What are the differences between SDH and PDH

Compared to traditional SONET or SDH, the next generation of DWDM (Dense Wavelength Division Multiplexing) is seen as a simple architecture with high scalability, capacity add / drop, multiple ring terminations, multi-services and multiple fabrics. In the following article we will compare SONET vs. SDH vs. DWDM and explain the exact differences.

SONET / SDH: Basics


SONET / SDH is the predominant technology used in most metro and long-distance networks. It refers to a group of fiber optic transmission rates that can carry digital signals with different capacities. Since their appearance on standardization bodies around 1990, SDH and its variant SONET (used in North America) have vastly improved the performance of fiber optic telecommunications networks. The basic unit of SDH is the synchronous transmission module Level-1 (STM-1). The basic unit of the SONET is the level-1 optical carrier module (OC-1). The other rates OC-3, OC-12, OC-18, OC-18, OC-24, OC-24, OC-36, OC-48, OC-96 and OC-192 are derived from this base rate.


TDM-based networks with PDH (Plesiochronous Digital Hierarchy) and SDH / SONET have long served as standard transport platforms for cellular traffic. PDH and SDH / SONET are optimized to handle bulk voice lines with maximum uptime, minimum delay and guaranteed service continuity. SDH was designed to replace the PDH system for interoperability between devices from different manufacturers. The signal hierarchy defined several line rates, among which STM-1 (155Mbps), STM-4 (622Mbps), STM-16 (2.5Gbps) and STM-64 (10Gbps) as well as STM-256 (40Gbps) are widely used.

DWDM: basics

DWDM is considered to be one of the best technologies for increasing the bandwidth compared to an existing fiber optic system. It makes it possible to create several "virtual fibers" over one physical fiber. It does this by transmitting different wavelengths (or colors) of light over a piece of fiber. DWDM was originally introduced by long-distance network operators because the expense of amplification, dispersion compensation, and regeneration made up the majority of the cost of network equipment in regional and national SONET networks. DWDM became increasingly popular in metro networks as local exchange operators expanded their networks. Apart from the depletion of the fiber, the volume of traffic is the most important economic factor for the use of DWDM technology in metro networks.

DWDM channel frequencies

DWDM works in the range between 1530 and 1565 nm, the so-called C-band, which corresponds to the low-loss window of the optical fiber. This is the area in which the erbium doped fiber amplifier (EDFA) operates. A grid of permissible wavelengths / operating frequencies per ITU-T, centered at a frequency of 193.1THz or a wavelength of 1553.3nm, and all types of frequencies at intervals of multiples of 25GHz (= 0.2nm) around this center frequency. Commercial systems can have channels at 2.5Gbps, 10Gbps and 40Gbps (the latter having recently become commercial) in addition to combinations of these channels in the same system. The higher the bit rate, the greater the need for power, which means that the lasers must have better signal-to-noise values. The distance between the amplifiers must be reduced, the gain must be higher, for example by using two optical DWDM amplifiers in series. Typically 64 DWDM channels at 10Gbps reach a maximum distance of about 1,500km with an amplifier spacing of almost 100km. Long distance transmission systems over 1,500km and up to 4,500km will also be commercially available with advanced and much more expensive systems.

Application of DWDM technology

The DWDM layer is protocol and bit rate independent, which means that it can transmit ATM (Asynchronous Transfer Mode), SONET and / or IP packets at the same time. WDM technology can also be used in passive optical networks (PONs), i.e. in access networks in which all transport, switching and routing are carried out in optical mode. With the inclusion of newer 3R devices (Reshape, Re-Time, Re-Transmit) that are integrated into the DWDM system, circuits can now be set up nationwide that only use DWDM devices. New performance monitoring functions are randomly built into these devices so that maintenance and repair of the link can be performed. With DWDM as the transmission method, the bandwidth of the existing fiber optic system is maximized.

SONET / SDH vs. DWDM: which is better?

The main differences between SDH and DWDM are listed below:

Number of topological structures for the same traffic More Fewer
costs Lower Higher
Application scenario The perfect choice when the design requires fewer than 4-10 OC-192 rings. Usually for the previous network structure. The perfect choice when the design calls for more than 10 OC-192 rings. Better for future network construction.
Number of optical fibers required More Fewer

The need for SONET granularity is due to the presence of low speed service requests, e.g. DS1 / DS3, and the insufficient maintenance of the sub-wavelengths within the DWDM platforms. DWDM aims to transmit the same traffic in as few topological structures (e.g. rings) as possible. DWDM does this at the expense of the ability for some visitors to travel greater distances than the SONET. For any simple example, the very best SONET design can create a bundle of topologically different OC-48 and OC-192 rings, while the most economical DWDM transport network can consist of just a single DWDM ring.

Traditionally, Internet traffic is over IP, which is then routed over ATM and SONET / SDH or IP over SDH to an optical layer (as shown in the figure below). The misconception that IP runs over ATM / SDH is the fact that IP traffic is small and needs to be combined with other services for cost-effective delivery. In contrast to this deception, the concept of IP over DWDM supports voice, video and data traffic and dedicates the rest to high-speed traffic. The elimination of layers (SONET / ATM) simplifies the task of network management and is cost effective.

Figure 1: Internet traffic on SONET / SDH and DWDM

SONET: the past

As expected, the SONET scenarios have a low initial cost. When the traffic volume is low, a SONET architecture is much more economical compared to the DWDM architecture. The modeling of FS shows that when designing a SONET overlay network with OC-3, OC-12, OC-48, and Gigabit Ethernet requirements, a SONET network is the perfect choice if the design is less than 4-10 OC-192 rings required.

DWDM: Today and in the future

As the volume of traffic increases, sooner or later DWDM will prevail and become the preferred network technology. The timing of this transition depends, among other things, on the spans, pricing and interface density. The differences between the types of demand are mainly due to the design efficiency of the interface cards of these two technologies in terms of density and price. The study by FS also shows that span spacing typically creates the additional need for regenerators, optical amplifiers and DCMs in the lines. Long spans tend to favor a DWDM architecture because of the efficient use of fibers and optical bypass capabilities at intermediate nodes.

In addition, higher fiber costs and situations where fiber restrictions are enforced lead to more consideration for DWDM than SONET because DWDM saves a tremendous amount of fiber within the optical network. DWDM systems could be planned for a large number of channels, but a pay-as-you-grow strategy can be used and channels added based on demand. The amplifier distance and total system power budget must be calculated from the start for the final number of channels.

Figure 2: SONET vs. DWDM


Different alternatives and their economic implications in designing the very same network is definitely an interesting topic. SONET is still doing better. However, these results do not apply in all situations. The implication is that in large network designs, the most optimized network need not necessarily have a single architecture. One part of the network can take on rings while another part implements point-to-point. Usually the core part of the network will justify a DWDM architecture.