Data Operations
Configuring the Cisco uBR900 Series Cable Access Routers 5
Figure 1 Typical Cisco Broadband Data Cable System
Operating Modes
The broadband data cable system uses multiple types of access control to ensure efficient use of
bandwidth over a wide range of loading conditions. Advanced queuing techniques and service
algorithms are used to define the acquisition and release of channels.
Cisco uBR900 series cable access routers support 64 or 256 Quadrature Amplitude Modulation
(QAM) downstream, and Quadrature Phase Shift Keying (QPSK) or 16 QAM upstream
transmission. This allows the CMTS system administrator to set the preferred modulation scheme
based on the quality of the cable plant.
Note In noisy plant environments, 16 QAM upstream and 256 QAM downstream modulation may
not be viable. In high-quality HFC networks capable of supporting 16 QAM formats in the upstream
direction, Cisco recommends using QPSK for fixed-slot short packets like maintenance or data
requests, and 16 QAM for variable length data packets. This results in the most efficient use of the
available upstream timeslots or minislots.
The system uses Transmission Control Protocol/Internet Protocol (TCP/IP) to transmit data. TCP/IP
transmits data in segments encased in IP datagrams, along with checksums to detect data corruption
and sequence numbers to ensure an ordered byte stream on the TCP connection between the Cisco
cable access router and the CMTS.
Cisco cable access routers also support multicast services—data streams sent to groups of
subscribers. These applications utilize the User Datagram Protocol (UDP) instead of TCP. Since
UDP does not mandate upstream acknowledgments, these applications can be very efficient in the
network. Additionally, restricting upstream throughput will have no effect on downstream UDP
streaming throughput.
Cisco uBR900 series
cable access router
18197
HFC
cable
plant
Internal backbone
and
worldwide internet
Cable System Headend
Upstream and downstream data interfaces
Operation support system interface
Downstream RF interface
Upstream RF interface
Residence or SOHO
subscriber site:
subscriber RF interface
Ethernet interface
Fiber
WAN
Switch/router
Servers
CMTS
rack equipment
Combiner Tx
Rx