Muxing over a single satellite

Using the Access Processor platform it is possible to multiplex several terrestrial services across a single satellite link. This is most often multiple E1’s over HSSI or EIA530 Satellite Modems.

The Access Processor is a modular switch, based on an ATM backplane. It’s possible to mux ATM and NON-ATM traffic together into a single ATM cellstream. This makes it ideal for doing “odd” data rates over satellite, that do not fit into the PDH/SDH Hierarchy. For example, its easy to transport 4 x E1 using an E3 PDH Mux. You would use 8.448Mbps and a G.703 Satellite Modem. If you wanted to Mux 2,3,5+ x E1 its pretty difficult unless you use the APSAT.

Terrestrial Interfaces Supported

With our Quad E1 Module its possible for us to Mux up to 32 x E1 per AP8000L Chassis, and we can daisy chain a second Chassis to offer up to 60 x E1.

Using the Metrodata APSAT also gives you the added benefit of being able to transport other services across the satellite link, for example HSSI Data, E1 Voice, 10/100BaseT Data/IP and also ASI based Video.

The interfaces we support on the Terrestrial side are:

• E1, T1, E2, E3, DS-3, OC-3/STM-1
• Ethernet 10/100BaseT
• Serial EIA530
• ASI Video
• HSSI (High Speed Serial Interface)

Satellite Modem Interfaces Supported

Metrodata have developed a portfolio of products for interfacing satellite and terrestrial networks. On the APSAT, we have the following interfaces for Satellite

• G.703 – Fixed Telco rates on the Modem are T1, E1, E2, E3, DS-3
• HSSI – Up to 90Mbps
• ASI – Up to 155.52Mbps
• EIA530 – Up to 20Mbps

ATM Over Satellite  

The APSAT uses ATM to Multiplex and therefore transport the traffic over the satellite link. With E1 transport, this uses ATM AAL.1 Circuit Emulation over a HSSI/ASI or EIA530 interface.

AAL.1 uses a payload of 47 bytes, and adds 1 byte of Sequence Number and 5 bytes of ATM Cell Header.

• An E1 is 2,048Mbps or 2,048,000bits per second
• 2,048,000 divided by 8 = 256,000 bytes per second
  
• 256,000 divided by 47 = 5,447 ATM Cells per second

In order to get the overhead calculation for using ATM, you need to add on the ATM Cell Header.

• 5,447 x 53 = 288,681 bytes per second
  
• 288,681 x 8 = 2,309,447 bits per second or 2.309Mbps

The APSAT HSSI and EIA530 ATM Interfaces use G.832 framing to give link integrity and performance monitoring, which adds additional overhead. A G.832 frame comprises 4296 bits, of which only 4240 bits are payload.

Therefore to transport an E1 AAL.1 payload (2.309Mbps) using HSSI or EIA530 will require the G.832 framing to be added. (If you use ASI then you do not have this G.832 framing).

• 2,309,447 x 4,296 divided by 4,240 = 2,339,949 bits per second including G.832 overhead
  

Then we suggest allowing a 1% overhead margin, to allow for miscalculations:

• 2,339,949 x 1.01 = 2,363,348 bits per second or 2.363Mbps of satellite capacity is required per E1
  

This is shown in the following table: