TETRA

TETRA is what those requirements produce.

The radio

Four channels per 25 kHz carrier with pi/4-DQPSK at 36 kbit/s, so a channel is about 9 kbit/s gross, of which the speech codec uses 4.567 kbit/s and the rest is protection. That is a quarter of GSM's per-channel rate, and it is deliberate: TETRA's priority is range and robustness, not rate. Its spectral efficiency, four channels in 25 kHz, is four times GSM's eight in 200 kHz, which matters because professional users are allocated narrow bands.

The frame structure is unusual in one respect: the control frame, frame 18 of every multiframe, is reserved for signalling, so there is always a known moment at which a radio can be reached or can ask for something. That is part of how call setup stays fast.

Trunking

A traditional professional radio system gave each user group its own channel: the fire service had one, the water board another. Half the channels were idle while another group queued.

Trunking pools them. A radio that wants to talk asks a control channel, and is assigned any free traffic channel for the duration. The gain is the trunking efficiency of [Channel Allocation, Cell Splitting, Sectorisation and Cell Breathing], and the program measures it: five groups of four channels carry 5.46 erlangs in all at 2 per cent blocking; the same twenty channels pooled carry 13.18, 141 per cent more.

The standard distinguishes how long a channel is held:

• Message trunked: the channel is held for the whole conversation, including the pauses between transmissions. Simple, and wasteful of the pauses.
• Transmission trunked: the channel is released at the end of every transmission and requested again for the next. Efficient, at the cost of a small delay and the risk that the channel is gone when the reply comes.
• Quasi-transmission trunked: a compromise, holding the channel briefly after each transmission in case the conversation continues.

The group call

This is the service that defines the system. A talkgroup is a set of radios that share an identity; a transmission to the group goes out once on one downlink channel, and every radio in the group in that cell hears it. The program counts what the alternatives would cost: twelve people talking to each other as individual calls would need 66 channels for everyone to hear everyone, a network conference would need twelve, and a TETRA group call needs one downlink and one uplink.

Around it sit the features that make it usable:

• Push to talk: the channel is half duplex and one radio at a time transmits, which is not a limitation but the discipline the users already have.
• Fast setup: because the group exists in advance and the control frame comes round every multiframe, the standard's target is well under half a second.
• Late entry: a radio switched on, or returning to coverage, during a group call joins it, because the group call is a state of the network rather than a connection between endpoints.
• Priority and pre-emption: calls carry priorities, and an emergency call can take a channel from a lower-priority call in progress.

Direct mode

Direct mode operation lets two radios talk on a dedicated channel with no network at all: in a tunnel, a basement, a building whose coverage has failed, or an area with no infrastructure. The range is that of a handset, so hundreds of metres to a few kilometres, and one radio may act as a gateway, relaying direct-mode users into the network, or a repeater, extending direct-mode range.

Nothing in GSM or DECT corresponds to this, and it is the plainest statement of what TETRA is for: a system whose users cannot accept that the infrastructure is a single point of failure.

TETRA, computed

The program computes the frame hierarchy and the per-channel rate; prices trunking against separate channel groups; counts the channels a group call saves; lists direct mode's properties; and tabulates what a public network cannot offer.

# TETRA: the frame, what trunking buys, and why a group call is not many calls.
import math

# 1. The TDMA structure, from EN 300 392-2 clause 4.5.2.
FRAME_MS, SLOTS, MULTI, HYPER = 56.67, 4, 18, 60
print("TETRA's TDMA structure (EN 300 392-2, 4.5.2):")
print("  1 TDMA frame  = %d timeslots      = %.2f ms, so a slot is %.2f ms"
      % (SLOTS, FRAME_MS, FRAME_MS / SLOTS))
print("  1 multiframe  = %d TDMA frames   = %.2f s (frame 18 is the control frame)"
      % (MULTI, MULTI * FRAME_MS / 1000))
print("  1 hyperframe  = %d multiframes   = %.1f s" % (HYPER, HYPER * MULTI * FRAME_MS / 1000))
print("  modulation rate %d kbit/s in a %d kHz carrier with pi/4-DQPSK, 4 channels a carrier"
      % (36, 25))
print("  so one channel has about %.1f kbit/s gross, against GSM's %.1f in 200 kHz"
      % (36 / 4, 270.833 / 8))
print("  spectral efficiency: TETRA %.2f channels a kHz, GSM %.2f"
      % (4 / 25, 8 / 200))

# 2. Trunking: why a shared pool of channels beats fixed assignments, which is
#    the whole idea of a trunked radio system. Erlang B again.
def erlang_b(c, a):
    b = 1.0
    for k in range(1, c + 1):
        b = a * b / (k + a * b)
    return b

print("\nTrunking: 5 user groups, 4 channels each, against one pool of 20 (2 per cent blocking):")
def capacity(c, target=0.02):
    lo, hi = 0.0, 1000.0
    for _ in range(60):
        mid = (lo + hi) / 2
        lo, hi = (mid, hi) if erlang_b(c, mid) < target else (lo, mid)
    return lo
sep, pooled = 5 * capacity(4), capacity(20)
print("  five separate groups of 4 channels: %.2f erlangs in all" % sep)
print("  one trunked pool of 20 channels:    %.2f erlangs, %.0f%% more" % (pooled, 100 * (pooled / sep - 1)))
print("  that is trunking, and it is what the T in TETRA stands for.")

# 3. A group call against many individual calls: how many channels each needs.
print("\nOne fire crew of 12 talking to each other:")
print("  individual calls, everyone to everyone: %d pairs, so %d channels for all to hear all"
      % (12 * 11 // 2, 12 * 11 // 2))
print("  a conference bridged in the network: 12 channels, one per member")
print("  a TETRA group call: 1 downlink channel for all, 1 uplink for whoever is speaking: 2")
print("  and the call sets up in under half a second, because the group already exists")

# 4. Direct mode: two radios with no network at all.
print("\nDirect mode operation (DMO), and why it matters:")
for point in ("two radios talk directly, on their own channel, with no base station",
              "it works in a tunnel, a basement, or where the network has failed",
              "range is that of a handset, so a few hundred metres to a few kilometres",
              "a radio can act as a gateway, relaying direct-mode users into the network"):
    print("  - %s" % point)

# 5. What TETRA has that a public network does not.
print("\nWhat a public cellular network cannot offer, and TETRA does:")
for feature, why in (("group call and broadcast call", "one transmission heard by a whole talkgroup"),
                     ("call setup in under 0.5 s", "a group already exists; no per-call negotiation"),
                     ("push to talk, half duplex", "one speaks, many listen: the discipline of radio"),
                     ("pre-emptive priority", "an emergency call takes a channel from an ordinary one"),
                     ("direct mode", "works with no infrastructure at all"),
                     ("air interface encryption plus end to end", "the operator is not trusted with the content")):
    print("  %-38s %s" % (feature, why))

The frame. A slot is 14.17 ms, a frame 56.67 ms, a multiframe 1.02 s with a control frame at its end, a hyperframe 61.2 s. Four channels in 25 kHz gives 0.16 channels per kHz against GSM's 0.04: four times the spectral efficiency, because the channels are slower.

Trunking. 141 per cent more traffic from the same twenty channels, purely by pooling them. That single number is why trunked radio replaced allocated channels, and it is the same trunking effect that makes a large cell better than several small ones for a given load.

The group call. Twelve people, 66 pairs, against two channels. A conference bridge in a public network would consume twelve channels and take seconds to build; TETRA consumes one downlink and one uplink and is ready before the speaker has finished pressing the key.

What is missing elsewhere. Six features, and every one of them follows from the users: group working, speed, discipline, priority, independence from infrastructure, and distrust of the operator with the content.

Distinctions

What it does not mean

Trunked does not mean high capacity per user. It means channels are pooled; each channel is slower than GSM's.

Push to talk is not a limitation of the radio. It is the working discipline of group communication, and the system is built around it.

Direct mode is not a fallback of last resort only. It is used routinely, for example inside a building where a crew is working.

A group call is not a conference call. It is one transmission to many radios, not many connections bridged together.

End-to-end encryption is not the same as air interface encryption. TETRA has both, and the second is what keeps the content from the operator as well as from an eavesdropper.

Quick revision

• TETRA: professional mobile radio; TDMA, 4 channels per carrier, 25 kHz for phase modulation, 36 kbit/s with pi/4-DQPSK, so about 9 kbit/s a channel.
• Frames: slot 14.17 ms, TDMA frame 4 slots = 56.67 ms, multiframe 18 frames = 1.02 s (frame 18 is the control frame), hyperframe 60 multiframes = 61.2 s.
• Trunking: channels pooled and assigned per call. Modes: message, transmission, quasi-transmission trunked. Program: 20 channels pooled carry 141 per cent more than five groups of four.
• Services: group call (one downlink for the whole talkgroup), broadcast call, push to talk, setup under half a second, late entry, priority and pre-emption, direct mode (DMO) with gateways and repeaters, voice plus data, end-to-end encryption.
• Program: a crew of 12 needs 66 channels as individual calls, 12 as a bridged conference, 2 as a TETRA group call.

Test yourself

1. What is TETRA and who uses it? TETRA, Terrestrial Trunked Radio, is an ETSI standard for professional mobile radio, used by emergency services, transport operators, utilities and other organisations whose work depends on radio. It is a trunked system carrying voice plus data, designed around group working, immediate call setup, priority, and operation with or without infrastructure, none of which a public cellular network provides.

2. Give TETRA's TDMA structure. The access scheme is TDMA with four physical channels per carrier, and for phase modulation the carrier bandwidth is 25 kHz with a modulation rate of 36 kbit/s using pi/4-DQPSK. One TDMA frame is four timeslots and lasts about 56.67 ms, so a slot is about 14.17 ms; one multiframe is 18 TDMA frames, 1.02 seconds, of which the eighteenth is a control frame reserved for signalling; and one hyperframe is 60 multiframes, 61.2 seconds.

3. What is trunking, and what does it achieve? In a trunked system the radio channels are held in a common pool and assigned to a call when it is made, instead of each user group having permanently allocated channels. Because peaks in different groups' demands do not coincide, a pool carries far more traffic than the same number of channels divided up: in the chapter's calculation, five groups of four channels carry 5.46 erlangs at 2 per cent blocking while the same twenty channels pooled carry 13.18, 141 per cent more.

4. Distinguish message, transmission and quasi-transmission trunking. In a message trunked system a channel is assigned for the whole conversation and held through the pauses between transmissions, which is simple but wastes the pauses. In a transmission trunked system the channel is released at the end of each transmission and requested again for the next, which uses the pauses for other users but adds a small delay and a risk that no channel is free for the reply. Quasi-transmission trunking holds the channel for a short time after each transmission in case the conversation continues, which is a compromise between the two.

5. Why is a group call fundamentally different from a conference call? A group call is one transmission addressed to a talkgroup: the base station sends it once on a single downlink channel and every radio of that group in the cell receives it, whatever their number, while one radio at a time transmits on a single uplink channel. A conference call is many separate connections bridged together in the network, costing one channel per participant and taking as long to build as the slowest setup. For a crew of twelve, the chapter counts 66 channels for individual calls, 12 for a bridged conference and 2 for a TETRA group call, and the group call is ready in under half a second because the group already exists.

6. What is direct mode operation, and why does it matter? In direct mode two or more radios communicate with each other on a dedicated channel without any base station or network. It matters because the users cannot accept infrastructure as a single point of failure: direct mode works in tunnels, basements and buildings where coverage fails, in areas with no network at all, and when the network itself has been destroyed or overloaded. Its range is that of a handset, so a few hundred metres to a few kilometres, and a radio may act as a gateway into the network or as a repeater to extend the direct-mode range.