سوئچز کے کردار اور ٹوپولوجیز: کون سی آپٹکس کہاں لگتی ہے

ہماری ٹیم ترجمے پر کام کر رہی ہے۔ یہ مضمون عارضی طور پر اردو میں دستیاب نہیں ہے، اس لیے اسے انگریزی میں دکھایا جا رہا ہے۔

The same 100G module is a cheap DAC in one rack, an SR4 across the room, a LR4 between buildings and a DWDM channel between cities. Which one a port gets is decided by the switch's role in the network and the topology around it. This page maps the classic campus tiers and the data-centre leaf–spine to the optics, distances and counts they imply — so a bill of materials can be read off the design.

Campus three-tier hierarchy and data-centre leaf–spine with the optics on each link

Campus: access, distribution, core

Tier	Role	Typical ports	Downlinks	Uplinks and optics
Access	connects end devices	24–48 × RJ45 (1G/mGig, PoE) + 2–4 SFP+/SFP28 uplinks	copper	10G/25G SR (in-building MMF), LR (between buildings), sometimes 1G LX
Distribution / aggregation	aggregates access switches per building or floor group	24–48 × SFP+/SFP28, 4–8 × QSFP28	10G/25G SR/LR	40G/100G SR4/LR4 to core; CWDM/DWDM if fibre is scarce
Core	interconnects distribution, routes	QSFP28/QSFP-DD	100G/400G	LR4/FR4/ER4 between sites; CWDM or DWDM over leased fibre
Collapsed core	small sites: distribution + core in one	mixed	—	—

Campus distances are metres to a few kilometres: SR on OM3/OM4 inside buildings, LR/LX on single-mode between them; the reach classes in PMD reference.

Data centre: leaf–spine (Clos)

Role	Also called	Ports	Downlinks	Uplinks
Leaf	ToR (top of rack), access	48 × SFP28/SFP56 + 8 × QSFP28/QSFP-DD, or 32 × QSFP-DD	servers: 25G/50G/100G — DAC in rack (≤ 3 m), AOC/SR to adjacent racks	to every spine: 100G/400G SR4/DR4/AOC (≤ 100 m)
Spine	aggregation, fabric	32–64 × QSFP28/QSFP-DD/OSFP	leaves	to super-spine or border: DR4/FR4 (500 m–2 km)
Super-spine / core	pod interconnect	high-radix 400G/800G	spines	DR/FR within the campus; ZR/ZR+ for DCI (Coherent)
Border leaf	edge to WAN/internet	mixed	—	LR/ER/CWDM/DWDM to carriers
Management	out-of-band	1G RJ45/SFP	BMCs, consoles	1G/10G LR to the OOB core

Rules of thumb: leaf–spine links are all the same speed and FEC; oversubscription (downlink capacity ÷ uplink capacity) of 3:1 is common for general compute, 1:1 for AI and storage (GPU fabrics).

Rail-optimised AI fabrics

GPU nodes connect each NIC to a different leaf, so most server links leave the rack — AOC and optics instead of DAC — and the cable count per GPU is one per tier (Cabling an AI cluster).

Storage and SAN

Fibre Channel switches form their own fabric (edge–core or core–edge–core) with 16/32/64GFC SW optics in a room and LW between rooms; Ethernet storage rides the leaf–spine with lossless classes (SAN design, Lossless Ethernet).

Distance → optic

Segment	Distance	Medium	Optic
Server to ToR	≤ 3 m	copper	DAC (Cable internals)
Server to end-of-row leaf	3–30 m	AOC or MMF	AOC, SR
Leaf to spine, same hall	10–100 m	OM4 or SMF	SR4/SR8, DR4/DR8
Between halls / buildings	100 m – 2 km	SMF	DR, FR4, LR/LX
Campus / metro	2–40 km	SMF	LR, ER, CWDM, 4WDM
Regional DCI	40–120 km	SMF, amplified	ZR/ZR+, DWDM transponders
Long haul	> 120 km	SMF, amplified	DWDM line systems (Network design & OTN)

Fibre-scarce paths use CWDM or DWDM to multiply capacity per strand; single-fibre BiDi where only one strand exists.

Counting optics from a design

Example: one pod of 16 racks, 32 servers per rack, 2 × 25G per server, 4 spines.

Item	Count
Server links (2 × 25G × 32 × 16)	1 024 DAC
Leaf uplinks (16 leaves × 4 spines × 2 × 100G)	128 links → 256 QSFP28 SR4/DR4 + 128 MPO trunks
Spine to super-spine (4 spines × 8 × 100G)	32 links → 64 modules
Border + OOB	8–16 LR modules, 48 1G
Spares (5 %)	~20

Multiply by two for the second fabric if the design is dual-plane.

Roles and policy

Role also decides how strict the platform is about optics: carrier and SAN gear tends to enforce OEM branding; data-centre leaf–spine on merchant silicon is often permissive; campus access varies by vendor (How each NOS validates a module, Vendor lock).

In CodingBox

A role-based bill of materials is a list of PMDs and part numbers; CodingBox verifies each delivered module against it (type, reach, wavelength, PN) and codes identities where the design calls for OEM-compatible modules (Check transceiver, Code database).

Turning a design into a purchase and a working rack — selection criteria, optics bill of materials, commissioning checklist and acceptance test: Selecting & commissioning; the L2/L3 mechanics behind these topologies: Forwarding basics.

The cabling under these topologies — structured cabling hierarchy, data-centre patterns (EoR, MoR, ToR, trunks and cassettes), fibre-count planning: Fibre network topologies.

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