Cable systems are usually what’s called a Hybrid Fibre/Coax plant (HFC). The town plan is subdivided into serving areas also known as neighbourhoods. Each serving area has a “node”, which contains the CMTS (cable modem termination system) unit and other RF (radio frequency) equipment for sending the signals into the coax cable. All the various nodes are then connected together into a larger network using fibre optic cable. One fibre carries the feed of TV channels into the node for distribution into the coax cable. Other fibres carry the data connections to and from the CMTS. Each node may contain more than one CMTS, and each CMTS may provide more than one downstream and more than one upstream channel. Provisioning software helps decide which modem uses which set of channels.

The cable network can increase the number of channels by physically segregating into more nodes because each node is an independent cable plant and can use the same frequency as another node without collision. The drawback of node-splitting is that Shaw cannot guarantee their customers that IPs will remain unchanged after the node is split. Additionally, a Shaw cable customer moving to a new location must move to a new node and to a new IP address. Skyway IP addresses are not affected by node-splitting and can be moved between locations and services. We achieve this by adding a router to the customer setup and routing an IP address block to that router.

Skyway recommends DOCSIS 3.0 services because they use the most recent firmware, are faster and they bond multiple channels which adds redundancy.

Cable Internet uses a modem at the customer site and a Cable Modem Termination System (CMTS) at the cable company that adheres to the DOCSIS 2.0 or DOCSIS 3.0 standards. The data flowing downstream towards the customer and upstream towards the Internet employ different radio frequencies so that they can co-exist on the same cable. In the downstream direction, an analog TV channel’s worth of frequency spectrum is dedicated to data use. This channel can carry up to 38 Mbps. The upstream channel is typically placed at a lower frequency in the spectrum and can carry approximately 27 Mbps. For example, Shaw Cable uses DOCSIS 2.0 to provide up to 25 Mbps over a single 38 Mbps channel shared by multiple users. For services of 50 mbps or higher Shaw uses DOCSIS 3.0, which allows bonding of multiple 38 Mbps channels, resulting in a far more reliable service.

The cable between a customer’s modem and the CMTS at the node is shared by the entire serving area. This means your data flowing towards you on the downstream channel is mixed with all other customers in the serving area. As long as the total usage demand in the serving area is less than the bandwidth available on the shared downstream channel, there is good performance. Once the demand on the downstream channel exceeds the capacity, congestion occurs, and the maximum speed each customer can achieve is limited. Since a downstream channel might be a single 38 Mbps DOCSIS 2.0, and the cable company may place 100 to 200 subscribers on that single downstream, each contracted up to a 25 Mbps, it’s easy to see how your speed is at the mercy of your neighbour’s use.

In the downstream direction there’s only one “talker”, the CMTS, so the CMTS uses various software algorithms to try to “fairly” allocate the limited bandwidth resource to the users. But in the upstream direction each of the 200+ modems on the channel could potentially be trying to transmit at once, causing collisions on the network.

To avoid these collisions the DOCSIS system uses a very complicated system of scheduling. A modem that wishes to send a packet upstream must wait until a Request Window is scheduled. Then, it may chirp a very brief transmit request on the upstream during this window. At that point it may collide with another modem doing the same; if that happens, neither request is heard. But assuming the request gets upstream to the CMTS, the CMTS will schedule a time when the upstream bandwidth will be reserved for for that packet from that modem. During that scheduled time only the particular requesting modem is authorized to transmit. The CMTS advises all modems of the upcoming transmit schedules by means of a special packet on the shared downstream. The transmitting modem waits and listens on the downstream to hear of its transmit schedule, and when it’s the right time, it transmits the packet on the upstream. Then the process repeats for the next packet. If there is more demand than capacity, the CMTS tries to hand out opportunities as best it can, and each modem gets a roughly equal slice of the total bandwidth.

Cable can achieve faster speeds but those speeds can vary widely; ADSL, while slower, is an inherently more stable technology. This is illustrated in the graphs below, showing network speeds achieved over a week of testing in the Lower Mainland for a 50 Mbps Down/ 3 Mbps Up Cable service in yellow and a 25 Mbps Down/ 5 Mbps Up ADSL service in blue. These two technologies have many similarities, but they also have significant differences which means that they each have their own strengths and weaknesses. Understanding these differences is important when choosing which technology to use for a particular business application.

With Cable, there’s a common channel that many customers share. Sync rates are fixed, and signals do not degrade with distance. But the available bandwidth is much less than the potential use of all subscribers so there’s often times where congestion prevents users from enjoying the full bandwidth they purchased. It’s difficult to guarantee or reserve bandwidth for a time-sensitive application like VoIP or video conferencing, but it’s a great system for web browsing, email and transferring large volumes of data.

With ADSL, each subscriber has their own dedicated channel. The channel is usually slower than the cable channels, and the actual speed is subject to the lay and length of of the phone line that carries it. But once you know the speed that the phone line can provide, you can be confident that that speed is consistent and stable, regardless of what your neighbours are doing. This is a good environment for VoIP or video conferencing, or for applying Quality of Service (QoS) priority queuing for these applications if you are also using the line for data.

Within a node, where channels cannot overlap each other, there is bandwidth sufficient for about 120 channels each taking a 6 MHz slot. These channels are shared not only by the CMTS, but also by the set-top-box provisioning system, by analog TV, and by the demands of Digital, HDTV and PPV. Each slot could be a CMTS downstream, a single analog TV channel, or 12 to 15 digital TV stations. This means if the cable company wants to offer 250 Mbps downstream out of a particular node, they either add nodes, or must choose to turn off 7 (250 Mbps/38 Mbps per channel) analog channels, or up to 105 (7 x 15) digital TV channels to make room. Since analog TV subscriber numbers are dropping and Internet subscribers are increasing, many cable companies are aggressively converting analog subscribers to digital in order to create space for data.