Archive for E1

WANs II: A New “Lease” on Life!

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 20, 2012 by jjrinehart

Private/Leased Line Network

One of the sayings I tend to use when explaining network concepts is a twist on the opening words of the book of Genesis, “In the beginning was the mainframe…” because so many things we take for granted today started with that piece of technology.  In terms of WAN connectivity, the original form of connectivity took place across specialized telephone lines that carried data rather than voice conversations.  Because the end-user/customer paid the IXC for the exclusive use of the line, they were referred to as private lines or point-to-point leased lines.  As mainframes were replaced by personal computers (and networks) these lines connected to devices that convert bits of data to electrical impulses that can be transmitted across these lines for long distances.  The technical term for this device is a Channel Service Unit/Data Service Unit, or CSU/DSU, which used to be an external device but are now integrated on interface modules on routers.  This is actually a good time to introduce two more terms, namely DCE and DTE.  A Data Terminal Equipment (DTE) device is the terminal or end-point sending and receiving information, usually a computer or router.  DCE devices, on the other hand, perform the conversion between raw data and the format needed for transmission, such as a CSU/DSU or modem.  The acronym stands for either Data Communications Equipment or Data Circuit-Terminating Equipment, depending on the publisher.  Cisco prefers the latter term as a general rule.

Private lines utilize electrical circuits to create the pathway between locations, as illustrated in the diagram above; this is in contrast to packet switched networks (which we will deal with later).  Depending on the part of the world you live in, there will be differing names and capacities, such as T1 (1.544 Mbps) or E1 (2.048 mbps), and even higher speeds.  These are typically copper connections, with very high speeds delivered on fiber optic connections (OC-X).  In North America these are the T1/DS1 and T3/DS3 standards, while the rest of the world utilizes the E1/E3 standards.  These lines are charged by mileage and often very expensive as a result, although very secure since a customer has private and full-time use of the connection, although if it ever sits idle that ends up being wasted bandwidth.  The structure of the framing and line coding ca be rather complex, and something you should definitely be familiar with as you pursue your certification studies.

Next time we will look at packet-switched WANs! Don’t you just love this stuff?

– Joe

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OSPF V: “You Have Chosen…Wisely” Path Selection Process

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 1, 2012 by jjrinehart

You Have Chosen…Wisely

In my opinion, the best movie in the Indiana Jones film franchise was Indiana Jones and the Last Crusade; aside from the pure enjoyment of the action scenes, the film gave great attention between “Indie” and his father.  One of the most-quoted lines from this movie, in pop culture at least, was the statement by the night (pictured above), “You have chosen…wisely.”  The whole concept of making the best possible choice ties in particularly well with OSPF, which puts it head-and-shoulders above Distance Vector routing protocols, since it truly can choose…wisely!  If you recall our earlier discussion about Link-State routing protocols, instead of depending on “mileage” (how far away something is”, the basis for route selection is cost, related to bandwidth.
OSPF uses cost in a cumulative manner–meaning that all of the costs of the links to a destination network are added up together.  If you have ever used a GPS for travel, this makes perfect sense, since the device would recommend the eight-lane Interstate highway (greater “bandwidth”) over the two-lane country road, regardless of the distance.  Ironically, you can even specify the route to avoid toll roads (a different spin on cost), to choose the best way to go.  If you think about it, this makes perfect sense, since you can go faster and have fewer stops on the bigger road, especially if you are tracking the route of travel from end-to-end.  In networking terms, then, OSPF devices will choose a 1.544 T1 link over the vastly inferior 56K link in choosing the best route.  What could be simpler?
Remember how strict and rule-oriented OSPF is?  Well, this applies to route preferences as well, meaning that there are additional selection criteria that will override the cost directive.  This fits into the hierarchy of OSPF areas, and creates the following list of route selection preferences:
1.  Intra-Area: Always choose the path within the area first.
2. Inter-Area: If no routes to the destination exist within the area, choose a path to another area but within the OSPF domain.
3. External Type 1: If no routes to the destination exist within the OSPF domain, choose an E1 route (remember that E1 routes count the cost to the external router in the metric)
4. External Type 2: If no E1 routes exist to the destination, choose an E2 route (remember that E2 routes do not count the cost to the external router in the metric, and redistributed routes are E2 by default)
As you can see, this can substantially change the route selection process.  Next time we will look at EIGRP, which is vastly simpler.
– Joe