Download Common WAN Components and Issues

Common WAN Components and Issues
Lesson overview.
In this lesson, we will cover:
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Common WAN components.
Common WAN issues.
Common WAN components.
Wide area network (WAN) connections may require components that are different than those
found within the local area network (LAN). Some of the most common components that may be
present with a WAN connection are explained below.
Copper line drivers or repeaters.
Copper line drivers, or repeaters, are used to allow network traffic to travel farther distances
over copper wire type networks (e.g., the PSTN network). They take an incoming signal and
regenerate it, boosting the strength of that signal. Then they send it back out, thus reducing
attenuation.
Highlights:
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Copper line drivers are used to allow network traffic to go farther distances over copper
wire type networks.
They take an incoming signal and regenerate it and then send it back out, reducing
attenuation.
Demarc.
Common to all WANs is the demarc. The term “demarc” is short for demarcation point. This is
the physical point where the responsibility of the telecommunication company (Telco) ends and
the customer's responsibility begins. The Telco takes care of the upstream end of the network
and the customer takes care of the downstream end of the network. The demarc may be simple
or it may be very complex, depending upon the size of the organization and the required
services.
Highlights:
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Demarc = the demarcation point: the physical point where the Telco's responsibility ends
and the customer’s begins.
The Telco takes care of the upstream end of the network.
The demarc may be simple or very complex, depending on the size of the organization
and the required services.
Network interface unit (NIU).
In the SOHO environment, the network interface unit (NIU) is usually the demarc. Also, in the
SOHO environment, the NIU is usually provided by the Internet service provider (ISP). An NIU
can be a cable modem, a DSL modem, or another piece of hardware that connects the
customer to the ISP. In an enterprise environment, although the NIU is often the client’s
responsibility, it must meet the ISP’s specifications.
Highlights:
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In the SOHO environment, the NIU is usually the demarc and is most often provided by
the ISP.
An NIU can be a cable modem, a DSL modem, or another piece of hardware that
connects the customer to the ISP.
In an enterprise environment, an NIU is often the client’s responsibility, but must meet
the ISP’s specifications.
Smartjack.
One type of NIU is the smartjack. It's an NIU that can provide feedback on conditions to an ISP.
Smartjacks help an ISP determine if a problem exists on its end of the demarc through the use
of remote loopback capabilities. Many smartjacks can also provide translation between
protocols, such as translating a serial PPP (Point-to-Point Protocol) communication stream into
Ethernet.
Highlights:
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A smartjack is an NIU that can provide feedback on conditions to the ISP to help it
determine if a problem exists on its end of the demarc through the use of remote
loopback capabilities.
Many smartjacks can provide translation between protocols.
Channel Service Unit/Digital Service Unit (CSU/DSU).
Channel service unit/digital service units (CSU/DSUs) are often found within enterprise
networks. A CSU/DSU is the interface point that provides the connection between a point-topoint line (e.g., a T1 line) and the device that is responsible for controlling network traffic, which
is usually a router. The CSU/DSU may be an external device or it may be a removable module
inside of a router. Only two CSU/DSUs may exist on a single point-to-point line—one at either
end of the connection.
Highlights:
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A CSU/DSU is the interface that provides the connection between a point-to-point line
and the device that is controlling network traffic.
The CSU/DSU may be an external device, or it may be a removable module inside a
router.
Common WAN issues.
The complexity of modern networking guarantees that network technicians will have to deal with
WAN connectivity problems. The causes are varied, but are often not all that difficult to
troubleshoot. Some of the most common of these issues are provided below.
Loss of Internet connectivity issues.
Many factors can lead to a loss of connectivity on either side of the demarc. Before contacting
the WAN provider, a technician should check the operational status of the LAN equipment. If the
issue is not found on the LAN side, technicians should contact the WAN provider. One of the
tests that the WAN provider will conduct is a loopback test to check its line for interference.
Highlights:
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Many factors can lead to a loss of connectivity—on both sides of the demarc.
Before contacting the WAN provider, it is wise to check the LAN equipment for its correct
operation.
DNS issues.
DNS issues are also common in a WAN environment. They may look like a loss of Internet
connectivity, but they aren’t. The users may complain that they cannot connect to an outside
source such as www.google.com. This may not actually be a connectivity issue, but a DNS
issue.
If using a local DNS server, a technician should verify the settings and make corrections
accordingly. If the network is using the WAN provider's DNS settings, technicians should
attempt to ping the IP address. If that works, then there is a WAN connection. The next test is to
use the ping utility with the fully qualified domain name (FQDN). If this fails, it will be necessary
to contact the WAN provider to resolve its DNS issue.
Highlights:
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DNS issues may look like a loss of Internet connectivity.
To resolve if using a local DNS server, verify the settings and make corrections
accordingly.
To correct if using the WAN provider’s DNS settings, determine if there is connectivity by
pinging the IP address; if that works, there is a WAN connection.
If there is connectivity, use the ping utility with the FQDN; if this fails, contact the WAN
provider to resolve this issue.
Interface issues.
Interface issues are also common. Errors on a router's WAN interface can indicate several
different issues. Monitoring an interface's status and reading the error reports may provide a
clue as to the issue. The most common issue that prevents a good connection is a speed or
duplex mismatch. A speed mismatch between interfaces will prevent the link from being
established. A duplex mismatch between the interfaces will create errors, such as output and
input errors.
If the network is experiencing discards and dropped packets, there are a couple of things to
consider. If the device is discarding incoming packets, more than likely, the device's CPU is
being overutilized. It may be time to upgrade. If the device is dropping outgoing packets, it is
likely that there is a bandwidth congestion issue, which may be caused by interference on the
line. Either network traffic is outpacing capacity or there may be an issue on the WAN provider's
side of the line.
Highlights:
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Errors on a router’s WAN interface can indicate several different issues. Monitoring an
interface’s status and reading the error reports may provide a clue as to the issue.
The most common issue that prevents a good connection is a speed or duplex
mismatch.
○ A speed mismatch between the interfaces will prevent a link from being
established.
○ A duplex mismatch between the interfaces will create errors.
Review discards and packet drops:
○ If the device is discarding incoming packets, the device’s CPU is being likely
being overutilized.
○ If the device is dropping outgoing packets, there is a bandwidth congestion issue.
Router configuration issues.
Router configurations are a common problem when establishing a new WAN connection. A
misconfiguration of the WAN interface of a router will lead to a WAN connection issue. If this is
suspected, technicians should verify the proper configuration settings with the WAN provider.
Highlights:
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A common problem when establishing a new WAN connection is a misconfiguration of
the WAN interface of a router.
If this is suspected, verify the proper configuration settings with the WAN provider.
Company policy and practice issues.
Unfortunately, company policy and practices sometimes get reported as WAN issues. Some
applications may be throttled (have their available bandwidth reduced) for quality of service
(QoS) reasons, leading to slow service, which can be a perceived WAN issue.
Additionally, acceptable use policies may restrict, or block, access to certain sites or types of
sites, which may also appear to the end user as a WAN issue. There's not much that a
technician can do to resolve this, but it is important to explain it to the end user.
Highlights:
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Some applications may be throttled for QoS reasons, leading to slow service.
Acceptable use policies may restrict or block access to certain sites or types of sites.
Satellite issues.
A satellite WAN connection may also become an issue. If a satellite WAN connection is used,
latency will increase due to the distances covered by the transmissions. Latency is the measure
of time between the sending of data and the receiving of the data. Careful application of QoS
techniques may help to mitigate the effects of latency on some applications; however, with a
satellite connection, there will be more latency than with other types of connections.
Highlights:
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If a satellite WAN connection is used, latency will increase due to the distances covered.
Careful application of QoS techniques may mitigate the effects of latency on some
applications.
Split horizon issues.
Split horizon is another potential issue. Split horizon is a technique used in routing to help
prevent routing loops. With split horizon, a router will not advertise a route to another network
out of the interface that it learned the route on. With a point-to-multipoint WAN connection, a
router may have difficulty with the split horizon. It will learn all of the routes available to it on the
same interface, but it cannot advertise those routes back out of that interface.
Creating logical sub-interfaces on the WAN interface will usually resolve this problem. The
logical sub-interfaces will appear to the router as individual interfaces, allowing the router to
advertise the routes back out of the WAN interface.
Highlights:
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Split horizon is a technique used in routing to help prevent routing loops in which a
router will not advertise a route to another network out of the interface it learned the
route on.
With a point-to-multipoint WAN connection, a router may have difficulty with split horizon;
creating logical sub-interfaces on the WAN interface will usually resolve this issue.
The logical sub-interfaces will appear to the router as individual interfaces, allowing it to
advertise the routes back out of the WAN interface.
What was covered.
Common WAN components.
There are several important components that help to make up a WAN connection. Some of
them are the WAN provider’s responsibility, including the copper line drivers or repeaters and
the NIU or smartjack. A shared responsibility would include the demarc, while the client’s
responsibility includes the CSU/DSU.
Common WAN issues.
Common WAN issues include: loss of Internet connectivity, DNS issues, interface issues, router
configuration issues, perceived issues due to company policies and practices, satellite issues,
and split horizon issues.