block from the codec through channel coding to the bursts and asks how much of a slot it fills; times a 100 kB file over each circuit bearer service; counts the characters that fit in a short message's 140 octets; and lists what makes an emergency call different.

# GSM's services, in numbers: what a speech channel costs, what the data
# bearer services carried, and what SMS fits into.
import math

# 1. From speech to a burst. GSM's full-rate speech codec makes 260 bits every
#    20 ms; channel coding takes that to 456 bits, carried as 4 half-bursts of
#    114 bits, interleaved over 8 bursts.
print("A full-rate GSM speech channel, block by block:")
speech_bits, block_ms = 260, 20.0
coded = 456
print("  the codec produces %d bits every %.0f ms: %.2f kbit/s" % (speech_bits, block_ms, speech_bits / block_ms))
print("  channel coding takes each block to %d bits: %.2f kbit/s on the air" % (coded, coded / block_ms))
print("  that is %d bursts of 114 payload bits, one per TDMA frame of 4.615 ms, spanning %.2f ms"
      % (coded / 114, coded / 114 * 4.615))
slot_kbps = 114 / 4.615                       # payload bits a burst, one burst a frame
print("  one slot carries %.2f kbit/s of payload; coded speech needs %.2f, so it fills %.0f%% of the slot"
      % (slot_kbps, coded / block_ms, 100 * (coded / block_ms) / slot_kbps))
print("  of that, the speech itself is %.0f%%: the rest is the protection that makes it survive the air"
      % (100 * speech_bits / coded))

# 2. The bearer services: data rates GSM offered on one slot, and how long a
#    file takes at each.
print("\nCircuit bearer services on one slot, and a 100 kB file:")
for name, kbps in (("300 bit/s", 0.3), ("1.2 kbit/s", 1.2), ("2.4 kbit/s", 2.4),
                   ("4.8 kbit/s", 4.8), ("9.6 kbit/s", 9.6), ("14.4 kbit/s", 14.4)):
    seconds = 100 * 1024 * 8 / (kbps * 1000)
    print("  %-12s %8.1f seconds, that is %5.1f minutes" % (name, seconds, seconds / 60))
print("  HSCSD and GPRS answer by bundling slots; EDGE by changing the modulation.")

# 3. Short messages. A short message is 140 octets of payload; in the 7-bit
#    alphabet that is 160 characters.
print("\nA short message:")
octets = 140
for bits, name in ((7, "the 7-bit GSM alphabet"), (8, "8-bit data"), (16, "UCS-2, for other scripts")):
    print("  %-26s %3d characters in %d octets" % (name, octets * 8 // bits, octets))
print("  a message of 160 characters sent on a signalling channel of 782 bit/s takes %.2f s of air time"
      % (octets * 8 / 782))

# 4. Emergency calls: why they are a teleservice of their own.
print("\nWhy emergency calls are a separate teleservice:")
for rule in ("dialled with a single number, recognised by the mobile itself",
             "placed without a SIM where regulation requires it",
             "not barred by call barring or by a full cell's access control",
             "routed to the nearest centre by the network, not by the subscriber"):
    print("  - %s" % rule)

Speech. The full-rate codec makes 260 bits every 20 ms, which is 13.00 kbit/s, and channel coding takes each block to 456 bits, 22.80 kbit/s. Those 456 bits are carried as four bursts of 114 payload bits, one per TDMA frame, spanning 18.46 ms, which is why the frame is 4.615 ms long ([The GSM Radio Interface: Carriers, the TDMA Frame and Bursts]). One slot carries 24.70 kbit/s of payload, so coded speech fills 92 per cent of it, and of what is transmitted only 57 per cent is speech: the rest is protection. That ratio is the price of a channel that must survive fading, interference and handover.

Data. A 100 kB file takes 45.5 minutes at 300 bit/s, 1.4 minutes at 9.6 kbit/s and 0.9 minutes at 14.4 kbit/s. For 1991 that was reasonable; by the time the web existed it was not, and the answers, bundling slots (HSCSD), packet switching (GPRS) and better modulation (EDGE), are [New Data Services: HSCSD, GPRS and EDGE].

Short messages. 140 octets hold 160 characters of the 7-bit alphabet, 140 of 8-bit data, or 70 in UCS-2, which is why a message in Devanagari or any non-Latin script costs twice as much of the allowance. At the SDCCH's rate a 160-character message is 1.43 seconds of air time, which is why an operator could give it away and then charge for it.

Distinctions

What it does not mean

A bearer service is not a teleservice with less. It is a different layer: the bearer carries bits, the teleservice defines what the terminals do with them.

Supplementary services are not features of the handset. They are network services; the handset only invokes and interrogates them.

SMS is not a small call. It travels on signalling channels, which is why it needs no call setup and works while a call is in progress.

GSM is not only the radio. The service definitions, the architecture and the signalling are as much a part of the standard as the air interface.

13 kbit/s of speech is not what the air carries. Channel coding takes it to 22.8 kbit/s, and only 57 per cent of what is transmitted is the speech itself.

Quick revision

• GSM: European digital standard for roaming, efficiency, security and digital services; the SIM separates subscription from handset.
• Bearer services (TS 22.002): transmission only; attributes in four groups, information transfer, access, interworking, general; transparent against non-transparent; 300 bit/s to 9.6 kbit/s, later 14.4.
• Teleservices (TS 22.003): telephony, emergency calls, SMS (MT/PP, MO/PP, cell broadcast), alternate speech/fax G3, automatic fax G3, voice group call, voice broadcast.
• Supplementary services (TS 22.004): CLIP, CLIR, CoLP, CoLR, CNAP, CD; CFU, CFB, CFNRy, CFNRc, ECT; CW, HOLD, CCBS; MPTY; CUG; AoCI, AoCC; BAOC, BOIC, BOIC-exHC, BAIC, BAIC-Roam, ACR; UUS. Operations: provision/withdrawal, registration/erasure, activation/deactivation, invocation (network or user), interrogation (status or data).
• Program: speech 260 bits / 20 ms = 13.00 kbit/s, coded 456 bits = 22.80 kbit/s, 4 bursts of 114 bits, 92 per cent of a slot, only 57 per cent of it speech; a 100 kB file takes 45.5 minutes at 300 bit/s and 0.9 at 14.4 kbit/s; SMS 140 octets = 160 / 140 / 70 characters at 7 / 8 / 16 bits.

Test yourself

1. What were GSM's goals? To replace the incompatible analogue national systems of Europe with a single digital standard, giving international roaming on one subscription; to use the spectrum more efficiently and so serve more subscribers; to be digital, which allows speech compression, error protection, encryption and authentication; to separate the subscription from the handset through the SIM; and to specify services and interfaces precisely enough for equipment from different manufacturers to interoperate.

2. Distinguish bearer services, teleservices and supplementary services with examples. A bearer service provides only the transmission of information between access points, with stated attributes and no interpretation of the content, for example a 9.6 kbit/s transparent circuit data service. A teleservice provides complete communication including the functions of the terminals, for example telephony, emergency calls, the short message service or facsimile. A supplementary service modifies or supplements a basic service and cannot be offered alone, for example call forwarding, call barring, call waiting, multi party or calling line identification presentation.

3. What attributes describe a bearer service? Four groups, as TS 22.002 states: information transfer attributes, which characterise the network's capability to transfer information from a user access point in one network to a user access point in another; access attributes, which describe how network functions are reached at the access point; interworking attributes, which describe the terminating network and its access point; and general attributes, which deal with the service in general. They include the rate, whether the transfer is transparent or non-transparent, and whether it is circuit or packet switched.

4. List GSM's teleservices and say why emergency calls are separate from telephony. Telephony, emergency calls, the short message service in its mobile terminated, mobile originated and cell broadcast forms, alternate speech and facsimile group 3, automatic facsimile group 3, the voice group call service and the voice broadcast service. Emergency calls are a separate teleservice because they must behave differently from ordinary telephony: they are dialled by a number the mobile itself recognises, they are permitted where an ordinary call would be barred or the cell would refuse access, in many countries they can be made without a valid subscription, and the network routes them to the appropriate emergency centre rather than to a subscriber-chosen destination.

5. Name the operations that apply to a supplementary service, and illustrate with call forwarding on busy. Provision and withdrawal by the operator; registration and erasure by the subscriber; activation and deactivation; invocation, either automatically by the network when a condition occurs or by the user through a control procedure; and interrogation, either a status check or a data request. Call forwarding on busy is provided by the operator, registered by the subscriber together with the number to forward to, activated as a result of that registration or by the subscriber, invoked automatically by the network when a call arrives while the mobile is busy, and interrogated by the subscriber as a data request to find out where calls are being sent.

6. How much of a GSM traffic channel is actual speech? The full-rate codec produces 260 bits every 20 ms, which is 13 kbit/s, and channel coding expands each block to 456 bits, which is 22.8 kbit/s on the air. Those 456 bits are sent as four bursts of 114 payload bits, one in each of four TDMA frames. One slot's payload capacity is about 24.7 kbit/s, so the coded speech fills about 92 per cent of it, and of the bits actually transmitted only 57 per cent are the speech itself; the remainder is the error protection that lets it survive fading and interference.