Chapter Seven
WANs and Remote
Connectivity
WAN Essentials
Wide Area Network
WAN link


Connection between one WAN site and
another site
A WAN link is typically described as point-topoint
Dedicated line

Continuously available link that is leased
through another carrier
Examples: p. 309
WAN Essentials
Figure 7-1: Differences in LAN and WAN connectivity
PSTN
Public Switched Telephone Network
Refers to the network of typical telephone
lines and carrier equipment that service
most homes
Also called plain old telephone service
(POTS)
PSTN
A dial-up connection uses a PSTN or other line
to access remote servers via modems at both
the source and destination
The Federal Communications Commission
(FCC) sets standards and policy for
telecommunications transmission equipment in
the United States

Even with 56 Kbps modems, FCC limits PSTN lines to
53 Kbps to reduce crosstalk
The place where two telephone systems meet is
the point of presence (POP)
 LD to local, etc.
PSTN
Figure 7-2: A typical PSTN connection to the Internet
Discussed on bottom of p. 311
X.25 and Frame Relay
X.25




Analog, packet-switched LAN technology optimized for longdistance data transmission
Mid-70s 64Kbps – 1992 2.048 Mbps
Developed as a more reliable connection for mainframes to
remote terminals
Not widely accepted in U.S.
Frame Relay





Updated, digital version of X.25 that also relies on packet
switching
Standardized in 1984
1.544 or 45 Mbps
Being replaced by newer, faster technologies
Depicted as clouds in diagrams
X.25 and Frame Relay
Contract with telecommunications company
states:



Endpoints
Amount of bandwidth
Less expensive than a dedicated T1
Figure 7-3:
A WAN
using frame
relay
X.25 and Frame Relay
SVCs (switched virtual circuits)

Connections established when parties need to
transmit, then dismantled once the transmission is
complete
PVCs (private virtual circuits)


Connections established before data needs to be
transmitted and maintained after transmission is
complete
Not dedicated like T-carrier services
CIR (committed information rate)

Guaranteed minimum amount of bandwidth selected
when leasing a frame relay circuit
ISDN (Integrated Services Digital
Network)
International standard for transmitting data
over digital lines
Established by the ITU
All ISDN connections are based on two types
of channels:

The B channel is the “bearer” channel
Circuit-switching – max 64 Kbps, multiple channels

The D channel is the “data” channel
Packet-switching – max 16 or 64 Kbps, single channel
Two types of ISDN connection: BRI & PRI
BRI (Basic Rate Interface)
A variety of ISDN using two 64-Kbps bearer
(B) channels and one 16-Kbps data (D)
channel, as indicated by the following
notation:

2B+D
Through bonding, the two 64-Kbps
channels can be combined to achieve an
effective throughput of 128-Kbps
Average monthly cost - $100-$250
BRI (Basic Rate Interface)
The Network Termination 1 (NT1) device connects
twisted-pair wiring at customer’s building with ISDN
terminal equipment (TE) via RJ-11 or RJ-45 data jacks
A terminal adapter (TA) converts digital signals into
analog signals for use with ISDN phones and other analog
devices
Figure 7-4: A BRI link
PRI (Primary Rate Interface)
A variety of ISDN using 23 B channels and one
64-Kbps D channel, as represented by the
following notation:

23B+D
PRI links use same kind of equipment as BRI
links, but require the services of an extra
network termination device—called a Network
Termination 2 (NT2)—to handle multiple ISDN
lines
Can achieve 1.544 Mbps
PRI (Primary Rate Interface)
Figure 7-5:
A PRI link
It is only feasible to use ISDN for the local loop
(from customer to POP) portion of a WAN link due to
18,000 linear feet limitation
T-Carriers
Broadband

Group of network connection types or transmission
technologies generally capable of reaching or
exceeding 1.544 Mbps throughput
Cable, DSL, & T-carriers
T-carriers



Term for any kind of leased line that follows the
standards for T1s, fractional T1s, T1Cs, T2s, T3s, or
T4s
T1 – 24 channels x 64 Kbps = 1.544 Mbps
AT&T developed in 1957
Types of T-Carriers
The most common T-carrier implementations are
T1 and T3
Signal level

ANSI standard for T-carrier technology that refers to
its Physical layer electrical signaling characteristics
DSO (digital signal, level 0)


Equivalent of one data or voice channel
International vs. NA standard - p. 318
Fractional T1

Arrangement allowing an organization to use only
some channels on a T1 line, paying for what they use
Types of T-Carriers
Figure 7-1: Carrier specifications
T-Carrier Costs
T1


$500-$2,000 installation
$500-$2,000 monthly access fees
Longer the distance, higher the access fees
Fractional T1

Only lease needed channels – in 64 Kbps increments
T3



$3,000 installation
Monthly access fees based on usage
Full usage could max at $18,000/month
T-Carrier Connectivity
Wiring

Can use unshielded or shielded twisted-pair copper wiring
CSU/DSU (Channel Service Unit/Data Service Unit)


CSU provides termination for the digital signal and ensures
connection integrity through error correction and line
monitoring
DSU converts the digital signal used by bridges, routers,
and multiplexers into the digital signal sent via the cabling
Figure 7-6: A CSU/DSU connecting a T1
T-Carrier Connectivity
Multiplexer

Device that combines multiple voice or data channels on
one line
Figure 7-7: Typical use of a multiplexer on a T1-connected data network
T-Carrier Connectivity
Routers and
bridges

On a typical
T1-connected
data network,
terminal
equipment will
consist of
bridges,
routers or a
combination of
the two
Figure 7-8: A router on a T1-connected network
DSL
Digital Subscriber Lines
Uses advanced data modulation techniques
to achieve extraordinary throughput over
regular phone lines
Like ISDN, DSL can span only limited
distances without the help of repeaters
Costs


T-1 throughput - $100/month
Consumer-grade (1/2 bandwidth) - $20/month
Types of DSL
Term xDSL refers to all DSL varieties, of which at
least eight currently exist
DSL types can be divided into two categories:


Asymmetrical – different speed up & down stream
Symmetrical – same speed up & down stream
To understand the difference between these two
categories, you must understand the concept of
downstream and upstream data transmission


Downstream – Carrier’s POP to customer
Upstream – Customer to Carrier’s POP
Types of DSL
Table 7-2: Comparison of DSL types
DSL Connectivity
DSL connectivity,
like ISDN, depends
on the PSTN
Inside carrier’s
POP, a device
called a DSL
access
multiplexer
(DSLAM)
aggregates multiple
DSL subscriber
lines and connects
them to a larger
carrier or to the
Internet backbone
Figure 7-9: A DSL connection
DSL Connectivity
Once inside
the customer’s
home or office,
the DSL line
must pass
through a DSL
modem
Voice signals
use very
small range
of
frequencies
between 0
and 35 KHz
Figure 7-10: A DSL modem
Cable
Cable
connections
require that the
customer use a
special cable
modem, a device
that modulates
and demodulates
signals for
transmission and
reception via
cable wiring
Not a true modem
– p. 327
Figure 7-11: A cable modem
Cable
Hybrid fiber-coax (HFC)


Very expensive fiber-optic link that can support high
frequencies
HFC upgrades to existing cable wiring are required before
current TV cable systems can serve as WAN links
Cable drop

Fiber-optic or coaxial cable connecting a neighborhood
cable node to a customer’s house
Head-end

Cable company’s central office, which connects cable
wiring to many nodes before it reaches customers’ sites
Cable
Figure 7-12: Cable infrastructure
SONET (Synchronous Optical
Network)
Developed in
1980s
Can provide data
transfer rates from
64 Kbps to 39.8
Gbps using the
same TDM
technique used by
T-carriers
Known
internationally as
SDH
(Synchronous
Digital Hierarchy)
SONET is self-
healing
Figure 7-13: SONET technology on a
long-distance WAN
Fiber-optics transmission media is used
SONET (Synchronous Optical
Network)
Data rate indicated by Optical Carrier (OC) level
Table 7-3: SONET OC levels
WAN Implementation:
Speed
Table 7-4a: A comparison of WAN technology transmission speeds
WAN Implementation:
Reliability
WAN implementations can roughly be divided as
follows:

Not very reliable, suited to individual or unimportant
transmissions:
PSTN dial-up

Sufficiently reliable, suited for day-to-day
transmissions:
ISDN, T1, fractional T1, T3, DSL, cable, X.25, and frame
relay

Very reliable, suited to mission-critical applications:
SONET
WAN Implementation:
Security
Among other things, consider the following
issues:




WAN security depends in part on the encryption
measures each carrier provides for its lines
Enforce password-based authorization for LAN
and WAN access and teach users how to choose
difficult-to-decrypt passwords
Take the time to develop, publish, and enforce a
security policy for users in your organization
Maintain restricted access to network equipment
rooms and data centers
WAN Implementation:
Virtual Private Networks (VPNs)
VPNs are wide
area networks
logically defined
over public
transmission
systems that
serve an
organization’s
users, but
isolate that
organization’s
traffic from other
users on the
same public
lines
Figure 7-14: An example of a VPN
Remote Connectivity
Remote access methods:

Direct dial to the LAN
The computer dialing into the LAN becomes a remote node
on the network (RAS)

Direct dial to a workstation
Software running on both remote user’s computer and LAN
computer allows remote user to “take over” the LAN
workstation, a solution known as remote control
 pcAnywhere, MS System Management Server (SMS)

Internet/Web interface
Through a browser, a user at home or on the road connects
to a LAN whose files are made visible to the Web through
Web server software
Remote Connectivity
ICA (Independent Computing Architecture) client



Remote access client developed by Citrix Systems,
Inc.
Enables remote users to use virtually any LAN
application over any type of connection, public or
private
Windows 2000 Terminal Services
Remote Access Service (RAS)


One of the simplest dial-in servers
This software is included with Windows 2000 Server
Dial-Up Networking
Refers to the
process of
dialing into a
LAN’s
(private)
access
server or to
an ISP’s
(public)
access
server to log
onto a
network
Figure 7-15: Choosing a network connection type
SLIP and PPP
Communications protocol enabling a workstation to
connect to a server using a serial connection

Serial Line Internet Protocol (SLIP)
Can carry only IP packets
Supports only asynchronous transmission


Data transmitted/received doesn’t conform to timing scheme
Start and stop bits
Requires more configuration than PPP – rarely used
SLIP and PPP

Point-to-Point Protocol (PPP)
Can carry many different types of Network layer
packets
Supports both asynchronous and synchronous
transmission


Synchronous – data transmitted/received must conform
to timing scheme
Uses timing – not start and stop bits
Requires less configuration than SLIP