Chapter 7 - Madisonville Community College

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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
-
to
-
point

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 long
-
distance 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)

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

Figure 7
-
5:
A PRI link

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)

Table 7
-
3: SONET OC levels

Data rate indicated by Optical Carrier (OC) level

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