by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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IV. Cable-TV Networks
Prior to the passage of the Telecom Act’96 in February 1996 by the U.S. Congress, cable-TV networks were limited to providing TV service to the general public. One of the key objectives of the Telecom Act’96 was to
break the monopoly in local telephone service provided by the RBOCs Regional Bell Operating Companies and other telephone operators in a particular telephone service area. There are currently three access solutions
available to provide competitive telephone service. These are wireless network, cable-TV network, and unbundling of PSTN access through telephone wire connection.
Although the cable-TV network operators were very enthusiastic about getting into telephone service as a new business opportunity following the passage of Telecom Act’96, their initial enthusiasm has been dampened by
the realization that it will require a significant amount of investment to offer traditional telephony service over a cable-TV network and be very difficult to offer cheaper telephone service than the PSTN operators,
particularly in the U.S.
However, these cable-TV operators soon realized that they could offer high speed Internet access, instead of telephony service, to cable-TV subscribers using the large amount of bandwidth available in their cable-TV
networks. At the present time, cable-TV network operators are already offering fast Internet access while telephone operators are still struggling with the type of technology to use such as ADSL to provide fast
Internet access to their subscribers. Eventually, as VOI and VTOA become mature technologies and widely used, then it will be possible for cable-TV operators to provide very cost-effective telephony service to their
customers.
We next look at how ATM technology can be used in cable-TV networks to provide fast Internet access. The cable-TV networks are currently capable of providing 10 to 30Mbps down-link capacity to the cable user.
However, the up-link from cable user to cable ISP is still problematic. In most cases, the up-link requires a modem connection over PSTN to cable ISP with low bandwidth capacity. This situation still maintains the
monopoly of PSTN operators. Fortunately the latest cable installations will allow up-link capability from 750Kbps to 1.5Mbps range over the same cable infrastructure. A typical cable networking infrastructure
providing fast Internet access is illustrated in Figure 4-5. ATM will play a role providing robust backbone data network after the head-end where the Internet traffic is separated from the TV broadcast signals.
Cable operators will likely have to rely on other companies to provide Internet content since they do not have the necessary expertise. It would be very difficult for the cable operators to compete against Internet service
provider giants such as AOL. Additionally, the cable user would prefer to have a choice in terms of selecting
One of the drawbacks of the cable network is that it is a shared medium and its bandwidth capacity is not scalable. Hence, as the number of users increase it will not be possible to provide 20 to 30 Mbps maximum
access bandwidth for each user. In this case, it is most likely that cable operators will choose a pricing structure which puts a premium on guaranteed maximum access bandwidth.
V. Integration of Various Transport Technologies
One of the strong features of ATM is to provide internetworking among various networks such as LAN and WAN networks, thus expanding the reach of these networks beyond their original domains. In addition, by
adapting SONETSDH as its primary physical transport medium, ATM is able to leverage vast amounts of SONETSDH installed base to offer internetworking capability on such a grand scale.
Figure 4-5 Cable-ATM inter-working for fast Internet access and video service.
Use of SONETSDH for ATM Transport
The SONETSDH transport technologies have been chosen to be the physical layer for ATM broadband applications by the ATM Forum and ITU-T standard organizations. The ATM protocol sits on top of the
SONETSDH-based physical layer. The key factors for choosing the SONETSDH platform, even though SONETSDH is a synchronous transport system, for the ATM physical layer are:
• SONETSDH has been extensively installed around the world. Thus, ATM can take advantage of such a large installed base provided by the PSTN operators.
• It is a highly scalable transport technology due to use of fiber optics. It can handle traffic capacity from 1.5 Mbps DS1 rate to 10 Gbps OC-192 rate. As fiber optic technology advances it will be
possible to provide an even higher capacity. • It is a highly reliable transport technology.
The mapping of ATM cells into SONETSDH payload for various speeds has been standardized. Currently, the specifications define the following SONETSDH rates for ATM:
• DS1E1 1.52 Mbps • OC3c 155 Mbps
• OC12c 622 Mbps
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by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
Previous Table of Contents Next
LAN Internetworking
One of the key applications of ATM technology is to provide internetworking capability between various LAN technologies such as Ethernet, Fast Ethernet, and Token Ring. The LAN Emulation
LANE protocol for ATM was specifically designed to allow interconnection among these different LAN technologies. Hence, increasing connectivity among many LANs based on different LAN
technologies. Through the LANE protocol it is possible to establish large and geographically diverse Virtual LANs.
The LANE protocol also offers unprecedented bandwidth scalability through the underlying ATM network to the LAN environment. The current LAN technologies, shared or switched, offer limited
bandwidth per user due to sharing andor limitation of the physical layer transport technology being used. For example, a switched Ethernet is limited to 10 Mbps per user or Token Ring limits the
maximum bandwidth to 416 Mbps. On the other hand, due to use of SONETSDH as the physical layer transport, ATM can scale to 622 Mbps per user. It is expected that the 2.4 Gbps OC-48c rate per user
will also become available very soon.
For the campus environment, the following non SONETSDH ATM rates are also available:
• ATM 25 25 Mbps • ATM 100 100 Mbps
Frame Relay-ATM Interworking for WAN Applications
Prior to recognition of ATM as the preferred network platform for WAN application, Frame Relay gained significant market penetration for WAN application. The success of Frame Relay in the WAN
segment can be attributed to the following factors:
• Simplicity of the protocol. • Very efficient use of existing PSTN networks through T1E1 and fractional T1E1 interfaces.
Hence, low build-up cost. • Very cost-effective data transport service compared to other alternatives such as leased lines
The Frame Relay protocol has its origin from the X.25 protocol. In order to make it suitable for fast data traffic a significant amount of overhead associated with the X.25 protocol was eliminated. For example,
node-by-node error checking and flow control was delegated to end-to-end error checking and flow control instead.
At the present time, the majority of Internet traffic is being carried over Frame Relay networks. However, as the ATM technology becomes more affordable, it is expected that the Internet traffic will
shift to ATM networks. The key factor for the switch from Frame Relay to ATM is due to limitation of scalability for higher bandwidths associated with Frame Relay technology.
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