EEEN 462 — Analog Telephone Local Loop

1Introduction

The local loop (subscriber line) is the physical two-wire connection between a subscriber's telephone and the nearest telephone exchange (central office).

▶ Animated: The Local Loop Circuit

💡 On-hook: switchhook open → no DC loop current flows → exchange knows the line is idle.

local loop diagram — Fig. 1.1 — The local loop: copper twisted pair, max ≈5 km, feeding the central office.

telephone network hierarchy — Fig. 1.2 — Hierarchy: local loops → end offices → tandem trunks → regional offices.

Key Ideas at a Glance

2-wire analog loop−48 V DC feed from exchange 20–40 mA loop current300 – 3400 Hz voice band Twisted pair ≤ ~5.5 kmLoop signalling: DC + tones Hybrid: 2-wire ⇄ 4-wireEcho → need for hybrids

2History

Scroll through the evolution of the analog telephone local loop.

1876 — Alexander Graham Bell patents the telephone

First practical point-to-point voice transmission over a single pair of wires.

1878 — First commercial telephone exchange (New Haven, USA)

Manual switchboard: operators connect subscribers using patch cords.

1891 — Strowger automatic exchange patented

Step-by-step electromechanical switch, driven directly by dial pulses from the subscriber's telephone.

1919 — Rotary dial telephone standardised

The familiar round dial opens/closes the loop to send decimal digits as pulse trains.

1963 — Touch-Tone® (DTMF) introduced by Bell System

Dual-tone multi-frequency signalling: each digit = two simultaneous audio tones — faster, more reliable than pulses.

1970s–1990s — Electronic & digital exchanges

Stored-program control (SPC) exchanges; DTMF becomes the universal standard; analog loop still carries voice.

2000s — xDSL & the "last mile" revolution

Same copper twisted pair carries broadband data (ADSL/VDSL) — the analog local loop lives on.

antique telephone — Fig. 2.1 — Early candlestick telephone (pulse signalling era).

rotary telephone — Fig. 2.2 — The classic rotary-dial telephone (1919–1970s).

3The Dial-Pulse Telephone

Signalling by interrupting the DC loop current — "make/break" pulses at 10 pulses per second.

▶ Animated Rotary Dial — watch the pulses!

Loop-current waveform: digit N → N break pulses @ 10 pps · 61% break / 39% make

💡 Pulse train: digit N produces N current interruptions. Between digits: ≥ 300 ms inter-digit pause. Exchange counts pulses to identify each digit.

How the Exchange Sees a Dialled Number

Speed: 10 pulses/sBreak: 61 ms Make: 39 msTolerance: ±1.5%Max digit = 0 (10 pulses)

⚠️ Limitations of dial-pulse: slow (≈0.7 s per digit with pause), sensitive to line distortion, noisy (clicks), hard to automate — hence DTMF.

4The Multi-Frequency (DTMF) Telephone

Dual-Tone Multi-Frequency: every key press sends TWO pure tones simultaneously — one from the LOW group, one from the HIGH group.

▶ Interactive DTMF Keypad (with real audio!)

🔊 Turn sound on & press the keys — hear each tone pair.

Last key: —
f₁ = — Hz · f₂ = — Hz

Live spectrum of the two tones your key generates

💡 Tones are sent inside the voice band (697–1633 Hz), so DTMF travels over the same analog loop as speech — in-band signalling.

f₂ ↓ / f₁ →	697 Hz	770 Hz	852 Hz	941 Hz
1209 Hz	1	2	3	A
1336 Hz	4	5	6	B
1477 Hz	7	8	9	C
1633 Hz	*	0	#	D

Fig. 4.1 — DTMF frequency plan: 4 low + 4 high = 16 unique tone pairs

Digit duration: ≥ 50 msInter-digit gap: ≥ 45 ms Twist: low tone may be up to 4 dB louderA–D reserved for network control

Fig. 4.2 — Modern push-button DTMF keypad.

DTMF frequency grid — Fig. 4.3 — The standard 4×4 DTMF frequency assignment.

5Call Setup Phase

From lifting the handset to the called party answering — follow the animated sequence.

▶ Animated Call-Setup Sequence

1 · Caller off-hook

2 · Exchange detects loop current

3 · Dial tone returned

4 · Digits sent (DTMF)

5 · Exchange analyses digits

6 · Ring-back tone to caller

7 · Ringing AC sent to callee (25 Hz)

8 · Callee off-hook

9 · Ringing stops · Path completed ✔

💡 Setup = loop signalling. Off-hook closes the loop (DC current) → exchange returns dial tone → digits (DTMF) select the route → exchange applies 75 V, 25 Hz ringing AC to the called line and returns ring-back tone to the caller.

6Conversation Phase

The speech path is established: two-way analog audio, 300 – 3400 Hz, carried over the loop via hybrids at each end.

▶ Animated Speech Path & the 2-Wire ⇄ 4-Wire Hybrid

💡 Hybrid function: converts the 2-wire loop (both directions on one pair) to the 4-wire trunk (separate Tx/Rx pairs). A balanced hybrid cancels the transmitted signal from the received path — reducing echo.

🔧 During conversation: loop current stays closed (~20–40 mA); voice AC modulates this current; the exchange keeps the path and meters the call; either party may hang up first.

▶ The Analog Voice Wave on the Loop (live)

Voice band: 300 – 3400 Hz Transmitted as AC superimposed on DC loop current Companding (A-law/μ-law) improves S/N at low levels

7Call Tear-Down Phase

Ending the call: the loop opens, the exchange releases the path, and both lines return to idle.

▶ Animated Tear-Down Sequence

1 · Either party goes on-hook

2 · Loop current drops to 0

3 · Exchange detects "open" for ~200 ms

4 · Speech path released (switch matrix freed)

5 · Billing (metering) stops

6 · Both lines returned to idle state ✔

💡 Supervision: going on-hook opens the loop → loop current falls to zero → exchange confirms ~200 ms of open loop → releases switches, stops metering. A "clear-forward" is sent downstream so all exchanges in the chain release the path.

The Analog Telephone Local Loop