Specifications

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vendors must install much new equipment. One or more TV channels are reserved for data services; this
accommodates the downstream path to the users. The upstream path is more difficult. The CATV vendor
must replace the amplifiers used to distribute TV signal with ones capable of carrying signals in both
directions. At the CATV office these signals are converted from the cable format and routed to the
backbone data network. The cable network that feeds the subscriber must be divided into small groups to
minimize the number of customers sharing cable bandwidth.
Some early cable Internet systems were unidirectional. The cable was used for downstream data and a
conventional modem for upstream. This allowed the CATV vendor to offer high-speed data while it
upgraded the network for bi-directional data.
The CATV industry is working to standardize the interface so cable modems can be purchased in retail
stores like dialup modems. The industry is rapidly migrating to the DOCSIS Data-Over-Cable Interface
Specification created by CableLabs. Like DSL DOCSIS is an always-on connection, it is not necessary to
“dial” into the Internet. Typical CATV data rates are 700-10,000kbps down (toward the customer) with sub
megabit rate up.
The USENET news group comp.dcom.modems.cable is a good source of information. My experience with
cable is very limited is has only just arrived in our area and we are not a subscriber.
2.4 Other High Speed Services
The demand for high speed Internet access is driving network innovation. In addition to DSL and Cable
fixed wireless service that do not require access to expensive right of way is being deployed on a trial basis.
Satellite service is competing with wired service in some areas. The distance to and from geosynchronous
orbit adds significant latency making this type of service more appropriate for file downloading then
interactive browsing. The holy grail of broadband is fiber optics. It promises virtually unlimited speed. It is
being rolled out in several Greenfield areas. New residential development is a prime candidate for fiber
converged service; fiber provides broadcast television, telephone service, and broadband Internet access
over a single infrastructure. In new developments fiber is cost effective today. As prices fall more and more
homes and small businesses will have direct access to high-speed fiber.
2.5 When “Always On” doesn’t mean “Always On”
DSL and Cable modem are marketed as an “always on service.” Exactly what always on means depends on
how the service is implemented. The most “on” service consists of a static IP address. The DSL connection
looks like a LAN. One simply sends bits down the wire. Some DSL providers use a server to allocate IP
addresses called Dynamic Host Controller Protocol (DHCP). When a device connects for the first time it
asks for an IP address. DHCP issues the address for a limited period called a lease. When the lease is about
to expire it is automatically renewed. DHCP makes it much easier to manage the IP address pool then
manually assign static addresses. From the customers point of view it is an always-on service, lease
renewal is transparent to the user.
Some broadband carriers have implemented a technique call Point-to-Point-Protocol over Ethernet (PPPoE)
or Point-to-Point-Protocol over ATM (PPPoA). This simulates a dialup connection. This type of service is
typically offered to residential customers. It leverages existing ISP investment in dialup authentication and
billing. In typical usage once the customer is authenticated they are issued a dynamic IP address. This has
to be renewed periodically like DHCP. When the connection has been idle for a predetermined time the
user is disconnected. This allows more customers to be serviced from a given pool of IP addresses. PPP is
an encapsulation protocol, as such it add 8 bytes of overhead to every packet. Internet Packets can be up to
1500 bytes long so in most cases this is not significant.