The QSFP-DD (Quad Small Form-factor Pluggable Double Density) and QSFP28 (Quad Small Form-factor Pluggable 28) modules share the same basic form factor, but the QSFP-DD module has additional connectors and electrical lanes to support higher data rates. Here are the key size differences between QSFP-DD and QSFP28: 

1. Width:

• The width of both QSFP-DD and QSFP28 modules is approximately 18.35 mm (0.722 inches). They have the same basic width, which allows QSFP-DD modules to physically fit into QSFP28 ports.

2. Depth/Length:

• QSFP-DD modules are typically longer than QSFP28 modules. The added length is necessary to accommodate the additional electrical connectors and lanes. The length of a QSFP-DD module is around 96 mm (3.78 inches), while a QSFP28 module is around 72 mm (2.83 inches).

3. **Height:

• The height of QSFP-DD and QSFP28 modules is the same. Both modules have a height of approximately 8.5 mm (0.334 inches).

4. Electrical Connectors:

• QSFP-DD features a total of eight electrical connectors (four on each side), while QSFP28 has four electrical connectors (two on each side). The additional connectors in QSFP-DD enable support for double the number of electrical lanes, allowing for higher data rates.

5. Data Rate:

• QSFP-DD supports higher data rates compared to QSFP28. QSFP-DD can support data rates of 200 Gbps or 400 Gbps by utilizing double the number of lanes compared to QSFP28.

In summary, while QSFP-DD and QSFP28 have the same width and height, the QSFP-DD module is longer due to the additional electrical connectors required to support higher data rates. The backward compatibility of QSFP-DD with QSFP28 ports allows for flexibility in network upgrades.