Cisco Systems UBR900 specifications Operating Modes, Data Operations

Data Operations

Figure 1 Typical Cisco Broadband Data Cable System

WAN

Switch/router

	Combiner	Tx
CMTS
rack equipment		Rx

Fiber

Servers

Cisco uBR900 series cable access router

Internal backbone

and

worldwide internet

HFC

cable plant

Cable System Headend

Upstream and downstream data interfaces Operation support system interface Downstream RF interface

Upstream RF interface

18197

Residence or SOHO subscriber site: subscriber RF interface Ethernet interface

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.

Configuring the Cisco uBR900 Series Cable Access Routers 5