NOC Interview advise

eten · July 2010

Hello guys.

I've recently got an interview for a NOC position from a medium sized fiber optic company. I myself have never been worked in a NOC before, and they will be conducting a technical evaluation on me.... as usual.

Is there anything in particular I should prepare for, besides the general network admin/engineer questions that I find on the net? Their qualification lists SONET and HP Network Node where I have no knowledge on. Any last minute brush ups will be greatly appreciated.

Thanks.

filkenjitsu · July 2010

Hello!

Congrats on your interview!

I work in a Cellular Telecommunications NOC and I think my advice may be helpful.

I would say the main stuff to know would be:

The different sizes of trunk, for instance, what is a T1, what is an E1, the differences, how many DS0s make up a T1? a T3?

What is the OC system and what are the various bandwidths of each type, from OC-3 to OC-192 and above. How many T1s are in an OC-3 (many times OC trunks are channelized into T1 circuits or T3 circuits.

What is the physical construction of a fiber optic cable?

What are the three different fiber types? Single Mode, Step Index, and Graded Index. know what those mean.

What is Chromatic Dispersion?

What is Polarization Mode dispersion?

loss in fiber optic strands is called absorption.

What is DWDM?

Light sources? LED and Laser, what are the differences?

A Fiber Optic Multiplexer sits at each end of the fiber, it accepts the electical interfaces on one side and does the electrical to optical and back and forth conversions. It also combines multiple signals and sends the down one pipe or trunk.

Some multiplexers are called ROADM or Reconfigurable optical add drop multiplexers. It is a pretty common term.

I don't know, you may or may not need to know this stuff, but if you are asked "What do you know about fiber optics/fiber optic networks?" you will be ready!

Also, make sure to know what DWDM is as I said before, it is important.

The main thing you do in a NOC is remotely configure, administer, and reset equipment through some sort of Telnet, SSH command line or a GUI application. You pretty much respond to alarms of trouble, use your mouse to create a ticket from the alarm from your alarmer, it goes into Remedy or some other ticket system, you work the ticket, call the right people, rinse and repeat. You will work with many types of telcom and data equipment, so it is a great job with great experience. I am sure you will be working with MPLS and BGP networks.

Also, it is good to know telco terms as you are working with other telecom companies all the time.

Stuff like:

CO
MTSO
POP
Trunk
Circuit
etc.

filkenjitsu · July 2010

Many common Terms:

5ESS
A digital central office switching system made by Lucent, typically used to serve local subscribers.

2-wire
Two-wire loops support loop-start, ground-start, loop reverse-battery or customer-provided inband signaling. 2-wire Digital loops are a Verizon Wholesale Unbundled Loop service and provide a digital 2-wire enhanced channel. Learn more.

4-wire
Four-wire loops support duplex signaling in addition to all of the same capabilities as 2-wire loops. 4-wire Digital loops are a Verizon Wholesale Unbundled Loop service and provide a digital 4-wire enhanced channel. Learn more.

Access/Local Tandem Switch
Switching systems that concentrate and distribute traffic originating from or terminating at end offices in the access/local service area. See tandem switch.

Tandem Switch
Tandem is a telephony term meaning "to connect in series." Thus, a tandem switch connects one trunk to another. A tandem switch is an intermediate switch or connection between an originating telephone call or location and the final destination of the call.

Also, how does a T1 work?

The Electrical Interface
The T1 interface consists of two pairs of wires - a transmit data pair and a receive data pair. Timing information is embedded in the data.

T1 utilizes bipolar electrical pulses. Where most digital signals are either a ONE or a ZERO (unipolar operation), T1 signals can be one of three states. The ZERO voltage level is 0 volts, while the ONE voltage level can be either a positive or a negative voltage.
Encoding Methods
There are a number of different encoding methods used on T1 lines. Alternate Mark Inversion (AMI), Bipolar With 8-Bit Substitution (B8ZS), and High Density Bipolar Three Code (HDB3) will be discussed here.

AMI encoding causes the line to alternate between positive and negative pulses for successive 1's. The 0's code is no pulse at all. Thus, a data pattern of 11001011 would cause the following pattern on an AMI line: - +,-,0,0,+,0,-,+.

With this encoding technique there is a problem with long strings of 0's in the user's data which produce no transitions on the line. The receiving equipment needs to see transitions in order to maintain synchronization. Because of this problem, DS-1 specifications require that users limit the number of consecutive 0's in their data steam to less than 15.

With this scheme of encoding there should never be consecutive positive or negative pulses on the line (i.e., the following pattern should never occur: 0,+,-,+,+,-). If two successive positive or two successive negative pulses appear on the line, it is called a Bipolar Violation (BPV). Most T1 systems watch for this event and flag it as an error when it occurs.

B8ZS and HDB3 are both methods which permit the user to send any pattern of data without affecting the operation of the T1 line. Both of these encoding schemes make use of BPVs to indicate that the users data contains a long string of 0's.

B8ZS looks for a sequence of eight successive 0's and substitutes a pattern of two successive BPVs. The receiving station watches for this particular pattern of BPV's and removes them to recreate the original user data stream.

HDB3 is the scheme recommended by the CCITT. This scheme watches for a string of four successive 0's and substitutes a single BPV on the line.
T1 Framing Techniques
D4 Framing
The original framing format for T1 was D4 framing. A D4 frame consists of 192 data bits: 24 channels X 8 bits per channel and a single framing bit.

D4 defines a 12-bit framing sequence which is sent as the 193rd bit in 12 consecutive frames. These 12 frames together are referred to as a superframe.

The framing pattern is defined as 100011011100. This pattern repeats continuously and the receiving equipment locks onto it in order to properly synchronize with the incoming data.

In order to send supervisory information over a D4 link "bit robbing" is used. A voice signal is not significantly affected if the low-order bit in a byte is occasionally wrong. D4 framing makes use of this characteristic of voice and uses the least-significant bits in each channel of the 6th (A Bit) and 12th (B Bit) frames to send signalling information; on-hook, off-hook, dialing and busy status.

D4 framing requires that the 8th bit of every byte of every frame be set to a 1 when data is transmitted. This requirement guarantees the required 1's density on the link, regardless of the contents of the user data. This requirement reduces the bandwidth available to the user from 64 Kbps to 56 Kbps (7 bits/frame X 8,000 frames/second).
Extended Superframe (ESF) Framing
The Extended Superframe Format (ESF) extends the D4 superframe from 12 frames to 24 frames. ESF also redefines the 193rd bit location in order to add additional functionality.

In ESF the 193rd bit location serves three different purposes:

* Frame synchronization
* Error detection
* Maintenance communications (Facilities Data Link - FDL)

Within an ESF superframe, 24 bits are available for these functions. Six are used for synchronization, six are used for error detection, and twelve are used for maintenance communications.

In D4 framing, 12 bits are used per superframe for synchronization. In ESF framing, 6 bits are used per superframe for synchronization.

There is no link-level error checking available with D4 framing (except for bipolar violations). ESF framing utilizes a 6-bit Cyclic Redundancy Check (CRC) sequence to verify that the frame has been received without any bit errors. As a superframe is transmitted, a 6-bit CRC character is calculated for the frame. This character is then sent in the six CRC bit locations of the next superframe.

The receiving equipment uses the same algorithm to calculate the CRC on the received superframe and then compares the CRC value that it calculated with the CRC received in the next superframe. If the two compare, then there is a very high probability that there were no bit errors in transmission.

As was stated earlier, 12 bits are used for maintenance communications. These 12 bits give the maintenance communications channel a capacity of 4,000 bits per second. This function enables the operators at the network control center to interrogate the remote equipment for information on the performance of the link.

As with D4 framing ESF utilizes "robbed bits" for in-band signalling. ESF utilizes 4 frames per superframe for this signalling. The 6th (A bit), 12th (B bit), 18th (C bit), and 24th (D bit) frames are used for the robbed bits. The function of the robbed bits is the same as in D4 framing.
T1 Error
T1 has a number of other defined alarm and control signals. The alarm signals have different color designations and are used to indicate serious problems on the link. These alarm signals are defined as:

Red Alarm
This is a local equipment alarm. It indicates that the incoming signal has been corrupted for a number of seconds. The red alarm shows up visually on the equipment that detects the failure. This equipment will then begin sending a yellow alarm as its outbound signal.
Yellow Alarm
The yellow alarm alerts the network that a failure has been detected. The yellow alarm pattern has a number of different definitions. The most common D4 definition is to set 1 bit of every channel to a ZERO.
Blue Alarm
A blue alarm indicates the total absence of incoming signal. This alarm also serves to keep the circuit in synchronizations by sending continuous transitions (an all 1's pattern).

T1 Overview

filkenjitsu · July 2010

Just wanted to comment on my copy paste of T-1 info.

Just make sure you know what Extended Super Frame and B8ZS are. Those are what you will see 99 percent of the time.

eten · July 2010

Thanks for the insightful share. That was a lot more info than I thought I needed to prepare for. Interview is in a few days, but it looks like it is mostly memorization and barely any concepts.

NOC Interview advise

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