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# 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

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)