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SCPC vs. TDMA

FAQ: Shared Bandwidth vs. Efficiency for Rural Satellite Mobile Backhaul

SCPC vs TDMA

Satellite mobile backhaul is changing significantly as new technologies and usages are making their way into the market. Data traffic is increasing while voice still occupies 80% of the traffic in rural and remote areas.

To help dispel the myths about TDMA versus current SCPC solutions for rural satellite mobile backhaul, we will examine some of the most frequently asked questions.

What are the key requirements for satellite mobile backhaul in remote and rural areas?
The uptake of mobile Internet has further reinforced the need for higher data traffic. High throughput with excellent quality of service (low jitter, low latency) is becoming crucial for the growing data communications and the established voice traffic. Quality of service must be guaranteed. Any limitation on throughput due to peak traffic periods can immediately lead to dropped calls, degraded user experience, revenue loss and customer churn. All this has to fit within the mobile operators’ OPEX and CAPEX objectives.

Which solution is more efficient?
The satellite mobile backhaul challenges have forced TDMA vendors to come back to an SCPC implementation due to efficiency and throughput requirements. However, not all SCPC solutions are alike. Efficiency is dependent on modulation, coding, encapsulation methods, protocol efficiency and WAN optimization methods, just to name a few.

Recent SCPC solutions have higher order modulation capabilities (32APSK and 16-QAM), better frame encapsulation efficiency, and embedded IP header and payload compression, which can provide more than 60% bandwidth savings (outbound and return) versus competitive solutions. Superior modulation and spectral efficiency provide up to 70% additional usable throughput on the outbound and from 100% to 250% on the return. Comtech EF Data’s SCPC solutions support Adaptive Coding and Modulation (ACM), taking advantage of link conditions and converting link margin into additional user capacity when the link allows.

Which solution is more CAPEX effective?
TDMA solutions require a central hub, which are commonly expensive. CAPEX can quickly become prohibitive as TDMA systems require provisioning for remote BUCs that need to support larger shared return carriers even though individual sites need much less bandwidth than the total shared capacity. SCPC solutions do not require an expensive hub and you are only required to use a BUC to support the capacity of the individual site.

Example: TDMA systems require each site to transmit at the aggregate data rate of the shared carrier during its burst period. In the example where five sites are sharing a TDMA carrier and each requires 1 Mbps of user capacity, the aggregate data rate of the carrier would be 5 Mbps of user traffic. This does not include the roughly 20%-40% additional data required due to TDMA structure overhead. When we include this overhead, to achieve 1 Mbps per site, the aggregate carrier for all five sites would actually need to be somewhere between 6 Mbps and 7 Mbps. SCPC circuits only require users to close the link for the data capacity the site will be using directly. In this case, a 1 Mbps circuit that allows for smaller BUCs and antennas, and results in substantial CAPEX savings (more than 40%).

Which solution incurs less OPEX?
SCPC solutions are simple to operate and manage, without requiring trained on-site personnel. SCPC solutions do not require TDMA structure overhead inefficiencies, such as guard band, pre-amble or unique word structures. Therefore, SCPC is inherently more spectrally efficient than TDMA. TDMA communications require short coding blocks in order to keep bursts small enough to share the capacity while attempting to minimize the negative effects of jitter and additional delay. Unfortunately for TDMA, the tradeoff for low latency small blocks means that most of the capacity is being used by this TDMA overhead. Furthermore, a shorter coding block offers a less powerful coding engine. SCPC modems do not contend with other sites for capacity and can, therefore, use the appropriate coding block for the correct data rate and link conditions. This flexibility offers the best jitter and latency performance with higher spectral efficiency, providing substantial savings on OPEX related to satellite bandwidth (more than 30%).

Which solution processes real-time voice and data better?
Minimizing jitter and latency are crucial for accommodating real-time voice and data. SCPC provides minimal jitter (no waiting for bandwidth allocation) and latency (less than 25 ms for real-time traffic), offering performance and a user experience that TDMA cannot match.

MOS Variations
Affect of jitter on voice quality (on top of packet loss, call establishment and round trip delays)
Ref: ITU/BDT Workshop on “ICT Applications for Rural Communications”

How is bursty traffic handled by TDMA architectures?
TDMA architectures with shared bandwidth functionality are well suited for highly bursty traffic. However, these solutions are only effective when traffic levels at some remote sites are low enough to allow the peaking sites to “borrow” their capacity. Most mobile backhaul networks do not function this way. There are typically long segments of peak load during work hours. Even vast countries with multiple time zones will have a substantial number of peak hours that overlap. When sites are peaking and capacity is oversubscribed, TDMA systems cannot leverage unused capacity, there is none. Worse still, the calls that get dropped and services that are degraded are all happening during what should be the most profitable times for the mobile operator. A dropped call on off-peak hours does not have the same value as a dropped call or poor service during the higher revenue peak hours.

How easy is it to expand the solution?
Both TDMA and SCPC solutions can be configured in a hub- spoke architecture, which allows for easy project expansion.

High efficiency, better modulation, FEC, encapsulation and framing allow SCPC to outperform TDMA efficiency. Hub and spoke with high capacity (up to 160 Mbps) circuits are available on outbound links while circuits as small as 9.6 kbps to 25 Mbps are available on return links. SCPC point-to-point links are available in virtually any size and can accommodate further bandwidth reduction using technologies such as DoubleTalk® Carrier-in-Carrier®.

Which solution guarantees Quality of Service (QoS)?
The shared bandwidth technology requires that the remote stations compete with each other to get the necessary bandwidth for the voice and data traffic. This prevents any commitment to a reliable QoS, as the situation can immediately lead to serious quality degradation when most remote stations are experiencing peak traffic at the same time (which is often the case).

Summary
Below is a summary of the advantages of SCPC versus TDMA.

Advantage SCPC TDMA
Efficiency
CAPEX (including RF and Antenna)
OPEX
Accommodation of Bursty Traffic ✔* ✔**
Ease of Expansion = =
QoS
Service Quality
Low Latency
Low jitter
  • * regardless of the number of remotes that experience peak traffic
  • ** provided the remote stations do not experience peak traffic at the same time

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