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Point-to-Point Leased Lines / Frame Relay
WAN protocols used on point-to-point serial links provide
the basic function of delivery of
data across that one link. As a
CCNA, you will be required to understand and configure a variety of
protocols used on point-to-point links, including Link Access Procedure
Balanced (LAPB),High-Level Data Link Control (HDLC), and Point-to-Point
Protocol (PPP). Each of these WAN protocols has the following functions in
common:
1)LAPB, HDLC, and PPP provide for delivery of data across a single
point-to-point serial link.
2)LAPB, HDLC, and PPP deliver data on synchronous serial links. (PPP
supports asynchronous functions as well.)
Framing
Framing is one core feature of any synchronous serial data link protocol.
Each of these protocols defines framing so that receiving stations know
where the beginning of the frame is, what address is in the header, and the
point at which the packet begins. By doing so, the
router receiving data can distinguish between idle frames and data
frames. Synchronous links, rather than asynchronous links, are typically
used between routers.
Synchronous simply means that there is an imposed time ordering at the
sending and receiving ends of the link. Essentially, the sides agree to a
certain speed, but because it is very expensive to build devices that can
truly operate at exactly the same speed, the devices adjust their rates to
match a clock source. The process works almost like the scenes in spy
novels, when the spies synchronize their watches; in this case, the watches
or clocks are synchronized automatically multiple times per minute. Unlike
asynchronous links, in which no bits are sent during idle times, synchronous
data links define idle frames. These frames do nothing more than provide
plenty of signal transitions so that clocks can be adjusted on the receiving
end, consequently maintaining synchronization.
4-wire circuit A line from the telco with four wires, comprised of two
twisted-pair wires. Each pair is used to send in one direction, so a 4-wire
circuit allows full-duplex
communication.
2-wire circuit A line from the telco with two wires, comprised of one
twisted-pair wire. The pair is used to send in only one direction at a time,
so a 2-wire circuit allows only half-duplex communication.
A WAN is a computer
network that covers a broad area (i.e., any network whose communications
links cross metropolitan, regional, or national boundaries [1]). Or less
formally a network that uses routers and public communication links [1].
Contrast with personal area networks (PANs), local area networks (LANs),
campus area networks (CANs), or metropolitan area networks (MANs) that are
usually limited to a room, building, campus or specific metropolitan area
(e.g., a city) respectively. The largest and most well known example of this
type of networking is the Internet or the World Wide Web.
WANs are used to
connect LANs and other types of networking systems together, so that users
and computers in one location can communicate with users and computers in
other locations. Many WANs are built for one particular organization and are
private to that organization.
Others, built by
Internet service providers, provide connections from an organization's LAN
to the Internet. WANs are often built using leased lines from a carrier such
as AT&T, Verizon, BT, etc. At each end of the leased line, a router connects
to the LAN (local area network) on one side and a hub within the WAN on the
other. Leased lines or circuits can be very expensive. Instead of using
leased lines, WANs can also be built using less costly circuit switching or
packet switching methods.
Network protocols
including TCP/IP deliver transport and addressing functions. Protocols
including Packet over SONET/SDH, MPLS, ATM and Frame relays are often used
by service providers to deliver the circuits that are used in WANs. X.25 was
an important early WANSACHMO protocol, and is often considered to be the
"grandfather" of Frame Relay as many of the underlying protocols and
functions of X.25 are still in use today (with upgrades) by Frame Relay.
What Is Frame Relay?
Frame Relay is a connection-oriented, Layer 2 WAN connection
technology. It operates at
speeds from 56 kpbs to 45 Mbps. It is very flexible and offers a wide
array of deployment options. Frame Relay operates by statistically
multiplexing multiple
data
streams over a single physical
link. Each data stream is known as a virtual circuit (VC). Frame Relay VCs
come in two types: Permanent Virtual Circuits (PVCs) and Switched Virtual
Circuits (SVCs). Each VC is
tagged.
with an identifier to keep it
unique. The identifier, known as a Data Link Connection Identifier (DLCI),
is determined on a per-leg basis during the transmission. It must be
unique and agreed upon by two adjacent Frame Relay devices. As long as the
two agree, the value can be any valid number, and the number does not have
to be the same end to end. Valid DLCI numbers are 16-1007. For DLCI
purposes, 0-15 and 1008-1023 are reserved. The DLCI also defines the
logical connection between the Frame Relay (FR) switch and the customer
premises equipment (CPE).
Frame Relay devices fall into two possible roles, data terminal equipment
(DTE) or data circuit terminating equipment (DCE). The DTE/DCE
relationship is a Layer 2 (data link) layer relationship. DTE and DCE
relationships are normally electrical. The DTE/DCE relationship at Layer 1
is independent of that at Layer 2.
. DTEs are generally considered to be terminating equipment for a specific
network and are located at the customer premises.
. DCEs are carrier-owned internetworking devices. DCE equipment provides
clocking and switching services in a network; they are the devices that
actually transmit data through the WAN. In most cases, the devices are
packet switches.
Local Management Interface (LMI) is the means by which Frame Relay edge
devices maintain keepalive messages. The Frame Relay switch is responsible
for maintaining the status of the CPE device(s) to which it is attached.
LMI is the
communication by which the
switch monitors status. LMI implements a keepalive mechanism that verifies
connectivity between DCE and DTE and the fact that data can flow. A LMI
multicast capability, in conjunction with an LMI multicast addressing
mechanism, enables attached devices to learn local DLCIs as well as
provide global, rather than local, significance to those DLCIs. Finally,
LMI provides a status indicator that is constantly exchanged between
router
and switch. The LMI setting is configurable.
Frame Relay networks provide more features and benefits than simple
point-to-point WAN links, but to do that, Frame Relay protocols are more
detailed. Frame Relay networks are multiaccess networks, which means that
more than two devices can attach to the network, similar to
LANs. However, unlike LANs,
you cannot send a data link layer broadcast over Frame Relay. Therefore,
Frame Relay networks are called nonbroadcast multi-access (NBMA) networks.
Also, because Frame Relay is multiaccess, it requires the use of an
address that identifies to which remote router each frame is addressed.
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