What really matters to people willing to pay extra for fast connections to the Internet is how quickly their traffic is moved through the network, which could be called "personal throughput". It is easy but misleading to assume that this performance depends only on the line speeds (bit rates) on the wire connecting to the customer premises. In fact, all the parts of cable modem networks are shared by many users. The generosity or stinginess of the carrier in the design of those shared parts limits the personal throughput the user experiences.

On modern cable modem systems, the downstream traffic (from the Internet toward the customer) actually moves at 27 or 32 Mbps. This includes all the data going to all the cable modem customer locations served by that "section" of the cable system. It usually is necessary to divide the cable network into several sections for serving modem users so that performance is not degraded by the sharing of this downstream capacity. If one of these sections passes 1000 homes (a common case), and 10% of them have cable modems, and half of those are receiving traffic at a given moment (probably a worst case number), then 50 computers are sharing 27 Mbps for an average personal throughput of 540 kilobits per second (Kbps) each. This is not to say that each user gets a continuous 540 Kbps stream. Rather, each one gets brief, intermittent use of the full 27 Mbps. When fewer users are active, each gets a bigger share, and what a particular user experiences varies from subsecond to subsecond as the data from the Internet actually becomes available for delivery to the active users. During times of very light demand performance can approach the 10 Mbps limit of the Ethernet connection between the modem and the computer. Naturally, cable systems with more homes passed per section or a higher proportion of modem users will give poorer service.

Each cable modem listens to all the traffic and captures just the packets directed to that modem's address. This is put into a buffer at line speed and fed out to the computer at Ethernet speed. Similarly, the upstream traffic (from the customer toward the Internet) enters a buffer in the modem at Ethernet speed and is transmitted onto a shared channel on the cable at upstream line speed. The upstream speed is much lower, ranging from 320 Kbps to 10.24 Mbps per channel, due to some constraints, particularly external electrical noise getting onto the cable. There can be several different

(top, next column)

upstream channels supporting the same group of modem users served by a single downstream channel, to compensate for the limited capacity of each upstream link, but the total upstream capacity for a group of users will almost always be just a fraction of the downstream speed _ probably no more than 5 Mbps at most. This would give each of our 50 simultaneous users 100 Kbps upstream. This limited capacity is what leads cable modem operators to object to their customers operating webservers. The upstream traffic from a server at the user site is much greater than what most upstream channels were meant to carry, and seriously degrades service for the other users dependent on that channel.

At the cable system "head end" all the modem traffic is converted to conventional LAN traffic and aggregated for connection to local servers and to the Internet via WAN routers. At all locations upstream from the cable system headend, only its Internet address distinguishes cable modem traffic from the Internet traffic of users of any other access technology. The WAN "uplink" isn't part of the cable modem technology, but its ability to handle the load generated by the users will affect the personal throughput of all cable modem users. The WAN link tends to be an expensive part of the network, leading some system operators to try to get by with less than is really needed. This can undermine the performance of an otherwise well designed system. At this point the 50 simultaneous users in our example might be part of 500 competing for the use of a 45 Mbps uplink, leaving them with an average of less than 100 Kbps each. Statistical effects will keep it from being as awful as it sounds, but the degradation will be perceptible. Personal throughput will suffer, but the cable modems are not the problem. The same thing can happen to DSL customers as their traffic is aggregated at the DSLAM and at higher levels of traffic concentration.

As all communications engineers know well, the "weakest link" in a connection determines the upper limit of the performance.

Transfer Rate for a 10 Megabyte File:

   << Previous Page