Telecom domain fundamentals

A configured capability on shared infrastructure, metered or bundled, with devices and content wrapped around it. That distinction matters more than it sounds: because the product is not an object but a state of the network, every sale is an instruction to reconfigure something, and every cancellation is another. The margin comes from utilisation of an asset already paid for, so commercial agility — launching a new bundle, in a new market, this quarter — is worth more than almost any efficiency elsewhere. That is why an architect's attention belongs on the product catalogue and the order path rather than on the invoice renderer, which is the part outsiders assume is the system.

Because both ends of the business read from it. The commercial end needs it to define what can be sold, at what price, with what eligibility and what bundling rules. The technical end needs it to know what a sold product decomposes into on the network and how its usage should be rated. If those are two catalogues, they drift, and the drift is not cosmetic: marketing launches a plan, ordering knows the plan, and rating does not, so usage is either unrated or rated at a default tariff. A single catalogue driving ordering, provisioning and rating is what makes "launch a product without a release" possible, and its absence is the root cause behind most operator delivery complaints.

One product offering, with the two consumers of it annotated.

Product offering  PO-FIB-300-V   "Fibre 300 with Unlimited Voice"

  price
    recurring     INR  999   monthly, billed in advance
    one-off       INR 1500   installation, waived when term >= 12 months

  characteristics                     consumed by ORDERING
    bandwidth     300 Mbps            -> BNG rate-limit profile FIB300
    staticIp      false               -> skip the IPAM assignment step
    term          12 months           -> early-termination charge applies

  bundled service specs               consumed by PROVISIONING
    CFS-BB-ACCESS  -> RFS-GPON-PORT, RFS-BNG-SESSION
    CFS-VOICE-SIP  -> RFS-IMS-SUBSCRIBER

  usage rating rules                  consumed by RATING
    voice_domestic  included, fair use 3000 min, then INR 0.30/min
    voice_intl      INR 4.50/min, rated in 30-second increments
    data            unmetered, shaped to 2 Mbps beyond 3.3 TB

Read it as three views of one record. Ordering takes the characteristics and turns them into steps; provisioning takes the bundled specifications and turns them into network configuration; rating takes the usage rules and turns call records into charges. Nothing here is code, which is the point — a new offering is a catalogue change plus a price, not a release.

The failure this prevents is the most expensive routine failure in a BSS estate. When rating carries its own copy of the tariff, a plan launched on Monday is unrated until someone configures the rating engine, and unrated usage is revenue leakage that nobody notices because no error is raised. The mirror case is worse for the customer: rating falls back to a default tariff, invoices go out wrong, and the cost is a dispute queue plus a regulator interested in why.

Two details a candidate should offer unprompted. The catalogue is versioned and effective-dated, because customers stay on withdrawn offerings for years, so rating must resolve the tariff that applied on the date of the call rather than the current one. And the decomposition is a specification, not an instance: the catalogue says a fibre product needs a GPON port, while the inventory says which port this customer got.

Four entities that candidates collapse into one and interviewers separate deliberately. The customer is the legal party with the contract — a person or a company. The account is the billing arrangement, holding the currency, the billing cycle, the payment method and the balance, and one customer may have several. The subscriber is the user of a service, identified on the network rather than commercially, which is why a company can have a customer record, ten accounts and forty thousand subscribers. The service is the specific instance sold and provisioned — this line, this broadband circuit. Conflating them breaks immediately in the enterprise segment, where the payer, the user and the contracting entity are three different things.

Five identifiers, and the interview question is which travels with what.

subscriber "Priya Rao", one mobile line

  IMSI    404450123456789    identifies the SUBSCRIPTION on the network
                             MCC 404 | MNC 45 | MSIN 0123456789
                             lives on the SIM, used for authentication
                             and for HLR/UDM lookup. Rarely exposed.

  MSISDN  919845012345       identifies the DIALLABLE NUMBER
                             the commercial identity, printed on the bill,
                             portable between operators, mapped to an IMSI

  ICCID   8991450123456789012  identifies the SIM CARD itself
                             printed on the plastic, used in logistics
                             and stock management, not in call setup

  IMEI    35-209900-176148-1 identifies the HANDSET
                             burned into the device, travels with the
                             device across every SIM put into it

  IMPU/IMPI sip:+919845012345@ims.example  identifies the IMS identity
                             used for VoLTE registration and routing

The IMSI and the MSISDN are the pair that matters. The IMSI is the network's name for the subscription and effectively an internal key; the MSISDN is the customer's name for it and is portable, reassignable and recycled. Their mapping is many-to-one over time and is data, not identity — which is why using the MSISDN as a primary key fails, and why the IMSI is what a charging system keys on.

What travels with the handset is the IMEI, and only the IMEI. Move the SIM to a new phone and the IMSI, MSISDN and ICCID are unchanged while the IMEI changes; that asymmetry is what makes IMEI blocklists the mechanism for stolen devices, and what lets an operator report which handset models are on its network without asking anyone.

The ICCID is the one candidates omit and the one warehouse and eSIM flows depend on, because until the SIM is issued to somebody it is stock with a serial number and no subscriber at all. With eSIM the plastic disappears and the eUICC identifier plus a downloadable profile take its place, which turns SIM logistics into a provisioning problem — an improvement operationally and a new set of systems to build.

Because both are ratios the business optimises and both are computed from systems you build. ARPU — average revenue per user — falls whenever data becomes cheaper faster than volume grows, so operators chase revenue in bundles, content and enterprise services, each of which is a new product shape the catalogue must express without a release. Churn means a subscriber who leaves takes their acquisition cost with them, so retention offers must be applied mid-cycle, which is precisely the proration and mid-cycle-change machinery that is hardest to get right. When an interviewer asks what your system does for the business, these are the two numbers the answer connects to.