Mobile traffic still ends up on fibre. Behind every antenna is a fixed network, and operators also sell fixed broadband, TV and business connectivity. This module covers the "wired world".
1. Fixed access technologies
| Technology | Medium | Typical downstream / upstream | Distance | Notes |
|---|---|---|---|---|
| POTS / PSTN | copper pair | analogue voice | Old telephone line, 300–3400 Hz | |
| ISDN | copper | 2×64 kbit/s (BRI), 30×64 (PRI) | Digital voice + data | |
| ADSL | copper | up to 8 / 1 Mbit/s (ADSL2+: 24 / 1) | ≤ 5 km | Asymmetric DSL, DMT modulation |
| VDSL2 | copper | up to 100 / 50 Mbit/s (profile 17a) | ≤ ~1 km | FTTC (fibre to the cabinet), vectoring cancels crosstalk |
| G.fast / XG-FAST | copper | 0.1–1 Gbit/s | ≤ 250 m | Fibre almost to the building |
| Cable (HFC), DOCSIS | coaxial TV cable | DOCSIS 3.0 ~1 Gbit/s; 3.1 up to 10 / 1–2 Gbit/s; 4.0 up to 10 / 6 Gbit/s | Shared medium, CMTS at the head-end, cable modem at home | |
| FTTH / GPON | fibre | GPON 2.5 / 1.25 Gbit/s shared; XGS-PON 10 / 10; NG-PON2 40 / 10 | up to 20 km | Passive splitters, no electronics in the field |
| FTTH P2P (Active Ethernet) | dedicated fibre | 1–10 Gbit/s | Costlier, symmetric | |
| FWA | 4G/5G radio | 50–1000 Mbit/s | Fixed Wireless Access with CPE | |
| Satellite | 25–300 Mbit/s | Module 17 | ||
| WiMAX (802.16) | radio | Largely obsolete | ||
| Power-line (PLC), G.hn | electrical wiring | In-home |
Naming FTTx: FTTH (home) · FTTB (building) · FTTC (cabinet) · FTTN/FTTdp (node/distribution point) · FTTA (antenna, fibre to a cell site).
2. How a PON (passive optical network) works
OLT ───── feeder fibre ───── splitter 1:32 / 1:64 ─── drop fibres ─── ONT/ONU at homes
| Term | Meaning |
|---|---|
| OLT | Optical Line Terminal — the operator's equipment in the exchange |
| ONT / ONU | Optical Network Terminal / Unit — the box at the customer |
| ODN | Optical Distribution Network — fibres and splitters |
| Splitter ratio | 1:32, 1:64 (up to 1:128): users sharing one PON port |
| Downstream | Broadcast on 1490 nm (GPON), upstream 1310 nm (TDMA, each ONU a time slot); video overlay 1550 nm |
| TDM-PON | GPON, XG(S)-PON (time-division) |
| WDM-PON | One wavelength per user (future) |
| OMCI | ONU Management and Control Interface |
| Ranging | OLT measures each ONU's distance so upstream bursts do not collide |
| Budget | Optical power budget in dB (class B+ 28 dB, C+ 32 dB) |
| Fibre types | Single-mode (SMF, OS2, G.652) for telecom (9 µm core) vs multi-mode (OM3/OM4) inside buildings |
| Connectors | SC/APC (green) in access, LC in data centres, E2000; APC = angled polish to reduce reflections |
| Splicing | Fusion splice (low loss, 0.05 dB) vs mechanical |
| OTDR | Optical Time-Domain Reflectometer: finds breaks and loss along a fibre |
| Loss budget | ~0.35 dB/km at 1310 nm, ~0.2 dB/km at 1550 nm + connector/splice losses |
3. Optical transport (core and metro)
| Term | Meaning |
|---|---|
| Wavelength | Colour of light carrying data; C-band 1530–1565 nm |
| CWDM | Coarse WDM: up to 18 channels, 20 nm spacing — metro/mobile fronthaul |
| DWDM | Dense WDM: 40–96+ channels, 100/50 GHz spacing; up to ~Tbit/s per fibre |
| ROADM | Reconfigurable Optical Add/Drop Multiplexer: remote rerouting of wavelengths |
| OTN | Optical Transport Network (ITU G.709): OTU2/OTU4 containers for 10G/100G, with FEC |
| EDFA | Erbium-Doped Fibre Amplifier — amplifies light directly |
| Coherent optics | 100G/400G/800G with phase + amplitude modulation and DSP |
| Submarine cables | Transoceanic fibre; ~95+% of intercontinental traffic; landing stations, repeaters every 50–100 km |
| IRU / dark fibre / lambda | Ways to lease fibre capacity |
| Latency | Light in fibre ≈ 200,000 km/s → ~5 µs per km (≈ 1 ms per 100 km one way) |
4. Enterprise and carrier services
| Service | What it is |
|---|---|
| Leased line (E-Line / EPL) | Dedicated point-to-point link with fixed bandwidth |
| E-LAN / VPLS / EVPN | Multipoint Ethernet LAN service across sites |
| MPLS L3VPN | Private IP network over the operator's MPLS (customer routing via BGP/OSPF) |
| Internet Access (DIA) | Dedicated Internet Access with SLA |
| SD-WAN | Software-defined WAN: combine MPLS, internet and 4G/5G links with central policy and application-aware routing |
| SASE / SSE / ZTNA | Cloud-delivered security and zero-trust access |
| SIP trunking / hosted PBX / UCaaS / CCaaS | Voice and collaboration as a cloud service (Teams, Webex, Zoom Phone) |
| MPLS-TP / Carrier Ethernet | Packet transport replacing SDH |
| SLA parameters | Availability (99.9 % = 8.76 h/year downtime; 99.99 % ≈ 53 min; 99.999 % ≈ 5 min), latency, jitter, packet loss, MTTR |
| Colocation / IXP | Data-centre space / Internet Exchange Point: operators exchange traffic (CIX in Croatia, DE-CIX, AMS-IX) |
| CDN / peering / transit | Content near users / free mutual exchange / paid access to the global internet |
| AS number / BGP | Autonomous System identity; BGP hijacking and route leaks are threats; RPKI secures route origin |
5. Ethernet and IP essentials
| Topic | Key facts |
|---|---|
| Ethernet speeds | 10 Mbit/s → 100M → 1G → 10G → 25G → 40G → 100G → 400G → 800G |
| MAC address | 48-bit hardware address (6 bytes, e.g. 3C:22:FB:12:34:56) |
| VLAN (802.1Q) | 12-bit VLAN ID (1–4094); QinQ (802.1ad) double tagging |
| Spanning tree (STP/RSTP/MSTP) | Avoid loops in Ethernet |
| LACP (802.3ad) | Link aggregation |
| IPv4 | 32-bit address (e.g. 10.0.0.1), private ranges 10/8, 172.16/12, 192.168/16, CGNAT 100.64/10 |
| IPv6 | 128-bit, usually /64 per subnet, SLAAC, no NAT needed; mobile operators run IPv6-only + NAT64/464XLAT |
| Routing | Static, OSPF / IS-IS (inside an operator), BGP (between operators) |
| TCP / UDP / SCTP / QUIC | Reliable stream / datagram / multi-stream reliable (signalling) / UDP-based encrypted transport under HTTP/3 |
| Ports | 53 DNS · 80/443 HTTP(S) · 22 SSH · 5060/5061 SIP · 3868 Diameter · 2123 GTP-C · 2152 GTP-U · 2775 SMPP · 8805 PFCP · 2905 M3UA · 36412 S1AP · 38412 NGAP |
| MTU / fragmentation | Typical 1500 B; GTP tunnels add ~36–50 B so mobile MTU is usually ~1400–1450 |
| QoS | DSCP markings (EF for voice, AF classes), queueing, policing, shaping |
| BFD / OAM / SLM | Fast failure detection; delay measurement |
| NTP / PTP / SyncE | Time and frequency sync |
| IGMP / multicast | IPTV distribution |
| IPTV / OTT TV | Operator TV via multicast in managed network vs streamed OTT |
| DNSSEC / DoH / DoT | DNS security and privacy |
| DDoS | Volumetric (UDP amplification), protocol and application attacks; scrubbing centres, RTBH, BGP FlowSpec |
6. Data centre and cloud basics for telecom
| Term | Meaning |
|---|---|
| Leaf-spine | Data-centre fabric topology |
| VXLAN / EVPN | Overlay networks for multi-tenant clouds |
| SR-IOV / DPDK | Fast packet processing for UPF/vRouters |
| OpenStack / Kubernetes | Virtual infrastructure (VMs) / containers for CNFs |
| Telco cloud | Distributed data centres: central, regional, edge, far-edge |
| Helm / GitOps / CI-CD | Deployment automation |
| Observability | Metrics, logs, traces (Prometheus, Grafana, ELK) |
| YANG / NETCONF / gNMI | Model-driven configuration |
| Intent-based networking / closed-loop automation | Network corrects itself from monitored goals |
| ONAP, OSM, ETSI MANO | Orchestration platforms |
Remember this: fibre carries almost everything; PON serves homes, DWDM carries cores, MPLS/Carrier Ethernet carries services, and BGP glues operators together. Mobile networks are just a very demanding customer of this fixed infrastructure.