Data Communications Networking Devices - 4th Ed.pdf

4.9 BROADBAND MODEMS ________________________________________________________ 497Con®guration serverThe con®guration server supports the TCP/IP network formed on the CATVcable infrastructure. In doing so the server performs address resolution services aswell as managing IP addresses for the client computers using cable modems.RouterThe function of the router is to encapsulate data into standardized frames fortransport to external networks as well as to receive frames originating outside theCATV LAN.Operating rate comparisonTo obtain an appreciation of the potential capability of a 10 Mbps modem, Table4.34 provides a comparison of the ®le transfer time for a 10 Mbyte ®le at operatingrates from 9600 bps V.32 modem) to the LCP cable modem's 10 Mbps rate.Although the data in Table 4.34 indicates that the 10 Mbps operating rate of theLANcity LCP modem provides a signi®cant reduction in ®le transfer time, itshould be noted that the transfer time of 8 seconds is a mathematically derived timethat will not be obtainable even under the best of circumstances. In actuality, acable modem user will share the 10 Mbps transmission path to the headend withother users using the CSMA/CD access protocol. This means your actualthroughput will be governed by the number of other subscribers transmitting data.In addition, the occurrence of Ethernet frame collisions results in each Ethernetadapter using a random exponential backoff algorithm to generate a waiting timeprior to retransmitting a frame. Although this might increase ®le transfer time to 10to 20 seconds, or even 30 or more on a heavily utilized LAN, it still represents aconsiderable improvement over the V.34 28 800 bps transfer time of 46 minutes.Table 4.34Operating rate10 Mbyte ®le transfer time comparisonTransfer time9600 bps V.32 modem) 138 minutes14 400 bps V.32 bis modem) 93 minutes28 800 bps V.34 modem) 46 minutes64 000 bps ISDN B channel) 20 minutes1544 000 bps T1 DSU) 52 seconds4000 000 bps Token-Ring) 20 seconds10 000 000 bps Ethernet) 8 secondsIEEE 802.14 proposalIn a proposal to the IEEE 802.14 Working Group for the use of QAM for HFCdownstream transmission, Scienti®c Atlanta noted that QAM is non-proprietaryand was previously selected as the European Telecommunications Standard. In the