Andrew Afflerbach, PhD, P.E.
Chief Technology Officer
Understanding broadband performance involves understanding a range of factors in addition to speed. Consumers usually compare the performance of data connections by considering network speed (measured in bits per second) but this measurement can be deceptive because it is incomplete.
For example, a 150 Mbps cable modem connection may cost a residential consumer $90 per month, while a business-grade Metro Ethernet service that delivers 100 Mbps Internet capacity can exceed $800 per month – yet the internet delivered through the Metro Ethernet service provides better value for many types of applications. Why would a service with two-thirds the speed cost 10 times as much as the “faster” alternative?
The answer is that all bits (and megabits and gigabits) are not created equal. Factors such as latency, availability, and oversubscription rate affect the connection’s overall performance. In the example above, the 100 Mbps Metro Ethernet service’s total set of performance attributes provides a more robust and secure connection than a 150 Mbps cable modem.
These are some of the performance factors a consumer – or a government or policymaker – should consider in evaluating a connection:
- Symmetry: Cable modem and DSL services are typically asymmetrical, meaning that their upload (from user to network) and download (from network to user) speeds are different. The download speed is generally greater than the upload speed by a factor of 10 or more. Metro Ethernet services, on the other hand, are typically symmetrical, meaning that upload and download speeds are the same. For businesses that transfer large data or video files, asymmetrical services often present bottlenecks to both internal users and external customers. A user on a typical cable modem service can download a 5 gigabyte (GB) file in less than 10 minutes but uploading the same file would take more than 90 minutes – which would not be acceptable to a business creating and distributing large files.
- Oversubscription to the internet: Internet users don’t all access the internet at the same time, or with the same requirements. For that reason, ISPs provide only a portion of their networks’ total potential demand. For example, an ISP that has 1,000 subscribers with 10 Mbps service might contract for an aggregate 100 Mbps connection rather than the maximum 10,000 Mbps connection that would be necessary in the unlikely event all users were online at the same time and using the full capacity of their connections. The ratio of a network’s maximum potential demand to its contracted rates is known as its oversubscription ratio. In this example, the oversubscription ratio is 100:1. Cable modem and DSL providers often have a 100:1 or greater oversubscription ratio for residential users and a 50:1 ratio for business users. These high oversubscription ratios may not always be evident to users, but at times of high use, oversubscription can bring a consumer connection to a crawl – just like traffic on the weekend versus traffic during a weekday rush hour. In contrast, with a Metro Ethernet service that includes internet access, the internet oversubscription ratio is often 10:1 or less.
- Availability of the data transport rate: Metro Ethernet providers specify a committed information rate (CIR), which is the guaranteed transport speed of the circuit connecting user locations. The network will be designed to sustain at least that rate for all users guaranteed that rate. By contrast, cable modem and DSL services are often “burstable,” meaning that users may at times experience the advertised data rates, but that the average speed will vary greatly based on how congested the distribution network is at that time. Performance parameters on a burstable service are rarely publicized or realized. Often a network operator cannot change this parameter without changing the network’s physical connections. During periods of heavy network use, burstable subscribers experience the same traffic discrepancies drivers on the road during rush hour do. (Availability is often confused with oversubscription to the internet, but they are different. Oversubscription applies to the network’s connection to the internet, and availability applies to the “uptime” of the connection or transport between user locations or from the access point to the internet. A service is available as long as it operates as promised. However, as mentioned, it may still be oversubscribed – in which case it is operating but may be operating well below the peak advertised speed).
- Maximum usage: A network service may have a maximum data usage (in bytes) or “data caps” for a given period. For example, many wireless service data plans specify the number of gigabytes that users can transmit during the month. The ISP may actually slow down a user’s connection speed as the usage limit is neared.
- Latency: Latency is the term that describes the delay between the instant a message is sent and the instant it is received. Latency can occur in multiple parts of the network—first in the ISP’s own network and then also on the internet, if the message traverses the internet to reach its destination. Latency for consumer-grade services (such as those on cable modem and DSL) is determined on a “best-effort” basis rather than at a guaranteed level. For Metro Ethernet and other higher-end transport services, latency is often a specified quality-of-service feature for which users pay extra. High latency can be a significant challenge for running certain applications. For example, satellite-based internet has extremely high latency because of propagation delays — the time it takes for a signal to reach the satellite. These delays prevent effective use of interactive services such as phone calls or video-conferencing.
- Connection Type: This attribute describes how a connection is made with other locations. For example, on a cable modem or DSL service, all connections to other locations are made through the internet with internet addressing schemes. This may require a user to set up a virtual private network (VPN) for secure communications among user locations. Establishing a VPN on the network imposes cost and requires expertise and software; it also has an impact on performance. In contrast, higher-end data services, such as Metro Ethernet, enable a user to send traffic over the provider’s network in virtual networks without connecting to the internet, set up direct point-to-point connections, and limit which locations may connect with one another.
- Security: Although security is primarily a function of encryption and other techniques applied by users or application providers, traffic over a private network is inherently more secure than traffic on a network that establishes connectivity over the internet. A user that is connecting multiple sites using standard, consumer-grade cable modem or DSL transmits packets over the internet. With a higher-end service such as Metro Ethernet connecting user sites, the transport would remain on a separate network and never traverse the public internet – offering greater security. In addition, higher-end services often have encryption options at the transport layer.
- Port Rate: This is the maximum speed that is physically possible at the physical demarcation point, at the customer premises, of the service provider’s network. For cable modem services, this is usually defined by the network’s DOCSIS version. However, not all connections with the same port rate are equal. A number of items determine the actual connection speed at a customer location, including any throttling or network control within the service provider network, oversubscription of the service provider network, and the condition of the last-mile physical infrastructure (for DSL, the condition and length of the copper; for cable modem, the condition and length of the coaxial cable and drop cable from the pole to the premises; for wireless, the lack of interference and obstructions and distance to the service provider access point).
In brief, don’t let speed be your sole measure of broadband performance. Like so much about the internet service industry, the reality is far more complex and nuanced than one simple metric.