---

Presented By:
NETWORLD INTEROP
courtesy:ceenet.org

---

read full presentation from
http://ceenetworkshops/lectures2002/Richard_Perlman/Cable%20Modems-2002.ppt

¢ Internet Access via Cable TV
¢ 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
¢ Changes in the Cable Network
¢ 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
¢ Changes in the Cable Network
¢ 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
¢ Changes in the Cable Network
¢ Changes in the Cable Network
¢ What is a Cable Modem?
¢ 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.
¢ How Fast is a Cable Modem?
¢ 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
¢ How Fast is a Cable Modem?
¢ 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.
¢ How Fast is a Cable Modem?
¢ 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
¢ Real-world performance
¢ 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.
¢ Real-world performance
¢ 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
¢ Cable Modem Technology
¢ It MOdulates and DEModulates signals
¢ Much more complicated than their telephone counterparts
¢ Cable modems can be part modem, part tuner, part encryption/decryption device, part bridge,

part router, part network interface card, part SNMP agent, and part Ethernet hub
¢ Cable Modem Technology
¢ Typically, a cable modem sends and receives data in two slightly different fashions
“ In the downstream direction
¢ he digital data is modulated and then placed on a typical 6 MHz television channel,

somewhere between 50 MHz and 750 MHz
¢ 64 QAM is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz

channel
¢ This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without

disturbing the cable television video signals.
¢ Cable Modem Technology
“ The upstream channel is more tricky
¢ Typically, in a two-way activated cable network, the upstream (also known as the reverse

path) is transmitted between 5 and 42 MHz
¢ This tends to be a noisy environment, with RF interference and impulse noise. Additionally,

interference is easily introduced in the home, due to loose connectors or poor cabling
¢ Since cable networks are tree and branch networks, all this noise gets added together as the

signals travel upstream, combining and increasing
¢ Due to this problem, most manufacturers use QPSK or a similar modulation scheme in the

upstream direction, because QPSK is more robust scheme than higher order modulation techniques in a

noisy environment
¢ The drawback is that QPSK is "slower" than QAM.
¢ Cable Modem Services
¢ The dominant service is high-speed Internet access
“ This enables the typical array of Internet services to be delivered at speeds far faster

than those offered by dial-up telephone modems
“ Other services will include
“ access to streaming audio and video servers, local content (community information and

services)
“ access to CD-ROM servers
“ a wide variety of other service offerings. New service ideas are being developed daily.
¢ Cost of Cable Modem Service
¢ In North America, cable operators are packaging high-speed data services much like they do

basic cable television service
¢ Typically charging $40 - $60 per month for an Internet service package
“ Includes software, unlimited Internet access, specialized content and rental of a cable

modem
¢ Cost of Cable Modem Service
¢ At the low end of this pricing scale, a very robust Internet service is available to

consumers for about the cost of a dial-up account with a local Internet service provider and a

second telephone line
¢ Even at $60 per month, cable is a far better value than ISDN.
¢ "Telco-Return" Modems
¢ Not really a cable technology
¢ Used more often with Direct Satellite video systems
¢ Satellite down page link is used for fast downstream transmission
¢ A telephone modem handles upstream communication over the public telephone network.
¢ Support for Multiple PCs
¢ A cable modem can provide Intenet access to multiple PCs, if they are connected via a local

area network (LAN)
¢ Cable modems typically have an Ethernet output, so they can connect to the LAN with a

standard Ethernet hub or router
¢ Each PC must have an assigned IP address
“ The cable ISP usually sells at a premium of $5-$10 a month per PC
“ NAT (Network Address Translation) can allow multiple PCs to "hide" behind a single IP

Address