Fibre network topologies and architectures

Two transceivers and a fibre make a link; a network is what you do with hundreds of them. The way links are arranged — point-to-point, ring, tree, mesh, star — decides how many fibres a route needs, what happens when a cable is cut, which optics each end carries and how the plant is documented. This page walks through the topologies used in data centres, campuses, access networks and transport, the structured-cabling hierarchy that standards prescribe, FTTx architectures, fibre-count planning and protection schemes — with the optics implied by each.

Fibre network topologies: point-to-point, protected ring, PON tree, WDM on one pair

Basic topologies

Topology	Structure	Fibres per link	Protection	Typical optics	Where
Point-to-point, duplex	A ↔ B on two fibres	2	none (or a second diverse pair)	SR/LR/ER, DAC/AOC	everywhere
Point-to-point, single-fibre BiDi	one fibre, two wavelengths	1	none	BiDi pairs (1310/1490, 1270/1330)	fibre-scarce routes, FTTx
Point-to-point WDM	N channels on one pair via mux/demux	2 for N services	per system	CWDM/DWDM modules (CWDM, DWDM)	metro, campus interconnect
Star / hub-and-spoke	every site to a central node	2 per spoke	dual-homing to two hubs	LR/ER by distance	campus, enterprise WAN
Ring	nodes in a loop, traffic both ways	2 (single ring) or 4 (dual)	inherent: second path	LR/ER, DWDM with OADMs	metro, carrier access, utilities
Tree (PON)	one OLT fibre split 1:32–1:128 to ONUs	1 feeder, 1 per subscriber	Type B/C protection optional	PON optics classes (How PON works)	FTTH
Mesh	many-to-many	many	rerouting	ROADM/coherent	core transport
Leaf–spine (folded Clos)	every leaf to every spine	2 per link	multipath by design	SR4/DR4/AOC (Roles & topologies)	data centre
Bus / linear add-drop	nodes along one route	2	none	OADM	pipelines, railways

Structured cabling hierarchy (TIA-568 / ISO 11801)

Element	Role	Fibre choice	Notes
Entrance facility	where carrier cables enter; transition outdoor → indoor	as carrier	outdoor cable may run ≤ 15 m indoors unless rated (NEC 770)
Equipment room / main cross-connect (MC)	core switches, ODF	OS2	all backbones terminate here
Backbone (vertical / campus)	MC ↔ intermediate (IC) ↔ horizontal cross-connects (HC)	OS2 (OM4 for short intra-building)	≤ 2 000 m campus SMF; 12–48 fibres per closet typical
Telecommunications room (TR/HC)	floor distribution	—	access switches
Horizontal cabling	TR → work area outlet, ≤ 90 m	copper mostly; fibre in centralized cabling (≤ 300 m OM3/OM4)	fibre-to-the-desk rare but exists
Data centre (TIA-942)	MDA → IDA → HDA → EDA (ZDA optional)	OM4/OS2 trunks with MPO	ToR, EoR, MoR patterns below

Data-centre cabling patterns

Pattern	Cabling	Optics
Top-of-rack (ToR)	servers to a switch in the same rack over DAC; switch uplinks to spines over structured fibre	DAC ≤ 3 m; SR4/DR4/AOC uplinks
End-of-row / middle-of-row (EoR/MoR)	servers cabled across the row to a switch at the end/middle	AOC or SR optics 10–30 m; more fibre, fewer switches
Structured MPO trunks + cassettes	12/24-fibre trunks between MDA and HDA/EDA, LC breakout in cassettes	any duplex optic; polarity method A/B/C (Breakout & MPO)
Direct point-to-point MPO	trunk straight between QSFP ports (SR4/DR4)	parallel optics; fewer connectors
Rail-optimised (AI)	each GPU NIC to a different leaf	AOC/DR4 mostly (Cabling an AI cluster)

FTTx architectures

Architecture	Fibre reaches	Last segment	Active equipment	Optics
FTTH — PON	the home	1 fibre from splitter	OLT + ONU	GPON/XGS-PON classes; splitters 1:32–1:64
FTTH — P2P Ethernet (AON)	the home	dedicated fibre per home	Ethernet switch + CPE	1G/10G BiDi single-fibre
FTTB	the building	copper (Ethernet/VDSL/G.fast) inside	building switch/DSLAM	1G/10G LR uplink
FTTC / FTTN	cabinet / node	VDSL over copper	street DSLAM	1G/10G LR, CWDM
FTTA	the antenna	fibre to the RRU	RRU/AAU	10/25G fronthaul optics (Fronthaul)
FTTx with WDM-PON / TWDM	the home	1 fibre, wavelength per user/group	NG-PON2	tunable ONU optics

Split architecture in PON: centralized (one 1:64 splitter at the cabinet — flexible, one splice point) vs cascaded/distributed (1:4 then 1:16 along the street — less feeder fibre, more closures, ~0.5 dB extra) (ODN classes).

Fibre-count planning

Rule	Reasoning
2 fibres per duplex service, 1 per BiDi/PON subscriber	baseline
Add 50–100 % spare on backbone routes	civil works cost dwarfs fibre cost; pulling later costs more
Never fewer than 12 fibres in any outdoor cable; 24–48 between buildings; 96–288 on metro trunks; 864+ on ducts feeding data centres	granularity of splicing and ribbon counts
Plan WDM instead of fibres where duct space is the constraint	8–96 services on one pair
Separate fibres (or cables) for protection paths	a second pair in the same cable does not survive a dig-up
Reserve fibres for monitoring (RFTS) and future PON splitters	—
Dark fibre lease vs wavelength/Ethernet service	dark fibre: you choose optics, you manage the plant; managed wavelength: provider's optics rules

Protection and diversity

Scheme	How	Switch time	Fibre cost
Unprotected	one path	—	1×
1+1	traffic sent on two diverse paths, receiver picks	< 50 ms	2×
1:1 / 1:N	standby path activated on failure	50 ms–seconds	2× or shared
Ring (ERPS G.8032, MPLS-TP, OTN, SNCP)	reroute the other way around the ring	< 50 ms	ring fibres
Mesh restoration	control plane finds a new path	seconds	shared spare capacity
PON Type B / C	redundant OLT port / trunk / full duplication	seconds	+1 feeder fibre
Dual-homing (LAG/MLAG/ECMP to two nodes)	L2/L3 multipath	sub-second	2× access links (Forwarding basics)

Diversity means physically separate routes: different ducts, different building entries, different street sides. Two cables in one trench share the excavator.

Latency

Medium	Delay	Note
Standard fibre	≈ 4.9 µs/km (n ≈ 1.47)	100 km ≈ 0.5 ms one way
Hollow-core fibre	≈ 3.3 µs/km	trading, HPC (Fibre types)
Copper / microwave / free space	≈ 3.3 µs/km	microwave beats fibre on straight paths
Add per hop	switches 1–10 µs, routers 10–100 µs, FEC/DSP in 400G+ optics 100–300 ns, coherent DSP µs	equipment, not glass

Documentation of a topology

Every fibre network needs: a route map (GIS), cable and fibre numbering, splice diagrams per closure, ODF port maps, cross-connect records, test results per fibre and a change log — kept current (Maintenance & restoration, Installation & routes).

In CodingBox

Topology decides the optics list: BiDi pairs for single-fibre spokes, PON classes for trees, CWDM/DWDM channels for shared pairs, parallel SR4/DR4 for MPO trunks. CodingBox verifies each delivered module against that list — wavelength, channel, PMD, power class — before it goes to the site (Check transceiver, Code database).

Keeping the topology knowable — identifier schemes, labels, splice schedules, GIS and records: Documentation & labelling; the boxes at the nodes of these topologies — transponders, ROADMs, OLTs, protection switches, media converters: Transport & access equipment.