SONET/SDH is now a legacy technology that is in maintenance mode.  That means no new development, and ever-increasing prices for spares will emerge. It is a bygone thing and is replaced by a more affordable, fast, and easily accessible technology. We are talking about Ethernet, the technology that has replaced SDH in more than one ways.

Technically, SONET/SDH is a TDM technology. Say for example, if there is no data to send, the bandwidth is wasted.  Ethernet, on the other hand, is very happy to provide stat mixing.  This generally leads to higher utilizations.  Since it is Ethernet, it’s easily interoperable with other Ethernet technologies, so it’s more flexible.  And at lower speeds (<100Gbps), it’s a commodity, so the price can’t be beat. SONET/SDH never made it there.

What is the topic all about?

Helping CSPs to use legacy networks like SDH while moving on to newer technologies like Ethernet.

The Audience Connect:

Who can be benefitted?

So!! You are a Communication Service Provider who have put-in huge investments for the set-up of Synchronised Digital Hierarchy Network (SDH) across the nation and want to continue to use them. At the same time you want to accommodate more nodes(devices), have better utilization of the resources, higher bit-rates(speed) and travel longer link distances using the newer technologies being developed for the last mile (customer end) like Ethernet. The question here is – Can we use the current SDH-based legacy systems at the Access/Transport layer while moving the last mile and Core Networks to the Ethernet technology? The answer is – Yes! How?? We answer the how as we read further.

 The knowledge gain:

What you will gain from this article – Here, we will talk about how the information (packets) flow through the High speed Ethernet last mile to SDH(Channelized) Access Network and switches back to Ethernet based Transmission Network and ends up in the Core Network of the Service Provider. We will also cover:

  • How to stitch an end to end circuit/connectivity from Access to Transmission to Core Networks where Last Mile, Transmission and Core are Ethernet but Access is legacy SDH channelized connectivity.
  • Possibilities of achieving the benefits of Ethernet’s higher speed and better bandwidth utilization without decommissioning legacy SDH networks.

Understanding communication network concepts:

  • What is a network?
  • Parts of network
    • Core
    • Access
    • Transmission

Understanding various communication technologies ( What is SDH / Ethernet etc..)

  • Plesiochronous Digital Hierarchy(PDH) is a technology used in telecommunications networks to transport large quantities of telephone calls and data over digital transport equipment such as fibre optic and microwave radio systems nearly synchronised.
  • Synchronous Digital Hierarchy (SDH) is the standardized protocol that was developed to improve over the PDH system (transfer of multiple digital bit streams over optical fiber using lasers or light-emitting diodes) synchronously for transporting large amounts of telephone calls and data traffic over the same fibre. The SDH standard defines a number of ‘Containers’ each corresponding to an existing PDH input rate. Information from the incoming PDH signal is placed into the relevant container. Each container then has some control information known as the ‘Path Overhead’ (POH). The container and the path overhead together form a ‘Virtual Container’ (VC).
  • Ethernet is a way of connecting computers together in a local area network (LAN), metropolitan area networks (MAN), and wide area networks, that is – basically multiple computers have access to each other and can send data at any time. Ethernet cables look like thick telephone cables and connect to boxes called hubs or switches. Each cable runs from a computer’s network interface card (NIC) to such a box. This cable is called 10BaseT or 100BaseT, or 1000BaseT Cable. There are different Ethernet standards being used in various speed requirements ranging from 10 Mbits per second to faster signals like 100Mbps or much faster speeds such as 1GBits or even 10GBits per second. Hence it is being used for the last Mile(user or customer end) and easily runs over legacy-SDH due to compatibility and can be again switched back to Ethernet based – Transmission and Core Networks.

Ethernet-over SDH

  • Customer interfaces are Ethernet, while Network interfaces are SDH
  • For existing networks the new EoS solution maps Ethernet frames to nx2Mbps using new concatenation technologies
  • This in effect translates into greater flexibility in provisioning, bandwidth adjustments
    • Bandwidth upgrades can happen through software simplifying capacity purchases
    • Lays the foundation for new applications such as time-of-day provisioning, bandwidth-on-demand, etc.

Functions needed for mapping Ethernet to SONET/SDH:

  • Framing protocol to encapsulate data packets to generate an SONET/SDH payload
  • Defined mapping of the SONET payload to SONET channels
    • SONET/SDH channels are either higher-order or lower order virtual tributaries (eg., VT’s or STS-n’s)
    • Capacity of one or more channels can be allocated
    • When multiple VTs are allocated, they need not be contiguous
  • Contiguity restriction lacks flexibility and is not desirable
  • Flow control mechanism to avoid packet drops due to mismatch in Ethernet port speed (10/100/1000 Mbps) and allocated SONET bandwidth (nx2 Mbps, nx51 Mbps)
  • Mechanism to increase/decrease allocated SONET/SDH bandwidth


  • Helping network engineers with view capacity utilization, reroute connections in case of fault.
  • Rate-controllable: Dedicated “leased-line” bandwidth adapted to user demands in multiples of SDH VCs at the finest granularities possible (multiples of “E1”, “DS1” or “DS3”)
  • Operators can significantly reduce equipment cost by building a unified TDM+IP network
  • One can work seamlessly with existing TDM (SDH or SONET) infrastructure
  • Use of integrated Ethernet eliminates the need for costly WAN interfaces (E1/E3/STM-1) on routers connecting to SDH/SONET
  • WAN bandwidth can be provided directly from customers’ Ethernet switches, potentially eliminating routers at customer sites
  • Software-control enables services like bandwidth-on-demand even for short time-periods
  • SDH grade sub-50 ms protection and hence has high levels of network reliability.


Use case – Running Ethernet over existing SDH network

How it is achieved?

  • EoS provides point-to-point LAN  services over an SDH backbone: 10/100/1000 Mbps Ethernet tributaries (client ports) directly on the SDH equipment used to provide “leased lines”
  • UIM to the rescue (brief implementation)
  • Oracle UIM provides the feature of mapping Ethernet over SDH and also provides visualization in its UI.


  • From the following screen shot, it could be clearly seen that how 10MBPS Ethernet signal is riding over SDH.
  • 10MBPS is distributed over 5 various VC12s and mapped for end to end connectivity.

For now, Ethernet is the most sustainable and affordable solution for network communication. It not only sorts out the challenges faced using SDH but also allows flexibility in its use.

Telecommunication is fast changing, and with times the older ways or technologies of communication either becomes obsolete or comes to an end. With Ethernet, the SDH technology is going through a similar fate and has taken over it.  We might come across many more innovations and methods used to communicate but until then it’s the Ethernet phase!