Cisco Systems UBR900 User Manual

 Configuring the Cisco uBR900 Series Cable Access Routers 19

The following sections describe the initial power-on and provisioning sequences in more detail, as 
well as the requirements that must be met by both the cable access router and the CMTS before 
provisioning can be successful.

Initial Power-On Sequence

When connected and first powered on, the Cisco uBR900 series cable access router performs the 
following boot procedures:

•

Boots the Read Only Memory (ROM) from the ROMMON partition of its flash memory.

•

Performs a self-test, initializes processor hardware, and boots the main operating system 
software—the Cisco IOS release image stored in NVRAM.

Next, the Cisco uBR900 series performs a series of DOCSIS-mandated procedures for automatic 
installation and configuration.  These procedures are summarized in Table 3 and in Figure 5.

.

Scan for a downstream channel and 
establish synchronization with the 
CMTS.

The Cisco uBR900 series acquires a downstream channel by 
matching the clock sync signal that is regularly sent out by the 
CMTS on the downstream channel. The cable access router  
saves the last operational frequency in non-volatile memory 
and  tries to reacquire the saved downstream channel the next 
time a request is made. 

An ideal downstream signal is one that synchronizes 

QAM symbol timing, FEC framing, MPEG packetization, and 
recognizes downstream sync MAC layer messages.

Obtain upsteam channel parameters.

The cable access router waits for an upstream channel 
descriptor (UCD) message from the CMTS and configures 
itself for the upstream frequence specified in that message. 

Start ranging for power adjustments.

The cable access router waits  for the next upstream bandwidth 
allocation map message (MAP) from the  CMTS to find the 
next shared request timeslot. The router then sends a ranging 
request message on the next available shared request timeslot, 
communicating its UID (its unique MAC address) using a 
temporary Service Identifier (SID) of 0 (zero) to indicate it has 
not yet been allocated an upstream channel. 

In reply to the cable access router’s ranging request, the CMTS 
sends a ranging response containing a temporary SID to be 
used for the initial router configuration and bandwidth 
allocation. As needed, the router adjusts its transmit power 
levels using the power increment value given by the CMTS in 
its ranging response message.

At this point, the cable access router has established 

connectivity with the CMTS but is not yet online. The next 
steps allocate “permanent” upstream and downstream 
frequencies, as well as the configuration required for IP 
network connectivity.