When you hear terms like SFP, DAC, CWDM, or 100G, it can feel confusing at first. These technologies are common in data centers, ISPs, and modern enterprise networks.
This guide breaks them down in a simple, practical way that anyone can understand.
SFP, SFP+, SFP28, QSFP - What Are These?
These are small, pluggable modules that go into switches, routers, servers, and transmission devices. They allow network equipment to connect using fiber or copper.
Common Types
- SFP (1G) – Used for 1 Gigabit connections
- SFP+ (10G) – Used for 10 Gigabit links
- SFP28 (25G) – Used for 25 Gigabit links
- QSFP+ (40G) – QSFP means Quad SFP, used for 40G
- QSFP28 (100G) – Used for 100G data center links
- QSFP56 (200G) – Used for high-throughput environments
- QSFP-DD (400G) – Used for ultra-high-throughput environments
- QSFP-DD800 (800G) – Used for super-ultra-high-throughput environments
 |
| Cable connector types and speeds |
These modules can operate over fiber or copper, depending on the specific type.
Understanding Network Speeds: 1G to 400G
Here’s a simple overview of common speeds and where they’re used:
Speed Technology Common Use
1G SFP, RJ45 Home ISPs, small offices
10G SFP+ ISPs, data centers, enterprise core
25G SFP28 Modern servers, cloud infrastructure
40G QSFP+ Aggregation layers
100G QSFP28 Data centers, carrier networks
200G QSFP56 High-performance computing
400G QSFP-DD Large ISP backbone, hyperscale cloud
Higher speeds provide the bandwidth required for cloud computing, virtualization, CDNs, and modern applications.
RJ45 - The Connector Everyone Knows
RJ45 is the standard Ethernet connector found in homes, offices, and many ISP networks. It’s used with copper cables such as Cat5e, Cat6, Cat6A, and Cat8.
 |
| RJ45 |
Key Points
- Commonly supports 1G
- Can support 10G over Cat6A or Cat8
- Not suitable for long distances due to higher signal loss compared to fiber
RJ45 is simple and cheap, but fiber is preferred as speeds and distances increase.
DAC Cables (Direct Attach Copper)
DAC cables are pre-assembled twinax copper cables with SFP or QSFP connectors on both ends. They don’t require separate transceivers or fiber.
 |
| SFP+ 10G Passive DAC Twinax Cable |
Why Use DAC?
- Very cheap compared to fiber
- Extremely low latency
- Ideal for short distances (typically 1–7 meters)
- Widely used in data centers for:
- Switch-to-server connections
- Switch-to-switch links
For short runs inside racks, DAC is often the best choice.
Optical (Fiber) Cables - The Backbone of High-Speed Networks
Optical cables (fiber optic cables) use light instead of electricity to transmit data. This allows much higher speeds, longer distances, and better resistance to interference than copper cables. Fiber is the standard choice for data centers, ISPs, backbone networks, and modern enterprise environments.
Single-Mode vs Multi-Mode Fiber
There are two main types of optical fiber:
Single-Mode Fiber (SMF)
- Very thin core (usually 9 µm)
- Designed for long distances
- Uses laser-based optics
- Commonly labeled OS2
- Used by:
- ISPs
- Backbone networks
- Long-distance data center interconnects
Distance: tens to hundreds of kilometers (with the right optics)
Multi-Mode Fiber (MMF)
- Larger core (50 µm or 62.5 µm)
- Designed for short distances
- Uses cheaper optics (VCSEL)
- Common types:
- OM3 (10G / 40G / 100G short range)
- OM4 (longer reach than OM3)
- OM5 (wideband, less common)
Distance: typically up to a few hundred meters
Common Fiber Cable Types
- LC–LC – Most common connector for SFP, SFP+, SFP28, QSFP
- MPO / MTP – Used for high-speed links (40G, 100G, 400G)
- Simplex – One fiber (rare in Ethernet)
- Duplex – Two fibers (Tx/Rx), most common
What Are CWDM and DWDM?
CWDM and DWDM are wavelength multiplexing technologies that allow multiple signals to travel over a single fiber. This dramatically reduces fiber costs.
CWDM (Coarse Wavelength Division Multiplexing)
- Uses fewer wavelengths
- 1270, 1290, 1310, 1330, 1350, 1370, 1390, 1410, 1430, 1450, 1470, 1490, 1510, 1530, 1550, 1570, 1590, 1610
- Lower cost equipment
- Medium-distance transmission
- Common in metro networks and ISP aggregation
DWDM (Dense Wavelength Division Multiplexing)
- Supports many more wavelengths (very high capacity)
- Works over very long distances
- Used for backbone, long-haul, and submarine fiber
- Expensive but extremely powerful
Think of CWDM as a local train and DWDM as a bullet train.
CWDM = Local Train, because a local train:
- Stops at fewer stations
- Is slower
- Uses simpler infrastructure
- Is cheaper to operate
That matches CWDM (Coarse Wavelength Division Multiplexing):
- Fewer wavelengths (typically 8–18 channels)
- Wider spacing between wavelengths
- Lower total capacity
- Short to medium distances (metro, access networks)
- Lower cost optics (often uncooled lasers)
- Little or no optical amplification
CWDM is ideal when you just need to move traffic across a city or between nearby sites without high complexity.
DWDM = Bullet Train, because a bullet train:
- Travels very fast
- Handles many passengers at once
- Covers long distances
- Requires precise, expensive infrastructure
That fits DWDM (Dense Wavelength Division Multiplexing):
- Many wavelengths (40, 80, 96+ channels)
- Very tight spacing between wavelengths
- Massive total capacity
- Long-haul distances (hundreds to thousands of km)
- Requires optical amplifiers (EDFA/Raman)
- Uses cooled, highly precise lasers
- Supports advanced modulation and error correction
DWDM is what powers ISP backbones, inter-city links, and submarine cables.
How Fiber Relates to SFP, CWDM, and DWDM
- SFP / QSFP modules plug into devices and connect to fiber cables
- CWDM and DWDM run over single-mode fiber
- One fiber pair can carry:
- A single 10G/100G signal
- Or dozens of wavelengths using DWDM
Fiber is what makes modern high-capacity large networks possible.
Quick Rule of Thumb
- Inside a rack: DAC
- Inside a data center: Multi-mode fiber
- Between buildings / cities: Single-mode fiber + CWDM
- Long-haul / backbone: Single-mode + DWDM
Credits
Inspired by this LinkedIn post, improved with support of ChatGPT, and reviewed by me.
No comments:
Post a Comment