CCNA Cisco Certified Network Associate Study Guide - FTP Server

490 Chapter 10 � Wide Area Networking Protocols
The basic idea behind Frame Relay networks is to allow users to communicate
between two DTE devices through DCE devices. The users should not
see the difference between connecting to and gathering resources from a
local server and a server at a remote site connected with Frame Relay.
Chances are this connection will be slower than a 10Mbps Ethernet LAN,
but the physical difference in the connection should be transparent to
the user.
Figure 10.4 illustrates everything that must happen in order for two DTE
devices to communicate. Here is how the process works:
1. The user’s network device sends a frame out on the local network. The
hardware address of the router (default gateway) will be in the header
of the frame.
2. The router picks up the frame, extracts the packet, and discards the
frame. It then looks at the destination IP address within the packet and
checks to see whether it knows how to get to the destination network
by looking in the routing table.
3. The router then forwards the data out the interface that it thinks can
find the remote network. (If it can’t find the network in its routing
table, it will discard the packet.) Because this will be a serial interface
encapsulated with Frame Relay, the router puts the packet onto the
Frame Relay network encapsulated within a Frame Relay frame. It
will add the DLCI number associated with the serial interface. DLCIs
identify the virtual circuit (PVC or SVC) to the routers and provider’s
switches participating in the Frame Relay network.
4. The channel service unit/data service unit (CSU/DSU) receives the digital
signal and encodes it into the type of digital signaling that the
switch at the Packet Switch Exchange (PSE) can understand. The PSE
receives the digital signal and extracts the 1s and 0s from the line.
5. The CSU/DSU is connected to a demarcation (demarc) installed by the
service provider, and its location is the service provider’s first point of
responsibility (last point on the receiving end). The demarc is typically
just an RJ-45 jack installed close to the router and CSU/DSU.
6. The demarc is typically a twisted-pair cable that connects to the local
loop. The local loop connects to the closest central office (CO), sometimes
called a point of presence (POP). The local loop can connect using
various physical mediums, but twisted-pair or fiber is very common.