Quiz: Introduction to Analog Switching Telecommunication Systems

Answer: (b). The fundamental purpose of switching is resource sharing: a limited pool of trunks and switch paths is shared among many subscribers, avoiding the N(N−1)/2 dedicated links a fully connected network would need.

Answer: (c) 1892. Almon Brown Strowger patented the automatic exchange in 1891; the first commercial installation was at La Porte, Indiana, in 1892. (1878 was the first manual exchange; 1938 the crossbar era.)

Answer: (c) Line finder. On off-hook, the line finder (or preselector) hunts for the calling line and seizes a free first selector, after which dial tone is returned. The connector (final selector) is the last stage; the marker and register are crossbar/common-control elements.

Answer: (b). A crossbar crosspoint is a mechanically latched contact operated by two electromagnets: the horizontal bar selects the inlet and the vertical bar the outlet; energizing one of each closes the crosspoint at their intersection. It remains latched for the call duration.

Answer: (b). The Erlang is a dimensionless unit: 1 E = one circuit busy for the whole observation period = 1 call-hour per hour = 36 CCS (cent-call-seconds). It measures average simultaneous circuit occupancy, not a count of calls.

Answer: (b) Concentration. Concentration reflects that only a small fraction of subscribers originate calls simultaneously, so a large line pool can share a smaller trunk pool. Distribution has equal inlets/outlets; expansion is the reverse (few in, many out).

Answer: (c). GoS is the blocking probability B — the fraction of offered calls that cannot be served immediately and are cleared (lost-calls-cleared assumption, as in the Erlang B model). Typical design targets: 0.1%–2%.

Answer: (b). Every pair of subscribers needs its own link: N(N−1)/2 ≈ 5×10⁷ lines for N = 10,000 — each used a tiny fraction of the time. A switch exploits the low simultaneous-usage factor, so a few thousand trunks serve the whole population at an acceptable blocking probability.

Answer: (b). In SxS the subscriber's dial pulses operate the selectors directly, stage by stage: setup is slow (10 pps), control equipment is dedicated for the entire call, and routing is rigid. Crossbar's marker (common control) decodes digits centrally, selects any free path quickly, and is released after setup.

Answer: (b). The speech store buffers the samples; the control store is written once per call by the processor and thereafter dictates, slot by slot, which speech-store location is read onto each outlet — i.e., it realizes the permutation of time slots.

Answer: (b). With at most N simultaneous connections, at most N of the N² crosspoints are ever closed simultaneously (and usually much fewer), giving utilization ≤ 1/N. Multi-stage networks with internal links (e.g. three-stage Clos) achieve huge crosspoint savings at the cost of a small, engineered probability of internal blocking.

Answer: (b). A blocking system has finite servers; when all are occupied, a new arrival is refused (lost-calls-cleared) under the Erlang B model. Blocking probability is the GoS. It is a deliberate trade-off: providing enough capacity for zero blocking would be uneconomical.

Answer: (a). Space division = separate paths in space (crosspoints/wipers); time division = one shared bus where samples are interleaved in time and the switch permutes slot assignments (TSI). Time division dramatically reduces physical hardware per conversation — the key idea behind PCM digital exchanges.

Answer: (b). Each dial pulse (10 pps) steps the wiper up one level; after five pulses it is at level 5. The wiper then rotates automatically ("hunts") until it finds a free outlet on that level, connecting to the next selector stage. Digit translation by a marker belongs to common-control systems, not SxS.

Answer: (b) 1,225. Each subscriber needs a dedicated link to every other subscriber. This quadratic growth is precisely why even modest networks use switched (shared-trunk) architectures.

Answer: (c) 18 Erlangs. Equivalently A = λh. Note the offered traffic (18 E) exceeds the number of trunks (12), so this group would block heavily — in practice the usage factor must be lower or more trunks provided (see Q17/Q18).

Answer: (c) 10 Erlangs. Total traffic carried in the hour is 360 × 100 = 36,000 call-seconds = 36,000/3,600 = 10 call-hours, i.e. 10 Erlangs.

Answer: (c) 9 trunks. From Erlang B tables/recursion B_k = A·B_k−1/(k + A·B_k−1): eight trunks give about 3% blocking (poor), nine trunks give about 0.9%, meeting the 1% target. Utilization = 4(1−0.009)/9 ≈ 44%.

Answer: (a). Offered traffic is reduced by the blocked fraction to give carried traffic; dividing by the number of servers gives the occupancy per server.

Answer: (c). In a single-stage matrix, a call blocks when its own inlet or outlet is already busy (a non-blocking requirement per inlet/outlet), because one inlet can feed only one output and vice versa. Here inputs 0–4, 6 and outputs 0,1,2,3,5,7 are busy; input 5 and output 4 are both free, so crosspoint (5,4) closes and the call proceeds. Blocking arises for, say, In0 → Out4, since In0 is already engaged. (Multi-stage internal blocking is a separate phenomenon.)