SDH (Synchronous Digital Hierarchy) and DWDM (Dense Wavelength Division Multiplexing) are both technologies used in the field of optical networking, but they serve different purposes and operate at different layers of the network. 

1. Purpose and Functionality:

• SDH (Synchronous Digital Hierarchy): SDH is a standardized technology for synchronous data transmission over optical fibers. It provides a synchronous and highly efficient way to transport digital data, including voice, data, and video. SDH is primarily used for the transport of high-capacity data over long distances.

• DWDM (Dense Wavelength Division Multiplexing): DWDM, on the other hand, is a technology that enables the simultaneous transmission of multiple signals at different wavelengths (or colors) over a single optical fiber. It is designed to increase the capacity of optical networks by utilizing multiple wavelengths on the same fiber.

2. Layer of the OSI Model:

• SDH: SDH operates at the data link layer (Layer 2) and the network layer (Layer 3) of the OSI model. It is concerned with the transport and multiplexing of digital signals.

• DWDM: DWDM operates at the physical layer (Layer 1) of the OSI model. It is focused on the transmission of optical signals over the physical medium (fiber) and the multiplexing/demultiplexing of wavelengths.

3. Multiplexing:

• SDH: SDH uses time-division multiplexing (TDM), where different signals are assigned specific time slots within a predefined time frame. This allows multiple signals to share the same transmission medium.

• DWDM: DWDM uses wavelength-division multiplexing (WDM), allowing multiple signals to coexist on the same fiber but using different wavelengths. Each wavelength operates independently, and the signals do not interfere with each other.

4. Capacity:

• SDH: SDH provides a standardized hierarchy of rates, such as STM-1 (155 Mbps), STM-4 (622 Mbps), STM-16 (2.5 Gbps), and so on. These rates define the capacity of the SDH links.

• DWDM: DWDM significantly increases the capacity of optical fibers by allowing multiple wavelengths to be transmitted simultaneously. It can support very high data rates and scalability to accommodate the growing demand for bandwidth.

5. Application:

• SDH: SDH is commonly used for traditional telecommunications applications, providing a reliable and standardized method for transporting voice and data.

• DWDM: DWDM is widely used in long-haul and metro optical networks to increase the capacity and efficiency of data transport. It is particularly effective for high-capacity data center interconnects and backbone networks.

In summary, while SDH is focused on efficient and synchronous transport of digital data, DWDM is geared towards maximizing the capacity of optical fibers by allowing multiple signals to be transmitted simultaneously at different wavelengths. They can complement each other in building high-capacity and efficient optical networks.