VoIP over Cable or DSL?

Two of the different types of VoIP installation include cable and DSL or Digital Subscriber Line. Cable installation requires high-speed cable broadband and a cable modem with VoIP to establish connectivity.  DSL VoIP requires a slightly different type of connectivity and infrastructure.

To help you understand the difference between the two types let’s start with the difference between a cable connection and a DSL connection.

Cable vs. DSL

When you establish a cable connection, high-speed broadband is delivered over the same type of coaxial cable you use to access televisions services.  A cable connection offers high-speed Internet and is an ‘always on’ connection.  The connection is delivered by the cable provider and then transmitted through a cable modem.

DSL connectivity is delivered over copper telephone lines which are twisted together and used for transferring voice and data signals.  A DSL Internet connection is established by delivering high-speed broadband over telephone lines with connectivity established using a DSL modem or router. DSL comes in many different flavors: ADSL, HDSL, IDSL, RADSL, SDSL, UDSL, VDSL… the list goes on. Each flavor comes with its own bandwidth and distance limitations.

DSL services, for example, are sensitive to the length of the copper pair used to access multiplexer (DSLAM). Downstream speeds gradually diminish with the distance. For specifics, you may consult this Cisco Press article.

Framing

Cable modems use Ethernet frames while DSL technologies rely on older Frame Relay or ATM switches that create additional overhead making your effective usable bandwidth a little lower than advertised since the cost of framing is passed on the end-user.

Asymmetrical Connections

Both cable and most DSL connections offer asymmetric service in which download speed greatly exceeds upload. From the voice or video calling perspective, however, you are going to focus on the lowest as your total available bandwidth for voice and data. For example, Comcast’s 22/5Mbps service is, therefore, a 5Mbps service. That is all the bandwidth available for your voice and video calls as well as outbound data. But for many small businesses that are not heavily relying on telecommunications, this could be ok – 5 Mbps. A restaurant or a coffee shop does not send 10Mb image files like a medical office would.

Duplex

Most modern cable and ADSL service providers offer full-duplex connectivity. In layman’s terms, this means that you can send and receive at the same time. That said, however, some cable operators still utilize pre-DOCSIS 3.0 cable technology which does not employ frequency division to achieve full-duplex communications. This is not something you will notice that unless your bandwidth utilization starts to challenge the limits of the service you had purchased.

Bandwidth Guarantee

Another problem is that bandwidth is not guaranteed by either telephone or cable companies to their respective DSL or cable customers. There is no prioritization or class of service offered with the cable or DSL. This means that if you do push your service to the limits, you will certainly begin to experience voice quality problems with the only choices remaining: upgrade your connection or curb your usage.

Another problem is that the service provider model relies on oversubscription. What this means to you is that when the node servicing your business gets oversubscribed – service provider is not under any great pressure to upgrade their infrastructure to deliver more bandwidth. What you get with the 12/5Mbps service is not a commitment. It’s a MAXIMUM SPEED your transfer is allowed to reach.

Broadband Latency

Latency, of course, does not particularly have to do with either cable or DSL technology. It may be high or low depending on how a service provider’s network is actually built. VoIP calls are sensitive to latency and any latency in excess of 150 milliseconds may seriously impact audio quality. As a practical matter in our own experience, most small businesses utilizing AT&T ADSL had experienced much higher latency than those using Comcast cable. But this may not be true in other parts of the country and with other service providers.

Usage Caps

This concept dates back to the beginning of bandwidth management and has become really popular with mobile carriers. If you exceed the maximum allowable bandwidth under your plan – you will pay more. If you are a low-tech customer, you may have no idea why you are exceeding your bandwidth caps and being billed for more bandwidth than you were anticipating. Your best bet is to bring a competent consultant to identify the source of bandwidth and offer you options on either upgrading your service or managing bandwidth.

Monitoring

DSL or cable services are not being monitored. Most service providers do not offer the option to monitor service quality, bandwidth availability, etc. Both services are being treated as “best-effort” Internet access. Unfortunately “best-effort” is a subjective term. There is no service level agreement to hold your cable or phone company too.

Cost

While service prices vary in different parts of the country, DSL services tend to be slightly less expensive almost everywhere. This may have something to do with the fact that DSL is less expensive to provide because phone companies can reuse existing copper wiring that runs to every home.

Suitability and Reliability

Using DSL or cable to carry VoIP traffic without bandwidth guarantees is a gamble. While Internet access over Cable and DSL are far from the best connectivity options for most companies they have their niche market. Most very small businesses and home office users that either manage bandwidth well or have very low bandwidth requirements can use these Internet access services and save on expensive fiber-optic connections. The other option is to use separate dedicated cable and/or ADSL connections for VoIP and data traffic. In most cases, the savings and price-performance of two connections would not add up to the cost of a single dedicated T-1 circuit, not to mention the Fiber link.

Cable connections tend to be more reliable in most places where cable infrastructure is either new or has been upgraded. In Illinois, DSL in most areas uses older copper pairs that have been buried underground without much maintenance or overhaul for a long time. In areas where cable integrity is no longer sound for various reasons, you may experience service degradation or complete connection loss during thunderstorms or rainfalls.

Hopefully, this will provide you with a basic understanding of how DSL VoIP compares with cable VoIP.  From a practical perspective, the differences aren’t huge. A lot depends on how much bandwidth you are going to really need for your VoIP trunks and data. There are also factors that are completely outside of your control when you purchase service without a service level agreement (SLA), for example, the rate at which bandwidth is oversubscribed by your service provider. After all – both are technologies are designed around sharing bandwidth among users with no speed or latency guarantee.

With the shared bandwidth technologies your business gets what it pays for but if you are willing to take a chance then you can decide whether it’s worth the gamble. All we can do is give you the facts that might help you make that decision for your small business.