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Changes in the Cable Network
• The cable network was designed to deliver TV signals in one direction from the Head-End to the subscribers homes
• To provide TV services Cable Operators had to recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line
• Operators had to upgrade the cable network so that signals could flow in both directions
• Cable Operators assign a spectrum of signal frequencies to the cable network
• One spectrum is used for the signals that move from the Head-End towards the cable subscriber
• Another spectrum of signal frequencies are used for the signals that move from the cable subscriber towards the Head-End
• By replacing existing one way amplifiers with two way amplifiers Cable Operators are able to separate the upstream and downstream signals and amplify each direction separately in the right frequency range
How Fast is a Cable Modem?
• Cable modem speeds vary widely
– Depends on the cable modem system
– Cable network architecture
– Traffic load.
• In the downstream direction (from the network to the computer), network speeds can be up to 27 Mbps
– BUT, this is an aggregate amount of bandwidth that is shared by users.
• Few computers will be capable of connecting at such high speeds or have exclusive access to the network
– A more realistic number is 1 to 3 Mbps.
• In the upstream direction (from computer to network), speeds can be up to 10 Mbps.
– However, most modem producers have selected a more optimum speed between 500 Kbps and 2.5 Mbps
– AND, many cable operators limit the upstream bandwidth to 128 or 384kbs
• An asymmetric cable modem scheme is most common. The downstream channel has a much higher bandwidth allocation (faster data rate) than the upstream,
• primarily because Internet applications tend to be asymmetric in nature.
• Activities such as World Wide Web (http) navigating and newsgroups reading (nntp) send much more data down to the computer than to the network.
• Mouse clicks (URL requests) and e-mail messages are not bandwidth intensive in the upstream direction.
• Image files and streaming media (audio and video) are very bandwidth intensive in the downstream direction.
Real-world performance
• The theoretical performance of a Cable Modem is based upon all other devices being able to work at the same speed and performance as the Cable Modem
• However, in a similar way that the actual usable bandwidth on a 10Mbps Ethernet connection reduces to a 4Mbps, so too will the performance of a Cable Modem connection be reduced
• The Cable network itself will suffer the same problems of Internet performance as any other Internet Service Provider (ISP)
• Although performance to services on the cable network itself can be amazingly fast, access to 'the outside world' will be slowed down by the performance of other connections on the way.
• As usage on your segment grows (as more customers are added) the bandwidth must be shared by more people
• Adding more cable network segments is very expensive for the cable operator
• If you connect to a remote Internet site that itself has a connection speed equivalent to a T1 connection (1.5Mbps), then that is as fast as the data can be served to you, no matter how fast your receiving equipment is
Who Makes Cable Modems?
• 3Com, Cisco Systems, Com21, General Instrument, Motorola, Nortel Networks, Phasecom, Samsung, Terayon, Toshiba, Zenith
• And many others