The Wide Area Network WAN

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The Wide Area Network (WAN)
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What is WAN?
A WAN is a data communications network that
operates beyond a LAN's geographic scope.
It provides connectivity between organizations,
services, and remote users. WANs generally carry
voice, data, and video.
WAN uses data links such as Integrated Services
Digital Network (ISDN) and Frame Relay. These are
provided by carrier services to access bandwidth
over wide-area geographies.
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WAN Technologies and the OSI Model
WANs function at the three lowest layers of the
OSI reference model

• the physical layer
• the data link layer
• the network layer

WAN standards typically describe both
physical-layer delivery methods and data
link-layer requirements, including addressing,
flow control, and encapsulation
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An overview of the WAN cloud organizes WAN
provider services into three main types

• Call setup - Sets up and clears calls between
  telephone users. Also called signaling, call
  setup uses a separate telephone channel not used
  for other traffic. The most commonly used call
  setup is Signaling System 7 (SS7), which uses
  telephone control messages and signals between
  the transfer points along the way to the called
  destination.

• Time-division multiplexing (TDM) - Information
  from many sources has bandwidth allocation on a
  single medium. Circuit switching uses signaling
  to determine the call route, which is a dedicated
  path between the sender and the receiver. By
  multiplexing traffic into fixed time slots, TDM
  avoids congested facilities and variable delays.
  Basic telephone service and ISDN use TDM circuits

• Frame Relay - Information contained in frames
  shares bandwidth with other WAN Frame Relay
  subscribers. Frame Relay is a statistical
  multiplexed service, unlike TDM, which uses Layer
  2 identifiers and permanent virtual circuits. In
  addition, Frame Relay packet switching uses Layer
  3 routing with sender and receiver addressing
  contained in the packet

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When selecting an appropriate WAN solution, you
should discuss the costs and benefits of each
with your service providers.
When an organization subscribes to an outside WAN
service provider for network resources, the
provider gives connection requirements to the
subscriber. For instance, the type of equipment
to be used to receive services.
The following are the most commonly used terms
associated with the main parts of WAN services

• Customer premises equipment (CPE) - Devices
  physically located on the subscriber's premises.
  Includes both devices owned by the subscriber and
  devices leased to the subscriber by the service
  provider.

• Demarcation (or demarc) - The point at which the
  CPE ends and the local loop portion of the
  service begins. This often occurs at the POP of a
  building.

• Local loop (or "last-mile") - Cabling (usually
  copper wiring) that extends from the demarc into
  the WAN service provider's central office.

• CO switch - A switching facility that provides
  the nearest point of presence for the provider's
  WAN service.

• Toll network - The collective switches and
  facilities (called trunks) inside the WAN
  provider's cloud. The caller's traffic may cross
  a trunk to a primary center, then to a sectional
  center, and then to a regional- or
  international-carrier center as the call travels
  the long distance to its destination.

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A key interface in the customer site occurs
between the data terminal equipment (DTE) and the
data circuit-terminating equipment (DCE).
Typically, the DTE is the router. The DCE is the
device used to convert the user data from the DTE
into a form acceptable to the WAN service's
facility.
The DCE is either the attached modem, channel
service unit/data service unit (CSU/DSU), or
terminal adapter/network termination 1 (TA/NT1).
The WAN path between the DTEs is called the link,
circuit, channel, or line. The DCE primarily
provides an interface for the DTE into the
communication link in the WAN cloud. The DTE/DCE
interface acts as a boundary where responsibility
for the traffic passes between the WAN subscriber
and the WAN provider.
The DTE/DCE interface uses various protocols such
as HSSI and V.35. These protocols establish the
codes that the devices use to communicate with
each other. Setup operation and user traffic
paths are determined by this communication.
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A virtual circuit is a logical circuit, as
opposed to a point-to-point circuit. It is
created to ensure reliable communication between
two network devices.
Two types of virtual circuits exist. gtswitched
virtual circuits (SVCs) gtpermanent virtual
circuits (PVCs).
The switched virtual circuits (SVCs)
SVCs are virtual circuits that are dynamically
established on demand and terminated when
transmission is complete.

• Communication over an SVC consists of three
  phases.
• The establishment phase involves creating the
  virtual circuit between the source and
  destination devices.
• Data transfer involves transmitting data between
  the devices over the virtual circuit.
• The circuit-termination phase involves tearing
  down the virtual circuit between the source and
  destination devices.

SVCs are used in situations where data
transmission between devices is sporadic. The
circuit establishment and termination phases of
SVCs represent a small bandwidth overhead, but
this is usually much less than the overhead of
making virtual circuits constantly available.
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The permanent virtual circuits (PVCs)

• A PVC is a permanently established virtual
  circuit that consists of one mode called data
  transfer.
• PVCs are used in situations where 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.

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WANs use numerous types of devices, including the
following
Routers - which offer many services, including
LAN and WAN interface ports.
Modems - which interface voice-grade services.
Modems include CSUs/ DSUs and TA/NT1 devices that
interface ISDN services.
WAN switches - which connect to WAN bandwidth
for voice, data, and video communication.
Communication servers, which concentrate dial-in
and dial-out user communication.
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• Routers are devices that implement the network
  service.
• They provide interfaces for a wide range of links
  and subnetworks at a wide range of speeds.
• Routers are active and intelligent network
  devices and thus can participate in managing the
  network.
• Routers manage networks by providing dynamic
  control over resources and supporting the tasks
  and goals for networks.
• These goals are connectivity, reliable
  performance, management control, and flexibility.

• A WAN switch is a multiport networking device.
• It typically switches such traffic as Frame
  Relay, X.25, and Switched Multimegabit Data
  Service (SMDS).
• WAN switches typically operate at the data link
  layer of the OSI reference model.
• switches filter, forward, and flood frames based
  on the destination address of each frame.

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An ISDN Terminal Adapter (TA) is a device used to
convert standard electrical signals into the form
used by ISDN so that non-ISDN devices can connect
to the ISDN network. For example, a TA would be
used to connect a router serial port to a BRI
capable device.
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• A CSU/DSU is either a digital-interface device,
  or sometimes two separate digital devices.
• It adapts the physical interface on a DTE device
  (such as a terminal) to the interface of a DCE
  device (such as a switch) in a switched-carrier
  network
• Sometimes, CSUs/DSUs are integrated in the router
  box.

the placement of the CSU/DSU in a WAN
implementation
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WAN physical layer protocols describe how to
provide electrical, mechanical, operational, and
functional connections for WAN services. Most
WANs require an interconnection that is provided
by a communications service provider (such as an
RBOC), an alternative carrier (such as an
Internet service provider), or a post, telephone,
and telegraph (PTT) agency
The WAN physical layer also describes the
interface between the DTE and the DCE. Typically,
the DCE is the service provider, and the DTE is
the attached device
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The WAN data link layer defines how data is
encapsulated for transmission to remote sites.
WAN data-link protocols describe how frames are
carried between systems on a single data path
Common data-link encapsulations associated with
WAN lines

• Frame Relay - Can transmit data very rapidly
  compared to other WAN protocols. Uses simplified
  encapsulation with no error correction mechanisms
  over high-quality digital facilities.
• Point-to-Point Protocol (PPP) - Described by RFC
  1661, PPP was developed by the IETF. PPP contains
  a protocol field to identify the network-layer
  protocol.
• ISDN - A set of digital services that transmit
  voice and data over existing phone lines.
• Link Access Procedure, Balanced (LAPB) - For
  packet-switched networks, LAPB is used to
  encapsulate packets at Layer 2 of the X.25 stack.
  It can also be used over a point-to-point link if
  the link is unreliable or if there is an inherent
  delay associated with the link, such as in a
  satellite link. LAPB provides reliability and
  flow control on a point-to-point basis.
• Cisco/IETF - Used to encapsulate Frame Relay
  traffic. The Cisco option is proprietary and can
  be used only between Cisco routers.

• High-Level Data Link Control (HDLC) - An ISO
  standard, HDLC might not be compatible between
  different vendors because of the way each vendor
  has chosen to implement it. HDLC supports both
  point-to-point and multipoint configurations.

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• Dedicated, full-time connectivity is provided by
  point-to-point serial links
• Connections are made using the router's
  synchronous serial ports with typical bandwidth
  use of up to 2 Mbps (E1) available through the
  use of a CSU/DSU.
• (issues to be considered)
• Different encapsulation methods at the data link
  layer provide flexibility and reliability for
  user traffic.
• Dedicated lines of this type are ideal for
  high-volume environments with a steady-rate
  traffic pattern.
• Use of available bandwidth is a concern because
  you have to pay for the line to be available even
  when the connection is idle.

Dedicated lines also are referred to as
point-to-point links. Their established path is
permanent and fixed for each remote network
reached through the carrier facilities. A
point-to-point link provides a single,
pre-established WAN communications path from the
customer premises through a carrier network, such
as a telephone company, to a remote network. The
service provider reserves point-to-point links
for the private use of the customer.
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• Packet switching is a WAN switching method in
  which network devices share a permanent virtual
  circuit (PVC) to transport packets from a source
  to a destination across a carrier network.
• A PVC is similar to a point-to-point link. Frame
  Relay, SMDS, and X.25 are all examples of
  packet-switched WAN technologies.
• Switched networks can carry variable-size frames
  (packets) or fixed-size cells. The most common
  packet-switched network type is Frame Relay.

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• Frame Relay was designed to be used over
  high-speed, high quality digital facilities.

• Frame Relay does not offer much error checking
  or reliability. It expects upper-layer protocols
  to attend to these issues.

• Frame Relay is a packet-switching data
  communications technology.
• It can connect multiple network devices on a
  multipoint WAN
• The design of Frame Relay WANs can affect
  certain aspects (such as split horizon) of
  higher-layer protocols such as IP, IPX, and
  Apple-Talk.
• Frame Relay is called a non-broadcast
  multi-access technology because it has no
  broadcast channel. Broadcasts are transmitted
  through Frame Relay by sending packets to all
  network destinations.
• Frame Relay defines the connection between a
  customer DTE and a carrier DCE. The DTE is
  typically a router, and the DCE is a Frame Relay
  switch. (In this case, DTE and DCE refer to
  the data link layer, not the physical layer.)
• Frame Relay access is typically at 56 kbps, 64
  kbps, or 1.544 Mbps.

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• Frame Relay is a cost-effective alternative to
  point-to-point WAN designs. A site can be
  connected to every other by a virtual circuit.
  Each router needs only one physical interface to
  the carrier. Frame Relay is implemented mostly as
  a carrier-provided service but can also be used
  for private networks

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• 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. (
  Connection on Demand )
• Used extensively in telephone company networks,
  circuit switching operates much like a normal
  telephone call.
• ISDN is an example of a circuit-switched WAN
  technology.
• Circuit-switched connections from one site to
  another are brought up when needed and generally
  require low bandwidth.
• Basic telephone service connections generally
  operate no faster than 56 kbps, and Basic ISDN
  connections (BRI) provide lines at 64 or 128
  kbps.
• Circuit-switched connections are used primarily
  to connect remote users and mobile users to
  corporate LANs.
• They are also used as backup lines for
  higher-speed circuits, such as Frame Relay and
  dedicated lines.

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Dial on Demand Routing (DDR)
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Telephone companies developed ISDN with the
intention of creating a totally digital network.
ISDN devices include the following

• Terminal Equipment 1 (TE1) -- Designates a
  device that is compatible with the ISDN network.
  A TE1 connects to an NT of either Type 1 or Type
  2.
• Terminal Equipment 2 (TE2) -- Designates a
  device that is not compatible with ISDN and
  requires a TA.
• TA-Converts standard electrical signals into
  the form used by ISDN so that non-ISDN devices
  can connect to the ISDN network.
• NT Type 1 (NT1) -- Connects four-wire ISDN
  subscriber wiring to the conventional two-wire
  local loop facility.
• NT Type 2 (NT2) -- Directs traffic to and from
  different subscriber devices and the NT1. The NT2
  is an intelligent device that performs switching
  and concentrating.

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ISDN interface reference points

• The S/T interface defines the interface between
  a TE1 and an NT. The S/T also is used to define
  the TA-to-NT interface.
• The R interface defines the interface between a
  TE2 and the TA.
• The U interface defines the two-wire interface
  between the NT and the ISDN cloud.

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• There are two ISDN services
• Basic Rate Interface (BRI) -
• Primary Rate Interface (PRI).
• ISDN BRI operates mostly over the copper
  twisted-pair telephone wiring in place today.
• ISDN BRI delivers a total bandwidth of a 144
  kbps line into three separate channels.
• The B (bearer) channels (2) - operate at 64
  kbps and are used to carry voice or data traffic.
  
• The D (delta) channel - is a 16-kbps signaling
  channel used to carry instructions that tell the
  telephone network how to handle each of the B
  channels.
• ISDN BRI often is referred to as 2BD.

ISDN- Basic Rate Interface (BRI)

• ISDN provides great flexibility to the network
  designer because of its capability to use each of
  the B channels for separate voice or data
  applications.
• For example, one ISDN 64-kbps B channel can
  download a long document from the corporate
  network while the other B channel browses a Web
  page.

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WAN communication is often called a service
because the network provider (often the telephone
company) charges users for the WAN services it
provides.
Circuit-switching vs. Packet-switching
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• relatively low throughput, high delay, and high
  error rates have characterized WAN communication
• WAN connections are also characterized by the
  cost of renting media (wire) from a service
  provider to connect two or more campuses
  together.
• The WAN infrastructure is often rented from a
  service provider. Therefore, WAN network designs
  must minimize the cost of bandwidth and optimize
  bandwidth efficiency

WAN Development Requirements

• Optimize WAN bandwidth
• Minimize cost
• Maximize the effective service to the end users

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Shared-media networks are being overtaxed because
of the following new network requirements

• Network usage has increased as enterprises
  utilize client/server, multimedia, and other
  applications to enhance productivity.
• The rate of change in application requirements
  will continue to accelerate with new development
  such as Internet "push" technologies.
• Due to the increased services they provide end
  users, applications are making increased demands
  on network services.
• An unprecedented number of connections are
  being established among offices of all sizes,
  remote users, mobile users, international sites,
  customers/suppliers, and the Internet.
• The explosive growth of corporate intranets and
  extranets has created a greater demand for
  bandwidth.
• The use of enterprise servers continues to grow
  to serve the business needs of organizations.

The projected 300 traffic increase over the next
five years will encourage enterprises' attempts
to further contain WAN costs.

• WAN connections generally handle important
  information and are optimized for price and
  performance.
• The routers connecting the campuses, for
  example, generally apply traffic optimization,
  multiple paths for redundancy, dial backup for
  disaster recovery, and quality of service (QoS)
  for critical applications.

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