Hybrid-fiber coax (HFC)/data over cable system interface specification (DOCSIS®) access networks are widely used to address the needs of residential and enterprise consumers for high speed Internet (HSI), video, and voice services. A hot question these days is: "Can HFC/DOCSIS access networks also support the backhaul needs of LTE small cells?" Since the characteristics of cable networks and the products used can vary significantly across markets, it is not always possible to have a clear yes/no answer. In performing techno-economic analyses for HFC -based backhaul applications, we have seen concern about meeting throughput and performance (packet delay/jitter/loss) requirements. And these can vary with the specific DOCSIS QoS mechanisms and bonding configurations.
SMALL CELL REQUIREMENTS
Small cells are fundamentally used to:
- Increase wireless network capacity in traffic ”hotspots”
- Provide coverage in areas where there is poor macro cell coverage
- Provide both capacity and coverage in targeted high-value areas
Operators are primarily motivated to deploy small cells to deliver higher user experience with better economics. Small cells are best suited for serving applications with high quality of service (QoS) requirements (video streaming, video calling, etc.) which need high throughput and low delay/jitter/packet loss. Environmental Small cells are typically used in indoor as well as outdoor environments. The type of environment has a major correlation with the anticipated user demand, and hence, the potential bandwidth requirement for traffic backhaul. Throughput Backhaul bandwidth requirements will depend upon:
- Spectrum used by the small cell (typically 5, 10, or 20 megahertz - MHz)
- Number of sites
- Use of hub or daisy-chain topology
Performance Many performance requirements -- such as packet delay, jitter and loss for voice, data and video -- do not present a fundamental challenge for an HFC/DOCSIS access network. On the other hand, control signaling performance requirements for LTE radio access networks are more stringent. During a handover (one cell handing off to another), the longer the handover preparations phase, the higher the chance of handover failure. This is a primary area of concern for HFC/DOCSIS-based backhaul.
HFC/DOCSIS NETWORK CHARACTERISTICS
Throughput HFC/DOCSIS 3.0 provides a channelized access network architecture. Downstream channels for data services typically achieve ~38 Mbps (@6 MHHz) or ~50 Mbps (@8 MHz ) per channel. An upstream channel may achieve typically ~28 Mbps (6.4MHz). Today there are only a handful of upstream channels, while there are well over 100 downstream channels. To improve aggregate throughput, channels can be bonded together and a service group concept used to define how DOCSIS channels are allocated to serve consumers over a defined geographic area. Note that dimensioning the upstream on the HFC/DOCSIS access network to support small cell backhaul is complex given the limited number of available upstream channels and lower throughput per channel. Also, should service group splits be required to reduce the size of the geographic serving area, this implies network changes and costly additional HFC/DOCSIS equipment. Performance Our analyses show important factors that contribute to performance issues in HFC/DOCSIS access networks include:
- DOCSIS QoS – although DOCSIS QoS mechanisms can be used to give preferential treatment to control traffic from small cells, the choice of the QoS mechanism can impact backhaul performance.
- Use of a single or bonded DOCSIS channels – slight increases in delays may be incurred when channels are “bonded”.
- Equipment used – design, implementation, and device components used can contribute to equipment performance differences between DOCSIS vendors.
- Consumer traffic – contention for bandwidth by residential and enterprise consumers, or by multiple small cells themselves.
- Fiber and coax lengths – while not a large contributor to delays, excessive lengths contribute to propagation delays in the HFC and IP networks.
We have seen that the feasibility of using HFC/DOCSIS access networks for traffic backhaul from small cells depends on many factors including:
- Use case
- Traffic mix
- Peak busy hour demand
- HFC/DOCSIS network characteristics
- Performance objectives
- QoS mechanisms
There are currently no commercial large-scale examples of HFC/DOCSIS access networks being used to backhaul traffic from small cells or macro cells. The feasibility and design/cost tradeoff of using HFC/DOCSIS networks for backhaul, however, can be determined through a detailed techno-economic analysis.
DOCSIS® for LTE small cell backhaul white paper
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