Is Circuit Emulation Bandwidth-Efficient?

Let’s face it. As an operator, you are faced with a situation of deciding between circuit emulation over Ethernet and carrying TDM data as  pure TDM transport; what would be your choice?

In fact! There seems to be some confusion regarding the extent of efficiency when circuit emulation is used to carry TDM data over Ethernet. I came across two application notes offering two different opinions about the efficiency. One from Vendor Orckit you can find the application note at the following link.

http://www.orckit.com/ptn_technologies/196.htm

The application note is titled “Circuit Emulation Bandwidth Efficiency”. The application note contends that Circuit emulation is highly bandwidth-efficient. It concludes that 4006 VC-12 can be transported in 10GE versus 4032 in STM-64. This gives an efficiency of 99 .4% they conclude.

My curiosity took me to another link where Fujitsu describes an efficiency of around 50% when using circuit emulation. The application note can be found at the following link

http://www.fujitsu.com/downloads/TEL/fnc/whitepapers/Fujitsu_Wireless_Backhaul.pdf

So what exactly is the true situation? And why are the calculations leading to such a big difference.

Let’s take Orckit white paper as an example.

The white paper compares the number of VC-12s that can be inserted in 10GE versus STM-64. Simple mathematics of having 63 x VC-12 in STM-1 leads to 4032 VC-12s in STM-64.

Let’s do the calculation for how many VC-12s can be carried inside one 10GE.

Single VC-12=36 Bytes

Overhead needed to carry a single VC-12 is calculated as following

Overhead for Ethernet Frame = 14 Bytes (6 Destination MAC + 6 Source MAC + 2 Ether type)

Checksum=4 Bytes

Interframe Gap = 12 Bytes

Preamble = 8 Bytes

MPLS+PW Label Bytes = 4+4=8 Bytes

CEP (RFC 4842) Overhead = 8 Bytes (Ignoring RTP bytes for this example)

Total Overhead = 54 Bytes

Calculating the number of VC-12 services in 10GE = 10,000,000/54*8*8000= 2893 Services

Comparing against SDH STM-64(4032), the bandwidth efficiency is around 72%

This is actually different then Orckit arrived figure. Looking deeper, I found out that Orckit combined 21 VC-12 as one service to arrive at the bandwidth efficiency of 99 %. However combining more VC-12 in one service will not be delay efficient. Since more real time services need to be buffered before transmission.

So the conclusion is that carrying TDM traffic inside Ethernet is not bandwidth efficient. While there might be ways to improve bandwidth efficiency by combing more TDM channels emulated in one Ethernet frame but that will not be delay-efficient for real time services. Operators need to be aware of these issues when going for circuit emulation. In my view, the circuit emulation would be beneficial in cases where there is a lot of Ethernet traffic but there is a requirement to carry a small amount of TDM data inside that Ethernet network. Using Ethernet to carry predominant TDM data is not a very good idea owing to the issues of bandwidth efficiency as discussed in this
post.

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