Upstream Transmission
The introduction of fiber trunking is the first step in cable TV networks' evolution to full-service RBB networks. The second major step is to create an upstream transmission capability in the HFC plant so that subscribers can send data as well as receive it. The termsupstream, reverse, and return path are used interchangeably to describe traffic and paths that go from the consumer to the head end.
Existing Return Paths
Previously, many return path amplifiers were installed by cable operators to meet franchise requirements, such as video from the city council chambers, for management communications for intelligent amplifiers and to poll set-top units to collect impulse pay-per-view (IPPV) billing information. IPPV information is retained in the set top and is polled periodically by the MSO to obtain invoicing information. IPPV and monitoring are low bit rate uses of the return, so little return bandwidth is required. Today, with higher bit rate requirements for data, more return bandwidth and more aggressive modulation schemes are required than are necessary for simple invoicing data collection.
The vast majority of systems today assign upstream frequencies in the range of 5 to 42 MHz below the frequencies used for broadcast television. Systems that split the upstream from downstream frequencies at frequencies below 54 MHz are referred to as low split.
Mid-split systems separate the upstream from downstream frequencies above 100 MHz. Exactly where the return and forward paths split is decided by the operators. In a representative mid-split system, the return path is in the range of 5 to 108 MHz, and the forward path is 174 to 450 MHz, noted as 5-108/174-450. A high-split system of 5-150/174-750 provides the greatest amount of return path.
Mid- and high-split systems are useful for two-way communication, but they infringe on frequencies used for legacy broadcast television. By starting the forward at 174 MHz, channels 2 through 6 are lost. Three broadcasters in any given market would lose their natural channel assignments. Consequently, low-split systems are the focus for RBB rollout. Mid- and high-split systems are being considered for custom networks installed by operators, specifically for industrial or governmental uses.
To provide reverse path or upstream support, amplifiers are needed on the coaxial cable in the upstream direction, and laser transmitters are needed in the fiber node to transmit back to the head end. Because return path frequencies are low, there is less signal loss in the return path than in the forward path, so fewer return path amplifiers are needed. The capital cost of return amplifiers is low compared with that of forward path lasers. Furthermore, low-cost Fabry-Perot lasers are typically used in the fiber nodes to illuminate the return path. Because of these relatively low-cost components, the capital cost of upgrading to two-way is only about $10 per home passed.
In addition, labor costs must be considered. Estimates from some operators cost about $300 per mile to improve signal fidelity for return path. Improving signal fidelity means to clear away noise, tighten the cable plant against leakage, equalize frequency response, and perform other installation preparation. The cost per home passed is less than $10, which, like capital costs, is relatively low. However, the work is labor-intensive and requires skilled labor. Given the thousands of miles that need to be installed, labor availability is a restrictive factor in creating and maintaining the return path. The availability of labor can be a significant problem because telephone companies compete for labor within the same pool.
Telephone Return
Due to return path impairment problems, some cable operators are choosing to defer enabling the return path and opting for telephone return instead, at least for an interim period. By using telephone return rather than cable return, two-way services for data and video on demand can be offered by the introduction of HFC only. Telephone return technology is well understood, and two-way service can be initiated by the MSO quickly.
Telephone return means that the consumer, or the subscriber modem, makes a telephone call to a terminal server when the consumer requires return path service. The terminal server may be located at the head end, a super head end, a telco facility, or a facility operated by an Internet service provider (ISP).
Viability of Telephone Return
Many observers contend that telephone return is not a viable long-term strategy for cable operators—it might not even be a profitable short-term one. Consider the following:
The customer might pay for message units. Telephone return is advocated in geographical areas that do not justify return path buildout. But these areas, primarily rural areas, are also likely to be farthest from the dial point of presence and therefore are charged for message units.
The consumer loses use of the telephone while it is being used for return path needs and might need an additional phone line to compensate.
Call setup is too long. Dialing takes a minute or so, which makes the TV viewing experience or Internet experience unsatisfying.
For TCP traffic, the slow return path limits the forward speed. The return path contains the acknowledgments that keep forward traffic moving. A 28.8 Kbps return path will limit the forward speed to 500 Kbps or so. This is a lot, but not the 27 Mb advertised for cable.
The cable operator must pay for the terminal server and access lines. If you count costs for rack space and management of multiple terminal servers, the cost differential between telephone return and enabling the cable return path narrows.
Because of the lack of satisfaction with telephone return and the relatively low cost of implementing two-way cable, systems using telephone return are expected to proliferate slowly. On the other hand, two-way cable systems are proliferating briskly, thereby obviating the need for telephone return.