Remote Access

Remote Access

 Remote users may be working at

branch offices or home offices, or they

may even be on the road with a laptop

or a handheld mobile device.

 Essentially, a remote user is any user

who is not presently working at the

company's central site.

Remote Access

 Remote access solutions come in all shapes

and sizes. Each company's solution typically

involves a combination of varied WAN

services. Most of these services are obtained

from a service provider, such as a regional

telecommunications company.

 Since the transmission facilities belong to a

service provider, your role is to select the

appropriate service, not actually to design

and maintain the WAN facilities themselves.

WAN’S

 A WAN is a data communications network

that covers a relatively broad geographic

area, often using transmission facilities

leased from service providers and telephone

companies.

 There are two basic methods of data

communications: asynchronous transmission

and synchronous transmission.

WAN’S

 Asynchronous Transmission:

Asynchronous means that no clock or

timing source is needed to keep both

the sender and the receiver

synchronized. Without the benefit of a

clock, the sender must signal the start

and stop of each character so that the

receiver knows when to expect data.

WAN’S

 Asynchronous transmission is often

described as "character-framed" or

"start/stop" communication because this

method frames each character with a start

and stop bit. Each character is typically a 7-

or 8-bit value that can represent a number, a

letter, a punctuation mark, etc. Each

character is preceded by a start bit and

followed by a stop bit, or in some cases, two

stop bits.

WAN’S

 Synchronous Transmission

Synchronous means "with time." In

terms of data transmission,

"synchronous" means that a common

timing signal is used between hosts. A

clock signal is either embedded in the

data stream or is sent separately to the

interfaces.

WAN’S

 A large amount of data (e.g., hundreds or

even thousands of bytes) can be preceded by

synchronization bits.

 In Ethernet, a field of synchronization bits

precedes the data payload. This field of

synchronization bits, called a preamble,

forms a pattern of alternating ones and zeros.

The receiver uses this pattern to synchronize

with the sender.

WAN’S

 Service providers offer a variety of

synchronous and asynchronous WAN

services. These services can be grouped into

three categories depending on their

connection type:

 dedicated connectivity-synchronous serial

 circuit-switched networks-asynchronous

serial, ISDN

 packet-switched networks-synchronous

serial.

Dedicated Connection

 A dedicated connection is a continuously

available point-to-point link between two

sites. Because of the expense associated

with building and maintaining transmission

facilities, dedicated connections are almost

always leased from the telephone company

or some other carrier network. Therefore, a

dedicated connection is often referred to as a

leased line.

Dedicated Connection

 A dedicated line is not actually a "line" at all.

Dedicated lines are switched circuits that

establish a fixed path through the carrier

network. Leased lines are circuits that are

reserved full-time by the carrier for the private

use of the customer.

 Leased lines also offer high speeds of up to

45 Mbps. Leased lines are ideal for high-

volume environments with steady-rate traffic

patterns.

Dedicated Connection

 If your organization's network must support a

constant flow of mission-critical data, such

as e-commerce or financial transactions, then

a high-speed leased line might best suit your

needs.

 Dedicated leased lines typically require

synchronous serial connections. Each leased

line connects to a synchronous serial port on

the router, via a channel service unit/data

service unit (CSU/DSU)

Dedicated Connection

 A CSU/DSU is classified as a data

communications equipment (DCE)

device. A DCE adapts the physical

interface on a data terminal equipment

(DTE) device to the signaling used by

the carrier network. A router is an

example of a DTE device.

Dedicated Connection

 The CSU/DSU provides signal timing for

communication and is used for

interfacing with the digital transmission

facility. Essentially, the CSU/DSU is

used by a router to connect to a digital

line in much the same way that a PC

uses a modem to connect to an analog

line.

Dedicated Connection

 Typical connections on a dedicated network

may operate at the following speeds:

 56 kbps

 64 kbps

 T1 (1.544 Mbps) US standard

 E1 (2.048 Mbps) European standard

 E3 (34.064 Mbps) European standard

 T3 (44.736 Mbps) US standard

Dedicated Connection

 Typically, a router's synchronous serial port

connects to a DCE (e.g., a CSU/DSU) using

one of the following standards:

 EIA/TIA-232 (RS-232)

 EIA/TIA-449

 V.35

 X.21

 EIA-530

Dedicated Connection

 When connecting a DTE (e.g., a router)

to an analog modem, you will typically

use EIA/TIA-232 compliant cabling and

interfaces. The EIA/TIA-232 standard is

very common. However, it provides

relatively low transmission speeds,

<64kbps.

Dedicated Connection

 When connecting a Cisco router to a

T1/E1 or fractional T1/E1 via a

CSU/DSU, you will use V.35 cabling

and interfaces, which are capable of

much higher throughput (over 2 Mbps).

Circuit-switched Network

 In a circuit-switched network, a dedicated

physical circuit is temporarily established for

each communication session. Switched

circuits are established by an initial set-up

signal. This call set-up process determines

the caller's ID and the destination's ID, as well

as the connection type. A teardown signal

brings the circuit down when transmission is

complete.

Circuit-switched Network

 Plain old telephone service (POTS) is

the most common circuit-switched

technology. With telephone service, the

circuit doesn't exist until you place a

call, but once the temporary circuit is

built, it is fully dedicated to your call.

Circuit-switched Network

 Circuit-switched connections provide mobile

and home users with access to the central

site or to an Internet Service Provider (ISP).

Corporate networks typically use circuit-

switched connections as backup links, or as

primary links for branch offices that exchange

low-volume or periodic traffic. In such

cases, a router must route traffic over the

switched circuit.

Circuit-switched Network

 Routers connected to circuit-switched

networks are configured to operate in a

specialized way, called dial-on demand

routing (DDR). A router configured for

DDR only places a call when it detects

traffic defined by a network

administrator as "interesting.“

Circuit-switched Network

 Typical circuit-switched connections

include:

 Asynchronous Dialup (POTS)

 ISDN Basic Rate Interface (BRI)

 ISDN Primary Rate Interface (PRI)

Asynchronous Dial-up

Connections

 Asynchronous serial connections offer

inexpensive WAN service via the existing

telephone network. In order for digital

devices, such as computers and routers, to

use analog telephone lines, modems are

required at each end of the connection.

 Modems convert digital data signals to

analog signals that can be transported over

the telephone company's local loops

asynchronously.

Asynchronous Dial-up

Connections

 Because modems can be used with virtually

any phone line, mobile and home users often

rely on asynchronous serial connections to

connect to a corporate network or ISP.

 Modems have one overwhelming drawback;

they do not provide high throughput. Today's

modems provide transmission speeds of only

56 kbps or less.

Asynchronous Dial-up

Connections

 Routers can also use asynchronous

serial connections to route traffic using

DDR. Because modems do not support

high transmission speeds,

asynchronous serial connections are

typically used as backup links.

Asynchronous Dial-up

Connections

 Some routers are designed with dozens

of asynchronous lines to support a large

number of dial-in users. Routers that act

as concentration points for dial-in and

dial-out calls are called access

servers. Throughout this course, the

term "access server" will be used to

refer to a router with at least one

asynchronous interface.

Asynchronous Dial-up

Connections

 To place or receive an asynchronous

serial call, a router must have at least

one asynchronous serial interface, such

as the AUX (Auxilary) port, which

connects to a modem (typically

external).

Integrated Services Digital

Network (ISDN)

 Integrated Services Digital Network

(ISDN) connections are typically

synchronous dial-up connections.

 Like asynchronous dial-up connections,

ISDN provides WAN access when

needed, rather than providing a

permanent link.

Integrated Services Digital

Network (ISDN)



ISDN offers more bandwidth than

asynchronous dial-up connections, and is

designed to carry data, voice, and other

traffic across a digital telephone network.

ISDN is commonly used with DDR to provide

remote access for small office/home office

(SOHO) applications, backup links, and

load sharing.

ISDN

 ISDN offers two levels of service, BRI and

PRI

 With BRI, there are two channels, called B

channels, designed to carry data. A third

channel, called the D channel, is used to

send call set-up and teardown signals.

 When both B channels are used together to

send data, ISDN BRI yields 128 kbps (more

than twice the top speed of POTS).

ISDN

 With PRI, there are 23 B channels on T1

used in North America and Japan. There are

30 B channels on E1 used in Europe and

other parts of the world. PRI employs a single

D channel as well.

 ISDN BRI requires straight through cables

with RJ-45 connections. ISDN PRI requires

crossover cables with RJ-48 connections for

T1 and DB-15 connections for E1.

Packet Switched Networks

 Unlike leased lines and circuit-switched

connections, packet switching does

not rely on a dedicated, point-to-point

connection through the carrier network.

Instead, data packets are routed across

the carrier network based on

addressing contained in the packet or

frame header.

Packet Switching

 This means that packet-switched WAN

facilities can be shared with other

customers, which allows service

providers to support multiple customers

over the same physical lines and

switches. Typically, customers connect

to the packet-switched network via a

leased line, such as a T1 or fractional

T1.

Packet Switching

 In a packet-switched network, the provider

configures its switching equipment to create

virtual circuits (VCs) that supply end-to-end

connectivity.

 Frame Relay is the most common packet-

switched WAN service in the United States,

although the older X.25 remains a prominent

packet-switching technology worldwide.

Packet Switching

 The cost of a packet-switched VC is generally

less than that of a leased line because the

WAN facilities are shared. VCs can be

permanent, or they can be built on demand.

 A Frame Relay VC offers speeds of up to

T3, making this packet-switched technology a

high-speed, cost-effective alternative to

leased lines. As well, a single synchronous

serial connection can support several logical

VCs in a point-to-multipoint configuration

Packet Switching

 This process of combining multiple data

conversations into a single physical line is

called multiplexing.

 Multiplexing in a packet-switched network is

made possible because a DTE (usually a

router) encapsulates the packet with

addressing information. The provider's

switches use the addressing to determine

how and where to deliver a specific packet.

Packet Switching

 In the case of Frame Relay, these addresses

are Data Link Control Identifiers, or DLCIs.

 The ability to multiplex means that a single

router port and CSU/DSU can support dozens

of VCs each leading to a different site.

Therefore, packet-switching makes a full- or

partial-mesh topology relatively affordable.

Packet Switching

 Frame Relay is a popular WAN service for

providing high-speed WAN connections to

branch offices and other remote sites.

However, Frame Relay does not offer the

degree of reliability, flexibility, and security

afforded by dedicated lines. Despite Frame

Relay's lower cost and multipoint capability,

dedicated lines are the preferred WAN

service for mission-critical traffic and

continuous, high-volume exchanges.

WAN Encapsulations

 Routers encapsulate packets with a Layer 2

frame before sending them across a WAN

link.

 Although there are several common WAN

encapsulations, most have similar anatomies.

 Most common WAN encapsulations are

derived from High-Level Data Link Control

(HDLC) and its forerunner Synchronous

Data Link Control (SDLC).

WAN Encapsulations

 By default, serial interfaces on a Cisco

router are set to encapsulate packets

using HDLC. You must manually

configure the interface for any other

type of encapsulation.

 The Figure shows which common data

link protocols are used with each of the

three WAN connection types.

WAN Encapsulations

 Common WAN protocols include the

following:

 PPP - Point-to-Point Protocol (PPP) is a

standards-based protocol for router-to-router

and host-to-network connections over

synchronous and asynchronous circuits.

 Serial Line Internet Protocol (SLIP) - SLIP

is the forerunner to PPP, and is used for

point-to-point serial connections using

TCP/IP.

WAN Encapsulations

 High-Level Data Link Control (HDLC)

- HDLC implementations are

proprietary, so Cisco's HDLC is typically

used only when connecting two Cisco

devices. When connecting routers from

different vendors, PPP (which is

standards-based) is used instead.

WAN Encapsulations

 X.25/LAPB - X.25 is an ITU-T standard

that defines the way connections

between DTE and DCE devices are

maintained for remote terminal access

and computer communications in public

data networks. X.25 provides extensive

error-detection and windowing features

because it was designed to operate

over error-prone analog copper circuits.

WAN Encapsulations

 Frame Relay - Frame Relay is a high-

performance, packet-switched, WAN

protocol that can be used over a variety

of network interfaces. Frame Relay is

streamlined to operate over highly

reliable digital transmission facilities.

WAN Encapsulations

 Asynchronous Transfer Mode (ATM)

- ATM is an international standard for

cell relay, in which multiple service

types (e.g., voice, video, or data) are

conveyed in fixed-length cells. ATM is

designed to take advantage of high-

speed transmission media such as

Synchronous Optical Network (SONET).

Choosing a WAN Connection

 There are other important factors to consider

when choosing a WAN service, including

ease of management, quality of service

(QoS), and reliability. You will probably find

that leased lines are easier to manage and

configure than packet-switched connections.

In terms of QoS, some applications, such as

Voice over IP (VoIP), require guaranteed

bandwidth, minimal delay, and high reliability,

which can make anything short of a leased

line problematic.

Identifying site requirements

and solutions

 When selecting WAN services, a networking

professional must evaluate the needs of each

site within a company. Individual worksites

within a company can be broadly categorized

as one of the following: a central site, a

branch office, or a telecommuter site.

 The term "telecommuter site" applies to both

mobile users and small office/home office

(SOHO) locations. These categorizations are

applied to the WAN depicted in the figure.

The Central Site

 The central site is the focal point of a

company's network.

 Typically, all remote sites and users

must connect to the central site to

access information, either intermittently

or continuously.

The Central Site

A central site's routers should have a

modular design so that interface

modules can be added (or swapped

out) as needed. The chassis of a

modular router allows you to install the

interfaces needed to support virtually

any media type.

The Central Site

 The central site's router must

accommodate circuit-switched

connections (e.g., ISDN/analog),

packet-switched connections (e.g.,

Frame Relay), and could feasibly have

a dedicated line to the ISP.

The Branch Office

 A branch office, commonly referred to

as a remote site, typically maintains at

least one WAN connection to the central

site, and may have several links to other

remote sites. Generally, branch-office

networks support fewer users than the

central site, and therefore require less

bandwidth.

The Branch Office

 Because remote-site traffic can be sporadic,

or bursty, you must carefully determine

whether it is more cost-effective to offer a

permanent or dialup solution.

 Telecommuters may also require access to

the branch office through various connection

types. Therefore, the branch office routers

should have the capability to support a variety

of WAN connections.

The Branch Office

 Typical WAN solutions for connecting

the branch office to the central site

include:

 Leased lines

 Frame Relay

 X.25

The Branch Office

 ISDN

 DSL ([digital subscriber line] - This

technology enables delivery of high-speed

data, voice, and multimedia over conventional

telephone wires. In order for a remote site to

connect to the corporate network without

traversing the public Internet, DSL typically

requires ATM at the central site.)

The Branch Office

 Wireless

 VPN ([Virtual Private Network] - This

technology typically requires that both

sites are already connected to the

public Internet.)

Telecommuters

 The improvement of WAN technologies,

notably DSL and cable modems, has

allowed many employees to do their

jobs remotely. As a result, the number

of telecommuters and small offices has

increased.

Telecommuters

 An asynchronous dialup solution

using the existing telephone network

and an analog modem is often the

solution for telecommuters because it is

easy to set up and the telephone

facilities are already installed.

 Typical WAN connections employed at

telecommuter sites include:

Telecommuters

 Asynchronous dialup

 ISDN BRI

 Cable modems

 DSL

 Wireless and Satellite

 VPN

Routers

 Cisco offers access servers, routers,

and other equipment that allow

connection to various WAN services.

Figure 1 highlights some of the products

that are suited for the various company

sites. Figure 2 lists the key features and

WAN options for each series of routers

Summary

 In this chapter, you learned about WAN

connections and how to determine the

requirements of a central site, a branch office,

and a telecommuter site. You also learned

how to select Cisco products to suit the

specific needs of each site and how to utilize

Cisco tools to select the proper equipment. In

addition, you learned how to identify and

connect the necessary components for

central-site, branch-office, and small-office

WAN solutions.