Aaron's Worthless Words

These are the notes I’ve taken as I read through the study materials.  Feel free to correct anything you see.

• Analog phone signaling
  • Misc
    • Ground = positive = tip
    • Battery = negative = ring
    • Signaling uses specific frequencies for specific events
  • Loop start signaling
    • When a circuit in the phone is completed (i.e., you take it off-hook), the CO detects it and provides services.
    • Susceptible to glare, where the phone requests dialtone at the same time that the CO sends a call.
      • Can connect two different calls if in a business with multiple lines
  • Ground start signaling
    • The circuit is temporarily completed to signal the CO for services
    • Doesn’t connect any call to any phone directly
    • Used in PBXes.
  • Supervisory signaling
    • On-hook:  Circuit is open
    • Off-hook:  Circuit is completed
    • Ringing:  AC current generated by CO to tell the phone to ring
  • Informational signaling
    • Gives information for the caller to use
    • Dial tone
    • Busy
    • Ringback: the ring you hear when you call
    • Confirmation:  the call is being attempted
    • Congestion:  no lines available to make the call
    • Receiver off-hook
    • Reorder:  can’t make the call
    • No such number:  can’t find the endpoint
  • Address signaling
    • Used to send digits
    • Dual-tone multifrequency (DTMF):  uses two electrical signals to indicate a digit; touch tone
    • Pulse:  flashes the circuit to indicate a digit; rotary dial
  • Disadvantages of analog signaling
    • Attenuation
    • Repeaters can’t differentiate between call and noise
    • One cable pair for each call; think about a pair for each call taking place in Manhattan right now

• Digitizing voice
  • Steps
    • Sampling: taking samples of the voice
      • Nyquist method:  sample rate = 2 x highest frequency
      • Human voices usually stay below 4000Hz, so a good sampling rate is 8000 samples/second.
      • Pulse-amplitude modulation (PAM)
    • Quantization:  assigning values to the sample
      • Assignment based on amplitude of the signal
      • Logarithmic scale for better accuracy at the more common amplitudes
    • Encoding:  converting quantization to binary
      • Pulse-code modulation (PCM)
      • 8 bits/sample * 8k samples/second = 64k bpbs
    • Compression:  optionally compress the binary information
  • Advantages
    • Transmitting numbers is less susceptible to attenuation
    • Multiple digital voice signals can use same pair
      • Time division multiplexing (TDM)

• Digital signaling
  • 24 channels * 8 bits/sample = 192 bits of voice
  • The T1 frame sends all 24 channels in one T1 frame with 1 bit for framing bit, so the T1 frame = 193 bits
  • 193 bits/frame * 8k frames/second = 1.544 Mbps
  • Channel associated signaling (CAS):  steals bits in a channel for signaling
    • The 8th bit of every 6th sample is stolen for signaling
    • Super frame (SF) uses 12 frames to synchronize a signal, so 12 samples are required to be received for synchronization (12/8000 second).
    • Extended super frame (ESF) uses 24 frames; 2000 bps for sync, 2000 bps for errors, 4000 bps for control and reporting
  • Common channel signaling (CAC):  uses a dedicated channel for signaling
    • Q.931 is a CAC signaling standard.

• The PSTN
  • Phone companies connect together using SS7 signaling (a CAC method), which is responsible for routing the call.
  • E.164 is an ITU standard for phone numbers.
    • Country code
    • National destination code
    • Subscriber number

Feel free to correct.  I feel like I’m missing a big piece here, so please fill in a gap if you see one.  Thanks.  :)

Study Questions

• How many area 0s (zero) can you have in an OSPF implementation

Just one.

• If my company merges with another company, and we’re both running OSPF, how can we get our networks routing together properly?

The easiest thing to do is to connect your two area 0s together through some physical link.  If you can, you can use virtual links to connect an ABR to another ABR to extend the zones together.

• How do you configure virtual links?

R1(config-router)#area 1 virtual-link 1.1.1.1

• That IP in the virtual link command looks like a loopback.  What’s up with that?

It’s the RID of the router to which you want to connect.

• Why wouldn’t I just use a GRE tunnel between my two routes and put that in zone 0?

That’s a good question.  I would probably do that instead of virtual links if I had the choice because it eliminates any weird problems you may see with the virtual links.  [Someone pipe up on this one, please.]

• What types of authentication can you do with virtual links?

None
Clear text
MD5

• I’ve configured frame-relay map ip 1.2.3.4 101 on my s0/0/0.1, but I can’t get a neighbor to come up.  What gives?

A non-broadcast medium can’t detect neighbors dynamically.  You need static neighbors, or you can add the broadcast keyword to the end of your map statement.

• What is the big problem with partial mesh frame relay topologies when OSPF comes into play?

Not all routers are connected directly, so some routes won’t see all the neighbors.  When OSPF routes propagate on broadcast medium, the next-hop is the router what propagated it; you’ll wind up seeing routes to routers to which you don’t connect.

• How do you get over the partial mesh problem?

You can statically configure frame relay maps pointing the IPs of the unconnected routers to the DLCI of a router that is connected to them (like a central hub router).

• What network types use DRs and BDRs?

Anything multiaccess, so the NBMA and BMA.

• Which network types can dynamically discover neighbors?

Broadcast and point-to-multipoint

• Why do you have to configure static neighbors on NBMA and point-to-multipoint non-broadcast links?

Since OSPF uses multicast to talk to neighbors, a router treats the packets like a broadcast.  Since these network types don’t have a broadcast capability, the only way a neighbor will be established would be through static statements.

• You have a hub-and-spoke topology over a frame relay cloud.  One of your hubs sees routes for all the networks at the other other hub sites.  Is all as well as it seems?

No.  The routes to the other hub networks will have their next hop set to the frame relay IP of each hub router.  Since one hub router can’t get directly to others, the router won’t be able to pass traffic to those sites at all.  You’ll need to statically map those IPs to the DLCI of the hub site for traffic to flow as expected.

What Command Was That

Which command(s)…

• …define a virtual link using the MD5 has of the key “test”?

R1(config-router)#area 1 virtual-link 1.1.1.1 authentication message-digest
R1(config-router)#area 1 virtual-link 1.1.1.1 message-digest-key 1 md5 test

• …configure a static OSPF neighbor?

R1(config-router)#neighbor 1.2.3.4 [cost X] [priority Y]

• …shows the status of a virtual link?

show ip ospf virtual-links
show ip ospf neighbor

• …shows the authentication type and youngest key for a virtual link?

show ip ospf virtual-links

• …displays the network type for an interface?

show ip ospf interface

I took and passed the ISCW test today.  I was super-nervous going into it, which is weird for me, but I finally calmed down after the first few questions.  Here’s my take.  I don’t want to get into any trouble so I’m not going to include very much detail.

The testing center wasn’t very good at all.  It’s in an old building on the busiest road in town, and the noise from the street was barely dampened by the 1960s building materials.  I can tell you that there are three different pipes in the walls since their vibrations resonated through the room every time somebody flushed or brewed some coffee.  There was also a little foot traffic, which can be expected anywhere; they were working through some software problems on another testing station and were very respectful, so it wasn’t too bad.  The worst part of the whole ordeal, though, was the Microsoft class I sat through while taking the test.  They were across the hall, but it sounded like they were in the room with me.  Usually, you hear the instructor yelling at the top of his lungs so the whole class can hear, but I could hear questions being asked and papers being moved.  I think I can go pass a test of AD replication, though.  I certainly won’t be using that facility for any more tests.

The test itself was fair and pretty close to where it should be.  The questions were well rounded and covered the book from front to back.  I missed a few due to my ADD kicking in and not letting me finish reading all the answers.  At least twice, I saw a more appropriate answer just as I released the mouse from the Next button. 

There were lots of interactive items- a lot more than I thought there would be or that there should be.  I can understand a few do-this-do-that things, but there were at least ten interactive questions, whether they be “put these in order”, “match the definition”, or “tell me what’s going on”.  Some of these had multiple parts that I had to click back and forth to get all the answers.  One of them in particular could have been more easily presented as an exhibit at the top of a question than a question that really zaps your time.  There were a few SDM questions, but I made it through those by clicking around until I found the info.

There were two simulations that were very straightforward and very easy.  The sim would present the scenario and tell you what the end result should be along with any details.  I found that some details had to be configured in the order the details were presented to finish the lab.  Not all of them, mind you, but enough of them to get annoying; I really expected something a little more robust.  The contextual help and autocomplete worked, though, so that’s a plus.

I had a big issue with time, and, if that happens to me, it can happen to anyone.  The test started with a multi-part interactive question that took me a long time to figure out through the nerves and discord.  I would guess that I got a simulation or interactive question in 8 of the first 11 questions, and, at one point, I looked at the clock to see I had 40 minutes and 38 questions left, so I started picking up the pace.  Luckily, after question 41, the testing gods showed mercy and ended the suffering.

Overall, I give the test an 8 out of 10.  It was very honest and frank with none of the nonesense of trying to trick me.  All of the problems I had were either from my lack of knowledge or my being so easily distracted this morning.  As Cisco goes, it’s not a bad test at all.

Send any ear plugs questions my way.