tdma vs scpc technical note

TDMA vs. SCPC

SatLink System Technical Notes

1

1. Background TDM/TDMA and SCPC are the main alternative technologies for satellite networking in

the world today. The modem and management technologies underlying both

approaches have been advancing rapidly in recent years, causing some confusion

as to which technology is better for a given set of networking requirements. This

technical note will explain the important trends and trade-offs.

These two alternative technologies are illustrated in Figure 1 and Figure 2 for a simple

star topology network, highlighting their key elements and configuration differences.

Figure 1: TDM/TDMA Network (Time Division Multiplexing with Time Division Multiple Access)

Figure 2: SCPC Network (Single Channel Per Carrier)

TDMA vs. SCPC Technical Note

2

SCPC uses a separate dedicated carrier to each remote terminal ("VSAT") to receive

information from the central site, and another dedicated carrier for each VSAT to

transmit information back to the central site. Both carriers are modulated in

"continuous mode". Usually some non-standard coding techniques are used since

SCPC technology was never standardized.

In contrast, TDM/TDMA technology uses a single high-speed TDM carrier transmitted

from the central site or "Hub", from which many VSATs can receive information. For

this TDM forward link, the DVB-S2 standard is most commonly used. It is also the most

flexible for multiplexing many concurrent streams of traffic to different sites, and the

most efficient with its support of Adaptive Coding and Modulation (ACM). ACM

dynamically adjusts the modulation and coding on the "virtual link" to each VSAT

individually, as local conditions (e.g., weather, interference) at the VSAT change.

To transmit back to the central site efficiently, the VSATs in a TDM/TDMA network are

synchronized, and they transmit information in "burst mode" within a series of short,

scheduled timeslots. Timeslots may be assigned across multiple TDMA carriers and

accessed using "fast frequency hopping". Timeslots are assigned to each VSAT

exclusively (i.e., without contention) based on their current traffic needs. This is called

Dynamic TDMA, and it is the most advanced form of TDM/TDMA. This technology is

fully standardized internationally by the DVB group under the DVB-RCS family of

standards.

TDM/TDMA networks allow all VSATs to dynamically share multiple TDMA carriers, as if

they were a single large pool of bandwidth. Each TDMA carrier group may contain

dozens of carriers, with up to 32 carriers per carrier group in a SatLink® network.

Therefore the "return link" may contain huge amounts of capacity, in aggregate.

In a SatLink TDM/TDMA network the TDMA carriers may operate at widely different

symbol rates (e.g., from 500 ksps to 5 Msps and even higher). To determine which

VSAT will use which timeslots on which carrier at any moment, SatLink has ACS

(Adaptive Carrier Selection). ACS is applied dynamically for each VSAT, given its

local weather conditions, configuration (e.g., antenna and BUC size) and service

policy (e.g., maximum rate requirements). ACS determines what carrier & symbol

rate will work best at the current signal levels of those available in the carrier group.

In addition, in a SatLink DVB-RCS2 ("2nd Generation") network, ACM per burst is

supported for each VSAT and on all TDMA carriers in the carrier group. This further

optimizes efficiency, throughput, and reliability for each VSAT and greatly simplifies

network operations. Any VSAT can use any MODCOD, on any carrier, if necessary.

DVB-RCS2 SatLink TDM/TDMA networks now surpass SCPC networks not only in

efficiency, but also in throughput and link availability for almost any conceivable

network configuration and satellite band (e.g., C, Ku, X, and Ka band).

2. Total Cost-of-Ownership Comparison Today, SCPC only makes economic or technical sense for very small networks; that is,

those with only a handful of remote sites and relative low-speed links, where total

transponder capacity requirements are low (e.g., just a few MHz) and opportunities

for bandwidth sharing are limited.


3

For most larger networks (e.g., with >20 sites and many MHz of transponder

capacity), the use of TDM/TDMA will result in much lower operating cost (OPEX) than

incurred with SCPC technology. Typically transponder requirements can be reduced

by 50% to 80% or more, while also increasing average link speeds, application

throughputs, and overall link availability for "free".

The relative financial advantage of TDM/TDMA vs. SCPC is shown in Figure 3.

Figure 3: Number of Sites Required to Justify TDM/TDMA vs. SCPC

In this cost analysis OPEX is the dominant consideration. The reason a TDM/TDMA

network is not easily justified when there are just a few sites is due to the fixed capital

cost of the Hub for a TDM/TDMA network, which is higher than the cost of a few

SCPC modems.

In the range of 20 to 50 sites, the total transponder OPEX, plus details of the network

traffic patterns and various user requirements, must be examined more closely to

determine which technology offers a lower cost of ownership.

Using teleport hub services to support a small number TDM/TDMA VSATs can also shift

the balance in favor of TDM/TDMA, enabling "virtual private" TDM/TDMA networks

with just a few VSATs that replace SCPC with good savings for network operators.

3. Statistical Multiplexing of IP Traffic Across Sites One major reason for the ascendency of TDM/TDMA over SCPC in the last decade is

that user traffic in satellite networks is increasingly all IP (Internet Protocol), regardless

of whether it is data, voice, or video. Most TDM/TDMA networks are optimized for

handling IP traffic of all types with excellent QoS. This is especially true of SatLink.

In particular, interactive IP data traffic (e.g., web browsing, email, downloads) is

notoriously bursty. This results in huge statistical multiplexing gains from using

bandwidth-on-demand. These efficiency gains can reduce aggregate bandwidth

To

tal C

os

t o

f O

wn

ers

hip

(OP

EX

+ A

mo

rtiz

ed

CA

PE

X)

Number of Sites in the Network

50 100 150 200


4

needs, and therefore transponder OPEX, by 10x or more, depending on two key

traffic pattern attributes:

the peak-to-average load ratios for traffic at each site (Tx and Rx) during the

peak hour, and

the variability in the timing of the peak hour, across the various sites.

Even video, audio, and voice traffic have peak load timings and magnitudes that

are variable throughout the month, the week, and the day. And their loads fluctuate

during the peak hour as well. Thus they all benefit from using bandwidth-on-demand

which matches actual load requirements second-by-second. This benefit is

especially applicable when using the most efficient new codecs for these traditional

"real-time" media types, when transporting them over IP links.

Furthermore, getting the maximum feasible peak speed for each VSAT site has

become very important for all types of IP applications and media. Having just 500

kbps or 1 Mbps per site as the maximum rate in a satellite network is no longer

acceptable to most user sites (with a few exceptions). User sites in many satellite

networks now demand peak load receive (Rx) rates of 10 to 40 Mbps or more, and

peak load transmit (Tx) rates of 3 to 20 Mbps, or more. And these rates are only going

to increase in coming years.

Higher peak transfer speeds increase the peak-to-average load ratio per site during

the peak hour, and therefore greatly favor the trend to use TDM/TDMA and replace

SCPC in all forms.

4. Hybrid TDM/SCPC Networks Due to the increasing dominance of IP traffic, many former SCPC networks have

already been converted to TDM/TDMA. However, some SCPC networks have

converted only "half-way", whereby a DVB-S2 TDM carrier is used on the forward link,

but SCPC links are used for return link communications. This is illustrated in Figure 4.


5

Figure 4: TDM/SCPC Hybrid Network

This hybrid configuration is called "TDM/SCPC" for convenience here. If using DVB-S2

it gets the full benefits of statistical multiplexing and ACM on the forward link, but

these benefits are non-existent on the return link in this hybrid network . Therefore, the

technical and business rationales for using the TDM/SCPC hybrid networks are weak

at best.

Nonetheless, the TDM/SCPC hybrid configuration is commonly promoted and used in

certain types of VSAT networks, in particular in cellular backhaul networks and in

some other types of networks where fast access to large amounts of capacity for the

return link (upstream) traffic must be guaranteed.

There are three possible reasons for the continued use of this form of SCPC:

A belief that SCPC ("continuous mode" ) will provide better modem efficiency

(in bps/Hz) than TDMA burst mode due to lower overhead and ability to use

higher-rate, more efficient MODCODs.

A belief that SCPC links are better at providing guaranteed capacity and will

operate more reliably against rain fades, interference, or congestion.

A belief that SCPC links will provide lower latency or less total delay.

A belief that SCPC links can be operated at a higher speed, when necessary,

for any or all sites within the satellite transponder footprint.

These beliefs (or some of them) are true with respect to the limitations of some

popular TDM/TDMA technologies. For those technologies, the hybrid TDM/SCPC

option is useful and may even be "cost effective" in networks with nearly constant

levels of traffic in the peak hour at each site, a consistent peak hour time each day.

However, in comparison to SatLink TDM/TDMA networks using the DVB-RCS2

standards, these conditions do not hold true. In fact, the opposite is true.

The SatLink DVB-RCS2 implementation exceeds or matches the modem performance


6

specifications of all but the most expensive SCPC modems with information rates > 24

Mbps for Tx.

In terms of total network efficiency, a SatLink DVB-RCS2 return link (operating in TDMA

burst mode !) will deliver 2x more in bps/Hz than some popular SCPC options, even

before adding in the benefits of statistical multiplexing with TDMA.

Table 1 compares the return link performance of SatLink DVB-RCS2 TDMA modems

against the SCPC return link modem options of two leading suppliers: Comtech EF

Data and iDirect.

Table 1: SatLink TDMA vs. SCPC Modems (for Return Links Only)

Attribute SatLink TDMA

(DVB-RCS2)

COMTECH

CDM-625 SCPC

Modems

iDirect's SCPC Option for

Evolution®

Return Link

Technology Used DVB-RCS2 standard

16-State Turbo Code

(Proprietary)

VersaFEC™ (Proprietary)

2D 16-State

MODCODs supported

QPSK 1/3 to 5/6

8PSK 2/3 to 5/6

16QAM 3/4 to 5/6

BPSK: .488

QPSK .533 to .803

8QAM .642 to .780

16QAM .731 to .853

QPSK 1/2 to 4/5 (Plus: 8PSK 4/5 & 6/7

but only if symbol

rate is >1.5 Msps)

Adaptive Carrier Selection

(ACS)

Yes (dynamically selects

best carrier & symbol

rate for each burst)

No (Fixed Carrier)

No (Fixed Carrier)

ACM on Return Link Yes, included

ACM per TDMA Burst (DVB-RCS2 standard)

Yes (Proprietary)

Extra Cost Option No

Minimum Return Link

Symbol Rate 125 ksps 18 ksps

128 ksps (1.5 Msps at 8PSK)

Maximum Return Link

Symbol Rate 8 Msps 12.5 Msps 15 Msps

Maximum Return Link Info Rate (after subtracting TDMA burst overheads

for SatLink DVB-RCS2 modems

24 Mbps at 16QAM 5/6

& 8 Msps

14 Mbps (in ACM mode)

24 Mbps at QPSK 4/5

& 15 Msps (QPSK required for this max)

Maximum Return Link (Tx) VSAT

IP Throughput (after physical layer overheads

w/ specified equipment)



15.6 Mbps (for X5 router)

20 Mbps (for 8000 series)

Return Link SNR required

for same bps/Hz & error rates (after all overheads)

See SatLink

modem performance

documentation

+/- 0.3 dB vs. SatLink

+/- 0.3dB vs. SatLink

Average Return Link

Spectral Efficiency (after SatLink's burst mode overheads

& required minimum carrier spacing

applied to each modem)

1. 6 to 2.6 bps/Hz with 8PSK & 16QAM at

1.12x carrier spacing

1.6 to 2.6 bps/Hz 8PSK & 16QAM at


0.8 to 1.3 bps/Hz

with QPSK at


Because no ACM,

see Section 4.2

Link Availability Best (with ACS & ACM)

Good (with ACM)

Poor, or will reduce

efficiency & speed

(without ACM)

Latencies & Delays <40% of COMTECH 16 milliseconds

(at 512 kbps

& 16QAM)

same as SatLink (at 512 kbps & QPSK

w/ 438 byte payload)

NOTE: We assumed here the COMTECH modem is configured to use their VersaFEC, which is required if


7

using their ACM feature. Their latency could be less if using a different FEC.

The COMTECH CDM-625 modem is normally used in a traditional SCPC network

where both forward and return links are SCPC. COMTECH also has the CDM-840

modem, with similar SCPC specs, but which works with a central site DVB-S2 TDM

modem for the forward link in multi-site multiplexed basis (i.e., TDM/SCPC hybrid).

The iDirect SCPC option can be used on any iDirect VSAT in their Evolution® product

line (with the DVB-S2 forward link) and that VSAT may be part of an iDirect

TDM/TDMA network, managed from the same NMS.

Sections below further explain the key points of Table 1 and why SatLink TDMA burst

modems using DVB-RCS2 match or exceed the SCPC modem performance of these

two suppliers.

4.1 SatLink's Advanced TDMA MODCOD Support with ACS & ACM

SatLink DVB-RCS2 burst modems are the first to provide high-order MODCODs (i.e.,

both 8PSK and 16QAM) with the newest, most advanced FEC (Forward Error

Correction) technologies using 16-state turbo codes for industry-leading efficiency

and total performance, with carrier spacing of only 1.12x symbol rate.

Furthermore, this is done to work at symbol rates up to 8 Msps, and IP throughput

rates up to 24 Mbps on Tx (from VSAT-to-Hub) on TDMA burst mode carriers. These

rates are more than 4x higher than many TDMA burst modems. And they work

concurrently with all TCP acceleration, IP routing, IP packet filtering, and QoS

features operating at these same speeds.

SatLink DVB-RCS2 burst modems are also the first and only to support both ACS and

ACM per burst, dynamically adapted to the immediate local conditions at the VSAT

at these very high speeds.

The combination of all this is powerful and unique in the satellite networking industry.

In addition, SatLink DVB-RCS2 modems have excellent signal-to-noise (SNR)

requirements, which puts them slightly lower than the very best SCPC modems for the

same spectrum efficiency (bps/Hz) and bit error rate results. And they have very low

burst overheads.

Therefore even after subtracting burst overheads, SatLink modem efficiency matches

or exceeds that of SCPC modems, in terms of the bps/Hz and Quasi-Error Free (QEF)

performance, at all comparable operating points for SNR (C/N or Eb/No.)

(Note: we assume here that the "long-burst" option is used on SatLink's DVB-RCS2

modems, which is fair when comparing with SCPC options. The differences in

required SNR vs. these other technologies are typically +/- 0.3 dB, depending on the

MODCODs used for comparison. All SatLink efficiency figures in bps/Hz shown have

burst overheads subtracted from their nominal efficiency and adjust for carrier

spacing differences.)

4.2 Lack of ACM Reduces iDirect's Average Efficiency

Because iDirect has no support for ACM on their SCPC return links, this means it is

unlikely they can operate at their most efficient MODCODs (8PSK 4/5 and 6/7). In


8

most Ku and Ka band networks, iDirect VSATs will need to use QPSK 1/2 to QPSK 4/5

to have sufficient fade margin with typical Ku or Ka band transponders and typical

VSAT configurations (0.98 to 1.8 meter antennas with 2W to 4W BUCs), to get 99.7%

availability or better (unless operating in a desert).

Also iDirect requires using QPSK in two important and common scenarios:

QPSK must be used to get the maximum data throughput rates at the VSAT

(because 8PSK is very "processing intensive" for them). This means that the

higher symbol rates normally allowed with iDirect's SCPC option (those from 10

Msps to 15 Msps) cannot be used with 8PSK.

QPSK must also be used for symbol rates less than 1.5 Msps. This is unfortunate

because SCPC links often need lower symbol rates than 1.5 Mbps.

Therefore iDirect SCPC links will use QSPK 1/2 to QPSK 4/5 most commonly (requiring

SNR levels of 2.2 to 6.0 dB, respectively, during fades). They will have corresponding

efficiencies of 0.8 to 1.3 bps/Hz, according to iDirect modem documentation.

In contrast, SatLink's modem efficiency in Ku and Ka band will average from 1.6 and

2.6 bps/Hz, by using 8PSK and 16QAM, or 2x better efficiency than iDirect's SCPC, with

typical transponders and VSAT configurations. This is because the use of ACM allows

"clear sky" link budgets (or nearly so) to prevail for >90% of the time in most regions of

the world, greatly increasing average network efficiency across all sites.

Furthermore, due to statistical multiplexing of IP traffic, the required capacity of the

TDMA carrier group, in Mbps, is much less than the sum of the required SCPC return

links would be (typically 2x to 8x less), depending on traffic patterns across the sites.

4.3 Link Availability Advantages of SatLink with ACS and ACM

When using ACS & ACM within a TDMA carrier group there are advantages not

possible when using ACM alone on individual SCPC links. This explains why SatLink's

overall link availability will be better than what COMTECH can obtain in any

comparable network. The differences will be most noticeable in Ku and Ka band

networks where rain fades can be large.

With ACS in a SatLink network, it is possible for the Hub to dynamically change the

carrier symbol rate used by a VSAT. This is done by selecting a different carrier in the

carrier group for its burst. When there is severe local fading at a VSAT, simply

reducing the MODCOD with ACM may not be sufficient. Also that reduces

bandwidth efficiency.

With ACM and ACS working together in a SatLink network, the MODCOD and the

symbol rate for a VSAT can both be reduced during a severe fade. This allows the

VSAT's return link to keep operating during the most severe fade (albeit at a lower

information rate). Maintaining return link connectivity during fades is as important as

maintaining forward link connectivity, since all monitoring of the VSAT (including the

ACM control feedback for the forward link) relies upon the return link.

The lowest link availability, though, will tend to occur when there is no ACM or ACS

available, as with the iDirect SCPC link (and by the way, also true with iDirect TDMA


9

links). In those cases, increased link availability can be obtained only by setting the

"fixed" MODCOD to be used all the time and the symbol rate as low as possible,

which hurts speed, throughput, and efficiency.

4.4 SatLink TDM/TDMA Delivers Lower Latencies than SCPC

TDMA return link carriers in a SatLink network using DVB-RCS2 standards deliver lower

latency than SCPC links. This is accomplished in three ways:

Being able to operate at high information rates, up to 24 Mbps, to reduce

serialization delays.

Using relatively small FEC block sizes (=burst sizes) compared to SCPC

modems. For example, only 540 symbols for the "small burst" and 1620 symbols

for the "long burst" vs. 4,000 symbols for the FEC blocks used by COMTECH's

VersaFEC®.

Using advanced methods for assigning bandwidth-on-demand so that there

are no delays for receiving capacity assignments once activity has begun.

The relatively low information rates common on many SCPC carriers (e.g., 500 kbps to

2 Mbps) used to avoid excessive amounts of dedicated capacity to a single site are

often a large source of delays from an end-user perspective when uploading emails,

photos, or other large data options.

The same is true for the TDM Forward Link carrier in a TDM/TDMA network. It will have

much lower latency than each of many SCPC forward link carriers operating lower

symbol rates and low information rates.

5. Use of DVB-S2 for "SCPC" Return Link Some suppliers (e.g., Gilat) are now using DVB-S2 TDM carriers as their "SCPC " return

link option, within the context of the hybrid TDM/SCPC configuration of Figure 4.

Using DVB-S2 in this way has some strong points and also creates some issues.

The strengths are that it is very efficient and supports ACM (unlike iDirect's proprietary

SCPC option). The weaknesses are:

It will have a high latency very low symbol rate carriers (e.g., < 500 ksps) even

when using the short frames option.

It is more expensive for each site, and requires one dedicated DVB-S2

receiver at the Hub site for each remote site using the SCPC option.

It is not feasible to switch rapidly between DVB-S2 (continuous mode carriers)

and a TDMA burst mode carriers in the event that site needs the option to use

either of two modes.

When compared to SatLink's implementation of DVB-RCS2 for TDMA (i.e., burst

mode), the efficiency of DVB-S2 modems are very similar. They are only slightly


10

better (less than 0.5 dB of SNR difference for the same bps/Hz, in the higher rate

MODCODs). Even the overhead differences are small (a few %) when comparing

similar FEC frame sizes. And because it is not possible to use "frequency hopping"

among DVB-S2 carriers there is no option for using ACS to adjust symbol rates

dynamically.

And, of course, as with all SCPC options on the return link, there is no ability to gain

from statistical multiplexing of traffic across multiple transmitting sites.

6. Advantages of SatLink TDM/TDMA for Cellular Backhaul Given the results above, the advantages of using a SatLink TDM/TDMA network for

cellular backhaul applications (vs. a TDM/SCPC network) can be significant.

Most of the advantage will come from statistical multiplexing the return link traffic

across multiple transmitting sites. With the rapid growth of cellular data, even return

link traffic is rapidly becoming mostly data traffic (e.g., for email, photo uploads,

video uploads, etc.), with a smaller and smaller share as cellular voice traffic.

Even the voice traffic in cellular networks, given the advance of 3G and 4G, is being

carried over IP with variable rate codecs, plus silence suppression in certain regions

of the world.

This means the statistical multiplexing gains of using TDMA for return link traffic may

easily exceed 2x for a satellite network with just 5 or more cell sites. That means 50%

less transponder capacity is required for the return link capacity.

SatLink networks also offer many efficiency advantages for the DVB-S2 forward link,

with MODCODs up to 32APSK (not offered by iDirect, nor on the COMTECH CDM-

625), which become feasible when investing in large remote site antennas. Thus

forward link capacities can reach well over 150 Mbps using only 45 Msps.

The ACM feature on SatLink DVB-S2 carriers also provides the most advanced QoS

and traffic engineering features to assure each cell site receives its necessary

capacity and quality for delay sensitive voice and video traffic during rain fades or

other forms of interference or congestion.

7. Option for an "SCPC-like" Dedicated Carrier in SatLink Sometimes in a customer network, a special technical or security reason may exist

that requires a dedicated return link carrier for a given VSAT site. Even though SatLink

does not support an SCPC option in "continuous mode", it is simple to assign a TDMA

carrier to a given SatLink VSAT for all its return link transmissions.

This is done easily in a SatLink network via a configuration in the SatLink NMS.

However, the VSAT will still use burst mode transmissions on this dedicated carrier. In

SatLink this is called the "SCPC-like" mode.

As we explained above there is no penalty from using burst mode transmissions for

creating this "SCPC-like" mode in a SatLink network. And it still allows that VSAT to use


11

ACM per burst, and all the advanced QoS features of SatLink.

Of course, preventing a VSAT from using ACS (i.e., hopping across carriers based on

immediate needs) is not usually a good network design strategy. For example, VSAT

transmissions could be impaired if interference or jamming at some frequency affects

its assigned carrier, or if it needs to operate at a higher or lower symbol rate to

transmit at higher speed or improve its SNR during rain fades.

Therefore, no efficiency, speed, or reliability benefits are gained from assigning a

dedicated return link carrier to a specific VSAT in a SatLink network.

8. Justification for Conversion from SCPC to TDM/TDMA We saw in Figure 3, earlier, that the total cost of ownership will usually favor using

TDM/TDMA over SCPC for networks with 20 sites or more. Likewise the conversion

from an old ("fully depreciated") SCPC network can usually be justified when the

number of sites is growing to 20 or 30 or more.

Conversion to TDM/TDMA is driven by the combination of following:

Increased business efficiency, revenue, and value from higher reliability,

speeds, throughputs, and more total network capacity.

OPEX savings from reduced transponder capacity requirements.

OPEX savings from reduced operations staff.

OPEX savings from reduced hardware and software maintenance fees on

SCPC equipment and software features.

Sometimes financial justifications can be found for a TDM/TDMA network upgrade

even if the network is composed of only 5 sites. The degree of savings will depend on

the following:

Average load per site (Gigabytes per hour) in the peak hour for Tx and Rx.

Degree of randomness in peak hour timing across sites.

Mix of traffic types (data, voice, video streaming, video conferencing).

Star topology only, or star and mesh topology needs.

Link availability requirements during worst month.

Satellite band(s) to be used (e.g., C, Ku, X, or Ka band).

Geography of the network (e.g., how many sites within tropical vs. temperate

vs. arid environments) or sites at low look angles to the satellite.

How widely scattered remote sites will be, and whether there will be diverse

VSAT antenna sizes, mobile VSATs, or other special site requirements.

STM will assist any potential customer with a fair and detailed analysis, including


12

documentation of all assumptions, to help them evaluate whether SCPC or

TDM/TDMA will deliver better performance relative to their network requirements,

and how both the OPEX and CAPEX for the network will compare under each

alternative. It is worth noting that networks with more than 50 sites easily justify

TDM/TDMA except in rare situations.

9. Bandwidth Cancellation Options for SCPC and TDMA In doing a cost comparison with SCPC it may be necessary to consider the optional

use of a bandwidth cancellation option, such as one that COMTECH provides for its

CMD-625 modems.

Bandwidth cancellation is a technique that allows forward and return link carriers to

share the same bandwidth (i.e., to overlap). This potentially reduces total

transponder requirements. When offered by COMTECH this feature has the trade

name of DoubleTalk™ (it is also called "Carrier-in-Carrier"). iDirect does not offer this

feature with their SCPC option.

Double-Talk is an expensive, extra cost option on the CDM-625 SCPC modems which

must be applied to all the modems in use in a COMTECH network (both central site

and remote) to fully benefit from this feature.

In theory, Double-Talk can reduce the total bandwidth required for a network by up

to 50% if the forward and return bandwidth requirements for each site are equal. This

is rarely the case. If not equal, the savings are only as much as the smaller carrier.

In practice, though, there are some side effects which further reduce the feasible

savings on transponder costs even more. These include:

Reduction in the MODCODs that can be used reliably (due to some

interference and generally lower SNR levels that result from overlapping the

carriers) and therefore some loss of efficiency .

Increase in transponder power required per MHz, and since more power is not

free this may keep transponder costs higher than expected.

Bandwidth cancellation options are also available for TDM/TDMA networks, but work

in a different manner. The cancellation equipment resides only at the Hub site. This

option is not frequently used in TDM/TDMA networks because of the complexities it

adds to the network and the large asymmetry in the bandwidth requirements of

forward vs. return link capacity. Also the same reasons mentioned above for SCPC

networks reduce the attainable gains in TDM/TDMA networks.

As noted earlier, using TDM/TDMA (vs. traditional SCPC) can easily reduce the total

capacity requirements of a network by 10x due to statistical multiplexing. Therefore

saving just 15% to 45% of the SCPC transponder capacity by using bandwidth

cancellation is a small reduction vs. saving 90% or more using TDM/TDMA.


www.stmi.com

STM Group, Inc. | 2 Faraday | Irvine, CA 92618, USA | T +1 949 273 6800 | FAX +1 949 273 6020 STM Norway AS | Vollsveien 21 | 1366 Lysaker, Norway | T +47 6753 5337 | FAX +47 6753 5335 All specifications and features subject to change without notice. SatLink and the STM logo are a registered trademarks of STM Group, Inc. Document # 200660; Revision B - 130206 © 2013

13

It is particularly important to note the "Double-Talk" bandwidth cancellation feature

for the COMTECH modems is only available when using traditional SCPC links, on

both forward and return links. It does not work in the hybrid TDM/SCPC mode.

Therefore all statistical multiplexing gains are lost when using "Double-Talk" on both

the forward and return traffic.

Therefore use of bandwidth cancellation does not fundamentally alter the significant

OPEX advantages of TDM/TDMA over SCPC, but it does significantly increase the

CAPEX of SCPC networks (by as much as 6x more) depending on the information

rates of the links.

The price of just ten (10) CDM-625 modems – for five (5) SCPC links – operating at 5

Mbps with the Double-Talk feature can easily justify the purchase of a TDM/TDMA

Hub.

Thus, the cost advantages of TDM/TDMA presented earlier still apply when

comparing against SCPC with bandwidth cancellation.

tdma vs scpc technical note

Documents

carrier tdma

tdma carrier group

satlink tdmtdma network

tdmtdma technology

multiple tdma carriers

scpc technology

satlink network

carrier symbol rate