Configuring the T3 CSU Compatibility Mode
Subrating a T3 interface reduces the maximum allowable peak rate by limiting the HDLC-encapsulated payload. Subrate modes configure the PIC to connect with channel service units (CSUs) that use proprietary methods of multiplexing.
You can configure T3 interfaces to be compatible with a Digital Link, Kentrox, or Larscom CSUs. For T3 intelligent queuing (IQ) channels only, you can also configure Adtran or Verilink CSU compatibility.
To subrate an E3 interface to be compatible with a Kentrox CSU, you must have an IQ or IQE based PIC. Non-IQ or IQE PICs allow a commit of the configuration, but the interfaces remain at the full E3 rate for the Kentrox compatibility mode.
4-port and 2-port channelized DS3(T3) IQ PICs do not support Adtran and Verilink compatibility modes. If configured, the default mode is applied on both the interfaces.
To configure a T3 interface so that it is compatible with the CSU at the remote end of the line, include the compatibility statement at the [edit interfaces interface-name t3-options] hierarchy level:
The subrate of a T3 interface must exactly match that of the remote CSU. To specify the subrate, include the subrate statement in the configuration:
For Adtran CSUs, specify the subrate as a number from 1 through 588 that exactly matches the value configured on the CSU. A subrate value of 588 corresponds to 44.2 Mbps, or 100 percent of the HDLC-encapsulated payload. A subrate value of 1 corresponds to 44.2 / 588, which is 75.17 Kbps, or 0.17 percent of the HDLC-encapsulated payload.
For Digital Link CSUs, specify the subrate as the data rate you configured on the CSU in the format xKb or x.xMb. For Digital Link CSUs, you can specify the subrate value to match the data rate configured on the CSU in the format xkb or x.xMb. You can configure the subrate values shown in Table 1.
For Kentrox CSUs, specify the subrate as a number from 1 through 69 that exactly matches the value configured on the CSU. A subrate value of 69 corresponds to 34.995097 Mbps, or 79.17 percent of the HDLC-encapsulated payload (44.2 Mbps). A subrate value of 1 corresponds to 999.958 Kbps, which is 2.26 percent of the HDLC-encapsulated payload. Each increment of the subrate value corresponds to a rate increment of about 0.5 Mbps.
For Larscom CSUs, specify the subrate as a number from 1 through 14 that exactly matches the value configured on the CSU. A subrate value of 14 corresponds to 44.2 Mbps, or 100 percent of the HDLC-encapsulated payload. A subrate value of 1 corresponds to 44.2 / 14, which is 3.16 Mbps, 7.15 percent of the HDLC-encapsulated payload.
For Verilink CSUs, specify the subrate as a number from 1 through 28 that exactly matches the value configured on the CSU. To calculate the maximum allowable peak rate, multiply the configured subrate by 1.578 Mbps. For example, a subrate value of 28 corresponds to 28 x 1.578 Mbps, which is 44.2 Mbps, 100 percent of the HDLC-encapsulated payload. A subrate value of 1 corresponds to 1.578 Mbps, 3.57 percent of the HDLC-encapsulated payload. A subrate value of 20 corresponds to 20 x 1.578 Mbps, which is 31.56 Mbps, 71.42 percent of the HDLC-encapsulated payload.
Verilink configuration is not functional if an IQ interface is paired with an IQE interface.
Verilink configuration on an IQE PIC is also not functional when the PIC is connected to any other vendor equipment that operates in Verilink Port B mode. The Verilink configuration on an IQE PIC works only when it is paired with another IQE PIC or any other vendor equipment that operates in Port A mode.
When subrate is configured under a T3 interface on a Channelized OC12 IQE PIC in SONET mode, link flaps are observed periodically when the interface is congested close to the subrate level. This behavior is expected. However, the flaps are not seen with shaping rates of traffic that are approximately 95 percent of the configured subrate. We recommend that when you configure the subrate on Channelized DS3/E3 Enhanced IQ (IQE) PICs, Channelized OC48/STM16 IQE PICs, Channelized OC12/STM4 IQE PICs, and Channelized OC3/STM1 PICs, configure the shaping rate of the interface to be the same as the subrate. For example, if you want to configure the subrate and shaping rate on a t3-fpc/pic/port:n interface, you can configure both the rates to be the same as follows:user@host# set interfaces t3-3/1/0:1 t3-options compatibility-mode digital-link subrate 18.0Mbuser@host# set class-of-service interfaces t3-3/1/0:1 shaping-rate 18m
Table 1: Subrate Values for T3 Digital Link Compatibility Mode
For information about subrating an E3 interface, see Configuring the E3 CSU Compatibility Mode.