SPECIFICATION OF VSAT SERVICES AND HARDWARE PROPOSAL

EMC Satcom Technologies
Vollsveien 21 N-1366
Lysaker, Norway
15 January 2015
TDMA vs. SCPC
Page 1
TDMA vs. SCPC
SatLink® Technical Note
Doc # 200660
15 January 2015
TDMA vs. SCPC
Page 2
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)
15 January 2015
TDMA vs. SCPC
Page 3
Figure 2: SCPC Network
(Single Channel Per Carrier)
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.
15 January 2015
TDMA vs. SCPC
Page 4
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).
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.
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.
Total Cost of Ownership
(OPEX + Amortized CAPEX)
Figure 3: Number of Sites Required to Justify TDM/TDMA vs. SCPC
50
100
150
200
Number of Sites in the Network
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.
15 January 2015
TDMA vs. SCPC
Page 5
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.
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
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-ondemand 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.
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.
15 January 2015
TDMA vs. SCPC
Page 6
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 specifications of all but the most
expensive SCPC modems with information rates > 24 Mbps for Tx.
15 January 2015
TDMA vs. SCPC
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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
Return Link
Technology Used
SatLink TDMA
(DVB-RCS2)
COMTECH
CDM-625 SCPC
Modems
iDirect's
SCPC Option for
®
Evolution
DVB-RCS2 standard
(Proprietary)
(Proprietary)
16-State Turbo Code
VersaFEC™
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
(Plus: 8PSK 4/5 & 6/7
but only if symbol rate is
>1.5 Msps)
Adaptive Carrier Selection
(ACS)
(dynamically selects
best carrier & symbol
No
No
(Fixed Carrier)
(Fixed Carrier)
QPSK 1/2 to 4/5
Yes
rate for each burst)
Yes, included
Yes
ACM per TDMA Burst
(DVB-RCS2 standard)
(Proprietary)
Extra Cost Option
Minimum Return Link
Symbol Rate
125 ksps
18 ksps
Maximum Return Link
Symbol Rate
8 Msps
12.5 Msps
Maximum Return Link Info Rate
24 Mbps
(after subtracting TDMA burst overheads
for SatLink DVB-RCS2 modems
at 16QAM 5/6
& 8 Msps
14 Mbps
(in ACM mode)
24 Mbps
14 Mbps
(after physical layer overheads
w/ specified equipment)
(in ACM mode)
(in ACM mode)
Return Link SNR required
for same bps/Hz & error rates
See SatLink
modem performance
documentation
ACM on Return Link
Maximum Return Link (Tx) VSAT
IP Throughput
(after all overheads)
Average Return Link
Spectral Efficiency
(after SatLink's burst mode overheads
& required minimum carrier spacing
applied to each modem)
15 January 2015
No
128 ksps
(1.5 Msps at 8PSK)
15 Msps
24 Mbps
at QPSK 4/5
& 15 Msps
(QPSK required for this max)
15.6 Mbps
(for X5 router)
20 Mbps
(for 8000 series)
+/- 0.3 dB
+/- 0.3dB
vs. SatLink
vs. SatLink
0.8 to 1.3 bps/Hz
1. 6 to 2.6 bps/Hz
1.6 to 2.6 bps/Hz
with 8PSK & 16QAM at
1.12x carrier spacing
8PSK & 16QAM at
1.25x carrier spacing
with QPSK at
1.2x carrier spacing
Because no ACM,
see Section 0
TDMA vs. SCPC
Page 8
Attribute
Link Availability
SatLink TDMA
(DVB-RCS2)
COMTECH
CDM-625 SCPC
Modems
Best
Good
(with ACS & ACM)
(with ACM)
16 milliseconds
Latencies & Delays
<40% of COMTECH
(at 512 kbps
& 16QAM)
iDirect's
SCPC Option for
®
Evolution
Poor, or will reduce
efficiency & speed
(without ACM)
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 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.
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 VSATto-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.)
15 January 2015
TDMA vs. SCPC
Page 9
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 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.
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 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.
15 January 2015
TDMA vs. SCPC
Page 10
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.
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 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.
15 January 2015
TDMA vs. SCPC
Page 11
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.
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 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.
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TDMA vs. SCPC
Page 12
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.
EMC will assist any potential customer with a fair and detailed analysis, including 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.
15 January 2015
TDMA vs. SCPC
Page 13