Configuring the SCC Clock Rate
Configuring the SCC Clock Rate
Communciation between a DSL WIC and the host in a router takes place through a device called a serial
communications controller (SCC). Whenever the host wants to transmit data or send any control traffic
to the DSL WIC, it uses an SCC. Similarly, when a DSL WIC wants to forward incoming data from the
line to the host, it also uses an SCC. Each DSL WIC installed in a router uses two SCCs. One SCC,
SCC-A, is used for ATM adaptation layer 5 (AAL5) data traffic, while the other, SCC-B, is used for ATM
adaptation layer 2 (AAL2) data traffic and for control traffic. The speed at which an SCC transfers data
between the host and the WIC depends on the clock rate with which the SCC has been configured. This
clock rate is configured by the user, and it is always synchronous. The SCC clock rate is the same
whether the WIC is sending or receiving data through the SCC. For an asynchronous DSL (ADSL) WIC,
the SCC clock rate should be set slightly higher than the larger of the DSL line rates (upstream or
downstream). It is recommended that the SCC clock rate always be set higher than the DSL line rate to
accommodate any SCC overhead.
SCC Clock Rate Configuration
The following example is a configuration of SCC clock rates on an ATM interface. Clock rates are set
with the clock rate aal5 command and the clock rate aal2 command. On Cisco 1700 series routers, valid
clock rates are in the range from 4 Mbps to 8 Mbps. The clock rate values are entered as bits per second,
as shown in the example.
interface ATM0/0
no ip address
clock rate aal5 5300000
clock rate aal2 4000000
no atm ilmi-keepalive
bundle-enable
bundle ama-bundle12
!
dsl operating-mode auto
end
It is strongly recommended that on Cisco 1700 series routers, the SCC clock rate be set to the default
Note
value of 8 Mbps (8000000 bps).
Note
When an SCC clock rate is deconfigured on a Cisco 1700 series router by using the no form of the
command, it is reset to the default value of 8 Mbps.
SCC Clock Rate Verification
To verify the configuration of the SCC clock rate, use the show controller command. SCC-A represents
the clock rate for AAL5, while SCC-B represents the clock rate for AAL2.
Router#show controller atm0/0
Interface: ATM0/0, Hardware: DSLSAR (with Alcatel ADSL Module), State: up
IDB: 82115298 Instance: 82116A4C reg_dslsar:68030000 wic_regs: 68030080
PHY Inst:8213862C Ser0Inst: 8210F690 Ser1Inst: 8211281C us_bwidth:864
Slot: 0 Unit: 0 Subunit: 0 pkt Size: 4528
VCperVP: 256 max_vp: 256 max_vc: 65536 total vc: 1
rct_size:65536 vpivcibit:16 connTblVCI:8 vpi_bits: 8
vpvc_sel:3 enabled: 0 throttled: 0 cell drops: 0
Configuring an ADSL WAN Interface Card on Cisco 1700 Series Routers
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OL-3317-03
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