7.5 Serial Interfaces

Serial interfaces come in several different types and speeds. These interfaces are based on the DS hierarchy from DS-0 to DS-3 of the circuit-switched network types discussed in Chapter 2. These interfaces can be either unchannelized or channelized. Unchannelized DS hierarchy interfaces are referred to as concatenated because all the channels on the interface are linked together, forming one large bandwidth interface.

Interfaces that are multiplexed into smaller streams are channelized. These channels can be bonded together to transmit data for a logical unit at a certain bandwidth. The root for these channels is the DS-0 or 64Kbps circuit that is used both in the global and the U.S. DS hierarchy. DS-1, E1, E3, and DS-3 are all very common implementations of the DS hierarchy, each having a certain number of DS0s. These DS-0s can be grouped together to form a channel of a required bandwidth. If a logical unit only needs to be capable of 512Kbps, it can be created by assigning eight DS-0s.

When nonconcatenated, the channels must be assigned to create a single logical unit made up of all the assigned channels. More detail on the DS hierarchy can be found in Chapter 2. The following sections describe how to configure physical, logical, and channelized T3s and bit error rate test (BERT) interfaces.

7.5.1 Physical T3 Configuration

The T3 physical parameters are configured in the same manner as the SONET interfaces. Remember that nonconcatenated SONET was derived from the need to multiplex DS circuits. The most common physical properties are configured under the physical interface and the t3-options hierarchy. Under the [chassis fpc (#) pic (#)] command, the whole PIC can be configured for nonconcatenation in the same manner as the SONET PICs. By default, the T3 interfaces are concatenated. With a T3 PIC in FPC 2 PIC position 1, the hierarchy and command ( set no-concatenate ) used to change concatenation are shown below:

 [edit chassis fpc 2 pic 1]  lab@Boston# set ? Possible completions: + apply-groups         Groups from which to inherit configuration data   framing              Framing mode   no-concatenate       Don't concatenate channels   vtmapping            VT mapping mode [edit chassis fpc 2 pic 1] user@Boston# 

The default encapsulation for T3 interfaces is PPP, and the default clocking is internal. These two parameters are configured directly in the [edit interfaces (interface) ] hierarchy. The t3-options hierarchy under the interface contains T3-specific parameters that can be modified, such as BERT parameters, parity configuration, 16- or 32-bit checksum, and compatibility-mode configuration, among others, as shown below:

 [edit interfaces t3-2/1/0]  lab@Boston# set t3-options ? Possible completions: + apply-groups         Groups from which to inherit configuration data   bert-algorithm       Set BERT algorithm   bert-error-rate      Bit error rate to use in BERT test (10^-n) (0..7)   bert-period          Length of BERT test (1..240 seconds)   cbit-parity          Enable C-bit parity mode > compatibility-mode   Set CSU compatibility mode   fcs                  Frame checksum   feac-loop-respond    Respond to FEAC loop requests   idle-cycle-flag      Value to transmit in idle cycles   long-buildout        Set hardware to drive line longer than 255 feet   loop-timing          Set loop-timing for all T1 channel under CT3   loopback             Loopback mode   no-cbit-parity       Don't enable C-bit parity mode   no-feac-loop-respond  Don't respond to FEAC loop requests   no-long-buildout     Don't set hardware to drive line longer than 255 feet   no-loop-timing       Don't set loop-timing for all T1 channel under CT3   no-payload-scrambler  Don't enable payload scrambling   payload-scrambler    Enable payload scrambling   start-end-flag       Set start-end flags on transmission [edit interfaces t3-2/1/0] user@Boston# 

Note

Do not forget to set clocking on one side of the T3 link to external .

7.5.2 T3 Logical Configuration

With a concatenated interface, logical unit configuration is no different than it is on single-interface ports. A single unit is used for the family addressing and protocol assignment. Shown here is a concatenated T3 interface configured on router Boston. Configuring the unit and family inet address is the same as on the previous interfaces in SONET. Notice the clocking and encapsulation parameters.

 [edit interfaces t3-2/1/0]  lab@Boston# show clocking external; encapsulation cisco-hdlc; unit 0 {     family inet {         address 192.168.100.1/30;     } } [edit interfaces t3-2/1/0] lab@Boston# 

The output of a show interfaces (interfaces) command is shown below:

 lab@Boston> show interfaces t3-2/1/0  Physical interface: t3-2/1/0, Enabled, Physical link is Up   Interface index: 16, SNMP ifIndex: 44   Link-level type: Cisco-HDLC, MTU: 4474, Clocking: External, Speed: T3,   Loopback: None, FCS: 16, Mode: C/Bit parity   Device flags   : Present Running   Interface flags: Point-To-Point SNMP-Traps   Link flags     : Keepalives   Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3   Keepalive: Input: 348 (00:00:01 ago), Output: 360 (00:00:09 ago)   Input rate     : 0 bps (0 pps)   Output rate    : 0 bps (0 pps)   Active alarms  : None   Active defects : None Logical interface t3-2/1/0.0 (Index 7) (SNMP ifIndex 57)     Flags: Point-To-Point SNMP-Traps Encapsulation: Cisco-HDLC     Protocol inet, MTU: 4470, Flags: None       Addresses, Flags: Is-Preferred Is-Primary         Destination: 192.168.100.0/30, Local: 192.168.100.1 lab@Boston> 

7.5.3 T3 Channelized Configuration

Configuring a channelized interface is a little more complicated. Since a channelized T3 is actually 28 T1s, these T1s can be individually configured. Logical units are made from groups of DS-0s or DS-1s that are bound together to form a logical interface. Since the T3 interface defaults to concatenated, the first configuration step is to deconcatenate the interface. This is done on the entire PIC. If you have a 4-port T3 PIC, all four ports will be changed to channelized mode through the use of the no-concatenate command. Below, PIC 1 of FPC 2 changed to channelized with the set no-concatenate command in the [edit chassis fpc (x) pic (x) ] hierarchy.

 [edit]  lab@Boston# edit chassis fpc 2 pic 1 [edit chassis fpc 2 pic 1] lab@Boston# set no-concatenate [edit chassis fpc 2 pic 1] lab@Boston# 

To configure a specific T1 in a T3, a colon and the T1 number is added to the interface name . A T1 #2 located in interface t3-2/1/0 would be named t1-2/1/0:2 . This is the third (remember that T1 0 is the first) T1 of the T3 interface in position 2/1/0 . Enter the T1 configuration just as a regular interface, as shown below:

 [edit interfaces]  lab@Boston# edit t1-2/1/0:2 [edit interfaces t1-2/1/0:2] lab@Boston# 

Once in the T1 interface configuration hierarchy, the unit can be configured with protocol addressing on the logical units, as was shown in previous sections.

7.5.4 BERT Configuration

A BERT is used to test the quality of a link. One device loops the receive signal to the transmit link, which sends whatever comes into the interface back to the sender. The sender then can compare what it has sent to what it receives to ensure that there are no errors. Any errors are counted and reported . In Figure 7-15, two T3 interfaces, A and B, are connected. The arrows indicate the path the data takes, from Tx (transmit) on one, to Rx (receive) on the other.

Figure 7-15. A BERT Configuration

If an administrator had reason to believe that the link was not passing traffic properly, one of the T3 interfaces could be looped to connect the Rx to the Tx. This would resend the same data that came in the interface back to the sender, as Figure 7-16 illustrates. In this example, any data that was sent to T3 A from T3 B would be sent back to T3 B.

Figure 7-16. Testing a Looped Interface with a BERT

This loop allows T3 B to put out a stream of characters and compare what was sent to what is received, which in turn allows testing for errors across the full link. The BER is the Bit Error Rate found during testing. DS-3 B should receive the stream of 123456 exactly as it sent it.

To test a line with the error tester, the far-end connection must be in looped mode. The BERT configuration takes place in the t3-options of the physical interface, as is shown below. Here are the possible command completions of the set command (use set b? to show only BERT options):

 [edit interfaces t3-2/1/0 t3-options]  lab@chicago# set b? Possible completions:   bert-algorithm       Set BERT algorithm   bert-error-rate      Bit error rate to use in BERT test (10^-n) (0..7)   bert-period          Length of BERT test (1..240 seconds) [edit interfaces t3-2/1/0 t3-options] lab@chicago# 

The BERT algorithm is the data pattern you are sending. It is important to know the pattern of 1s and 0s that is sent to ensure that it is received properly. In addition, the transmitted pattern can be observed along different parts of the link using test equipment, which assists in determining where the pattern is getting corrupted, if appropriate. The following output demonstrates the possible algorithms that can be selected:

 lab@chicago# set bert-algorithm ?  Possible completions:   all-ones-repeating   Repeating one bits   all-zeros-repeating  Repeating zero bits   alternating-double-ones-zeros  Alternating pairs of ones and zeros   alternating-ones-zeros  Alternating ones and zeros   pseudo-2e10          Pattern is 2^10 - 1   pseudo-2e11-o152     Pattern is 2^11 -1 (per O.152 standard)   pseudo-2e15-o151     Pattern is 2^15 - 1 (per O.152 standard)   pseudo-2e17          Pattern is 2^17 - 1   pseudo-2e18          Pattern is 2^18 - 1   pseudo-2e20-o151     Pattern is 2^20 - 1 (per O.151 standard)   pseudo-2e20-o153     Pattern is 2^20 - 1 (per O.153 standard)   pseudo-2e21          Pattern is 2^21 - 1   pseudo-2e22          Pattern is 2^22 - 1   pseudo-2e23-o151     Pattern is 2^23 (per O.151 standard) [edit interfaces t3-2/1/0 t3-options] lab@chicago# 

Next, the BERT time period is configured. This is how long the test will last. The default is 10 seconds and can be up to 240 seconds. The example below sets it to 30 seconds and shows that the alternating-double-ones-zeros BERT algorithm has also been set.

 [edit interfaces t3-2/1/0 t3-options]  lab@chicago# set bert-period 30 [edit interfaces t3-2/1/0 t3-options] lab@chicago# show bert-algorithm alternating-double-ones-zeros; bert-period 30; [edit interfaces t3-2/1/0 t3-options] lab@chicago# 

The last configurable item, the bit error rate, allows for errors to be inserted intentionally. It is set to errors by default, but ranges from to 7 . If 5 is entered, the BER becomes 10 ^“5 , or 1/100,000, and the BERT process would insert 1 error every 100,000 bits. Inserting errors at a specific rate can be helpful in determining the accuracy of the error checking equipment along the connection. In addition, if the error rate the interface receives is the same as that it sent out, then the connection is good both ways, and so is the error rate counter.

Once the configuration is complete, the interface must be disabled before the test can be run. The interface is disabled so that no other data will be present on the link while the interface is sending the test stream. To run the test, the test command completions you can use for that particular interface are as follows .

 lab@Boston> test interface t3-2/1/0 ?  Possible completions:   e1-bert-start             Start BERT   e1-bert-stop              Stop BERT   e3-bert-start             Start BERT   e3-bert-stop              Stop BERT   feac-loop-initiate        Initiate FEAC loopback   feac-loop-terminate       Terminate FEAC loopback   restart-auto-negotiation  Restart Auto Negotiation   t1-bert-start             Start BERT   t1-bert-stop              Stop BERT   t3-bert-start             Start BERT   t3-bert-stop              Stop BERT lab@Boston> 

Once you start BERT, you can show the interface with the extensive option to view the BERT results under the DS-3 BERT section, as is shown below. The show interfaces command output has been cut for brevity to show only the BERT section of the output.

 lab@Boston> show interfaces t3-2/1/0 extensive  Physical interface: t3-2/1/0, Administratively down, Physical link is Up   Interface index: 16, SNMP ifIndex: 44, Generation: 15 <<<output omitted for brevity>>>  DS3 BERT configuration:   BERT time period: 30 seconds, Elapsed: 15 seconds (in progress)   Algorithm: Double alternating ones/zeros, Repetitive (25), Induced Error rate: 10e-0   Bit count      :    679742966, Overflows: 0   Error bit count:       217553, Overflows: 0   Error rate: 10e-3.5   LOS status: OK, LOS count: 1, LOS seconds: 15  <<<output omitted for brevity>>> 

Notice the error rate. The errors are counted as 1 of every x. If an error occurs every 100 packets, that is 10 ^“2 ; that is, 1/100 is 10 to the power of negative 2. To stop the complete BERT process, use the bert-stop command shown below:

 lab@Chicago> test interface t3-2/1/0 t3-bert-stop 

A BERT can be extremely useful when dealing with copper media, which can be more error prone than an optical media type. It allows the transmission of a set pattern of bits to be sent and the pattern to be checked as it returns.