Wide Area Network Protocols
This page introduces various protocols and technologies used in
Wide Area Network (WAN) environments. Sections include
point-to-point links, circuit switching, packet switching, virtual
circuits, dialup services, and WAN devices.
To begin with, a WAN is a data communications network that covers
a relatively broad geographic area and often uses transmission
facilities provided by common carriers, such as telephone companies.
WAN technologies function at the lower three layers of the OSI
reference model: the physical layer, the data link layer, and the
network layer.
Point-to-Point Links
A point-to-point link provides a single, preestablished WAN
communications path from the customer premises through a carrier
network, such as a telephone company, to a remote network. A
point-to-point link is also known as a leased line because its
established path is permanent and fixed for each remote network
reached through the carrier facilities.
Circuit Switching
Circuit switching is a WAN switching method in which a dedicated
physical circuit is established, maintained, and terminated through
a carrier network for each communication session. Circuit
switching accommodates two types of transmissions: datagram
transmissions and data-stream transmissions. Used extensively
in telephone company networks, circuit switching operates much like
a normal telephone call. Integrated Services Digital Network
(ISDN) is an example of a circuit-switched WAN technology.
Packet Switching
Packet switching is a WAN switching method in which network
devices share a single point-to-point link to transport packets from
a source to a destination across a carrier network.
Statistical multiplexing is used to enable devices to share these
circuits. Asynchronous Transfer Mode (ATM), Frame Relay, Switched
Multimegabit Data Service (SMDS), and X.25 are examples of
packet-switched WAN technologies.
WAN Virtual Circuits
A virtual circuit is a logical circuit created to ensure reliable
communication between two network devices. Two types of
virtual circuits exist: switched virtual circuits (SVCs) and
permanent virtual circuits (PVCs).
SVCs are virtual circuits that are dynamically established
on demand and terminated when transmission is complete. SVCs
are used in situations in which data transmission between devices is
sporadic, largely because SVCs increase bandwidth used due to the
circuit establishment and termination phases, but decrease the cost
associated with constant virtual circuit availability.
A PVC is a permanently established virtual circuit that
consists of one mode: data transfer. PVCs are used in
situations in which data transfer between devices is constant.
PVCs decrease the bandwidth use associated with the establishment
and termination of virtual circuits, but increase costs due to
constant virtual circuit availability.
WAN Dialup Services
Dialup services offer cost-effective methods for connectivity
across WANs. Two popular dialup implementations are
dial-on-demand routing (DDR) and dial backup.
DDR is a technique whereby a router can dynamically
initiate and close a circuit-switched session as transmitting end
station demand. A router is configured to consider certain
traffic interesting (such as traffic from a particular protocol) and
other traffic uninteresting. DDR can be used to replace
point-to-point links and switched multiaccess WAN services.
Dial backup is a service that activates a backup serial
line under certain conditions. The secondary serial line can
act as a backup link that is used when the primary link fails or as
a source of additional bandwidth when the load on the primary link
reaches a certain threshold.
WAN Devices
WAN Switch
A WAN switch is a multiport internetworking device used in
carrier networks. These devices typically switch such traffic
as Frame Relay, X.25, and SMDS and operate at the data link layer of
the OSI reference model.
Access Server
An access server acts as a concentration point for dial-in and
dial-out connections.
Modem
A modem is a device that interprets digital and analog signals,
enabling data to be transmitted over voice-grade telephone lines.
CSU/DSU
A CSU/DSU (channel service unit/digital service unit) is a
digital-interface device that adapts the physical interface on a DTE
(Data Terminal Equipment) device to the interface of a DCE (Data
Circuit-Terminating) device in a switched-carrier network. The
CSU/DSU also provides signal timing for communication between these
devices.
ISDN Terminal Adapter
An ISDN terminal adapter is a device used to connect ISDN Basic
Rate Interface (BRI) connections to other interfaces, such as
EIA/TIA-232. A terminal adapter is essentially an ISDN modem.
High-Speed Serial Interface
The High-Speed Serial Interface (HSSI) is a DTE/DCE interface
developed by Cisco Systems to address the need for high-speed
communication over WAN links. HSSI defines both the electrical
and the physical DTE/DCE interfaces. It therefore corresponds to the
physical layer of the OSI reference model. The flexibility of
the HSSI clock and data-signaling protocol makes user (or vendor)
bandwidth allocation possible. The DCE controls the clock by
changing its speed or by deleting clock pulses. In this way,
the DCE can allocate bandwidth between applications. HSSI
assumes a peer-to-peer intelligence in the DCE and DTE. The
control protocol is simplified, with just two control signals
required ("DTE available" and "DCE available"). Both signals
must be asserted before the data circuit is valid. The DCE and
DTE are expected to be able to manage the networks behind their
interfaces. HSSI technical characteristics are summarized as
follows:
| Characteristic |
Value |
| Maximum signaling rate |
52 Mbps |
| Maximum cable length |
50 feet |
| Number of connector pins |
50 |
| Interface |
DTE-DCE |
| Electrical technology |
Differential ECL |
| Typical power consumption |
610 mW |
| Topology |
Point-to-point |
| Cable type |
Shielded twisted-pair wire |
WAN Connection Types:
| Leased Lines |
Typically referred to as a
point-to-point connection or dedicated connection. Is
is a pre-established WAN communications path from the CPE,
through the DCE switch, to the CPE of the remote site,
allowing DTE networks to communicate at any time with no
setup procedures before transmitting date. It uses
synchronous serial lines up to 45Mbps. |
| Circuit Switching |
Sets up like a phone call.
No data can transfer before the end-to-end connection is
established. Uses dial-up modems and ISDN. It is used for
low-bandwidth data transfers. |
| Packet Switching |
WAN switching method that allows
you to share bandwidth with other companies to save money.
As long as you are not constantly transmitting data and are
instead using bursty data data transfers, packet switching
can save you a lot of money. However, if you have a
constant data transfers, you will need to use a leased line.
Frame Relay and X.25 are packet switching technologies.
Speeds can vary from 56Kbps to 2.048Mbps. |
WAN Protocols:
|
Frame Relay |
A packet switching technology
that emerged in the early 1990's. Frame Relay is a
Date Link and Physical Layer specification that provides
high performance. Frame Relay assumes that the
facilities used are less error prone than when X.25 was
being implemented and that they use less overhead.
Frame Relay is more cost-effective than point-to-point links
and can run at speeds of 64Kbps to 45Mbps. Frame Relay
provides features for dynamic-bandwidth allocation and
congestion control. |
| X.25 |
ITU-T standard that defines how
connections between DTE and DCE are maintained for remote
terminal access and computer communications in PDNs(Public
Data Networks). X.25 specifies LAPB, a data link layer
protocol, and PLP, a network layer protocol. Frame
Relay has to some degree superseded X.25. |
|
ISDN |
Integrated Services Digital
Network is a set of digital services that transmit voice and
data over existing phone lines. ISDN can offer a
cost-effective solution for remote users who need a
higher-speed connection than an analog dial-up link offers.
ISDN is also a good choice as a backup link for other types
of links such as Frame Relay or a T-1 connection. |
|
LAPB |
Link Access Procedure Balanced
was created to be used as a connection-oriented protocol at
the Data Link layer for use with X.25. It can also be
used as a simple Data Link transport. LAPB has a
tremendous amount of overhead because of its strict timeout
and windowing techniques. You can use LAPB instead of
the lower-overhead HDLC if your links are very error prone.
However, that typically is not a problem anymore. |
|
HDLC |
High-Level Data Link Control was
derived from Synchronous Data Link Control (SDLC), which was
created by IBM as a Data Link connection protocol.
HDLC is a connection-oriented protocol at the Data Link
layer, but it has very little overhead compared to LAPB.
HDLC was not intended to encapsulate multiple Network layer
protocols across the same link. The HDLC header
carries no identification of the type of protocol being
carried inside the HDLC encapsulation. Because of
this, each vendor that uses HDLC has their own way of
identifying the Network layer protocol, which means that
each vendor's HDLC is proprietarty for their equipment. |
| SDLC |
Synchronous Data Link Control.
SNA data link layer communications protocol. SDLC is a
bit-oriented, full-duplex serial protocol that has spawned
numerous protocols, including HDLC and LAPB. |
|
PPP |
Point-to-Point protocol is an
industry standard protocol. Because many versions of
HDLC are proprietary, PPP can be used to create
point-to-point links between different vendor's equipment.
It uses a Network Control Protocol field in the Data Link
header to identify the Network layer protocol. It
allows authentication and multilink connections and can be
run over asynchronous and synchronous links. |
Click the Links to see protocol specific information.
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