Content Overview This module presents
an overview of WAN technologies. It introduces and explains WAN
terminology such as serial transmission, time division
multiplexing (TDM), demarcation, data terminal equipment (DTE)
and data circuit-terminating equipment (DCE). The development
and use of high-level data link control (HDLC) encapsulation as
well as methods to configure and troubleshoot a serial
interface are presented. Point-to-Point Protocol (PPP) is the
protocol of choice to implement over a serial WAN switched
connection. It can handle both synchronous and asynchronous
communication and includes error detection. Most importantly it
incorporates an authentication process using either CHAP or
PAP. PPP can be used on various physical media, including
twisted pair, fiber optic lines and satellite transmission.
Described in this module are configuration procedures for PPP,
available options, and troubleshooting concepts. Among the
options available is the ability of PPP to use PAP or CHAP
authentication. Students completing this module should be able
to: - Explain serial communication
- Describe
and give an example of TDM
- Identify the demarcation
point in a WAN
- Describe the functions of the DTE and
DCE
- Discuss the development of HDLC
encapsulation
- Use the encapsulation hdlc
command to configure HDLC
- Troubleshoot a serial
interface using the show interface and show
controllers commands
- Identify the advantages of
using PPP
- Explain the functions of the Link Control
Protocol (LCP) and the Network Control Protocol (NCP)
components of PPP
- Describe the parts of a PPP
frame
- Identify the three phases of a PPP
session
- Explain the difference between PAP and
CHAP
- List the steps in the PPP authentication
process
- Identify the various PPP configuration
options
- Configure PPP encapsulation
-
Configure CHAP and PAP authentication
- Use show
interface to verify the serial encapsulation
-
Troubleshoot any problems with the PPP configuration using
debug PPP
Content 3.1
Serial Point-to-Point Links 3.1.1 Introduction to
serial communication WAN technologies are based on serial
transmission at the physical layer. This means that the bits of
a frame are transmitted one at a time over the physical
medium. The bits that make up the Layer 2 frame are signaled
one at a time by physical layer processes onto the physical
medium. The signaling methods include Nonreturn to Zero Level
(NRZ-L), High Density Binary 3, (HDB3), and Alternative Mark
Inversion (AMI). These are examples of physical layer encoding
standards, similar to Manchester encoding for Ethernet. Among
other things, these signaling methods differentiate between one
serial communication method and another. Some of the many
different serial communications standards are the following:
- RS-232-E
- V.35
- High Speed Serial
Interface (HSSI)
Content 3.1 Serial
Point-to-Point Links 3.1.2 Time-division multiplexing
Time-Division Multiplexing (TDM) is the transmission of several
sources of information using one common channel, or signal, and
then the reconstruction of the original streams at the remote
end. In the example shown in the first figure, there are three
sources of information carried in turn down the output channel.
First, a chunk of information is taken from each input channel.
The size of this chunk may vary, but typically it is either a
bit or a byte at a time. Depending on whether bits or bytes are
used, this type of TDM is called bit-interleaving or
byte-interleaving. Each of the three input channels has its own
capacity. For the output channel to be able to accommodate all
the information from the three inputs, the capacity of the
output channel must be no less than the sum of the inputs. In
TDM, the output timeslot is always present whether or not the
TDM input has any information to transmit. TDM output can be
compared to a train with 32 railroad cars. Each is owned by a
different freight company and every day the train leaves with
the 32 cars attached. If one of the companies has product to
send, the car is loaded. If the company has nothing to send,
the car remains empty, but it is still part of the train. TDM
is a physical layer concept, it has no regard for the nature of
the information that is being multiplexed onto the output
channel. TDM is independent of the Layer 2 protocol that has
been used by the input channels. One TDM example is Integrated
Services Digital Network (ISDN). ISDN basic rate (BRI) has
three channels consisting of two 64 kbps B-channels (B1 and
B2), and a 16 kbps D-channel. The TDM has nine timeslots, which
are repeated. This allows the telco to actively manage and
troubleshoot the local loop as the demarcation point occurs
after the network terminating unit (NTU). Web Links
Time-Division Multiplexing http://whatis.techtarget.com/
definition/0,,sid9_gci214174,00.html
Content
3.1 Serial Point-to-Point Links 3.1.3
Demarcation point The demarcation point, or "demarc"
as it is commonly known, is the point in the network where the
responsibility of the service provider or "telco"
ends. In the United States, a telco provides the local loop
into the customer premises and the customer provides the
active equipment such as the channel service unit/data service
unit (CSU/DSU) on which the local loop is terminated. This
termination often occurs in a telecommunications closet and the
customer is responsible for maintaining, replacing, or
repairing the equipment. In other countries around the world,
the network terminating unit (NTU) is provided and managed by
the telco. This allows the telco to actively manage and
troubleshoot the local loop with the demarcation point
occurring after the NTU. The customer connects a customer
premises equipment (CPE) device, such as a router or frame
relay access device, into the NTU using a V.35 or RS-232 serial
interface.
Content 3.1 Serial Point-to-Point
Links 3.1.4 DTE-DCE A serial connection has a data
terminal equipment (DTE) device at one end of the connection
and a data communications equipment (DCE) device at the other
end. The connection between the two DCEs is the WAN service
provider transmission network. The CPE, which is generally a
router, is the DTE. Other DTE examples could be a terminal,
computer, printer, or fax machine. The DCE, commonly a modem or
CSU/DSU, is the device used to convert the user data from the
DTE into a form acceptable to the WAN service provider
transmission link. This signal is received at the remote DCE,
which decodes the signal back into a sequence of bits. This
sequence is then signaled to the remote DTE. Many standards
have been developed to allow DTEs to communicate with DCEs. The
Electronics Industry Association (EIA) and the International
Telecommunication Union Telecommunications Standardization
Sector (ITU-T) have been most active in the development of
these standards. The DTE-DCE interface for a particular
standard defines the following specifications:
- Mechanical/physical – Number of pins and connector
type
- Electrical – Defines voltage levels for 0
and 1
- Functional – Specifies the functions that
are performed by assigning meanings to each of the signaling
lines in the interface
- Procedural – Specifies
the sequence of events for transmitting data
If two
DTEs must be connected together, like two computers or two
routers in the lab, a special cable called a null-modem is
necessary to eliminate the need for a DCE. For synchronous
connections, where a clock signal is needed, either an external
device or one of the DTEs must generate the clock signal. The
synchronous serial port on a router is configured as DTE or DCE
depending on the attached cable, which is ordered as either DTE
or DCE to match the router configuration. If the port is