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24 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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Selecting Technologies and Devices
for Enterprise Networks


We are going to look at the technologies for remote
access and WAN components of an enterprise
network design

We will talk about physical and data link layer
protocols and enterprise network devices such as
access servers, routers and WAN switches

What you select will depend on bandwidth and
quality of service requirements, network topology,
business requirements and constraints and technical

Remote Access Technologies

Have become a more important ingredient
of many network designs

It is important to recognize the location and
number of full and part time telecommuters
and the extent that mobile users need to
access the network

How often and amount of time on the
network will drive what you give them

Point Protocol

Standard data
layer protocol for
transporting various network
protocols across serial, point
point links

Can be used for a single user or a group

can be used with ISDN, analog lines, digital
leased line, and other WAN technologies

Point Protocol (Cont’d)

Provides following services: Network
layer protocol
multiplexing, link configuration, link quality testing,
option negotiation, authentication, header
compression and error detection

Four functional layers:

Physical layer

encapsulation of network
layer datagrams

Link Control Protocol

Network Control Protocols

Multilink PPP and Multichassis
Multilink PPP

Adds support for channel aggregation to
PPP which can be used for load
and providing extra bandwidth

MPPP ensures packets arrive in order

Multichassis is a Cisco enhancement to
MPPP that allows channel aggregation
across multi
access servers at a
central site

Multilink PPP and Multichassis
Multilink PPP (Cont’d)

Multichassis MPPP allows multiple access
servers to be grouped into a single stack group

Uses Stack Group Bidding Protocol (SGBP)
to elect a server to handle aggregation for an

SGBP can be weighted so that CPU intensive
process are offloaded to routers designated as
offload servers

Password Authentication Protocol and
Challenge Handshake Authentication

Supports both PAP and CHAP authentication

CHAP is more secure than PAP. PAP can be
used if remote station does not support CHAP

CHAP is superior. PAP sends password in
plain text. An intruder can use a protocol
analyzer to capture and use it

CHAP uses a connect, challenge, hashed
response and accept/deny reply

Integrated Services Digital

PPP is often used with ISDN since it provides
data encapsulation, link integrity and
authentication for ISDN

Cost effective solution for high transmission
speed and quick connection requirements

Charge is a basic monthly fee and a usage time

MPPP can be used to aggregate ISDN channels

ISDN Components

Terminal equipment type 1 (TE1)(compliant)

ISDN terminal equipment Type 2 (TE2)
(non compliant)

NT1 devices (physical
layer & connect user)

NT2 devices (Layer 2 and 3 protocols)

Reference point interfaces

Cable Modem Remote Access

Another option that uses coax cable used for
cable TV

Much faster than analog and usually faster
than ISDN

some cable
network service providers offer
hybrid fiber/coax (HFC) systems that
connect CATV networks to the service
providers high
speed fiber
optic network

Challenges Associated with
Cable Modem Systems

It’s a new technology and there are many
standards that do not interoperate with each

the CATV infrastructure is designed for TV
signals in one direction

Usually more bandwidth is offered
downstream than upstream

Number of users on system

Digital Subscriber Line Remote

Growing in popularity with home users

Have to use a DSL modem or other device

DSL uses sophisticated modulation schemes
to offer much higher speeds than ISDN,
32Mbps for downstream traffic and 15Kbps
to 1.5 Mbps for upstream

Comes in different flavors (ADSL, HDSL,

Asymmetric Digital Subscriber

Has three channels

speed downstream from 1.5 to 9 Mbps

medium speed duplex from 16 Kbps to 640

POTS 64 Kbps channel for voice

Rate Digital Subscriber

Provides same upstream and downstream

1.544 over 2 wire pair or 2.048 Mbps over 3
wire pairs

Has a 12,000 to 15,000 operating distance
which is shorter than ADSL’s 18,000 feet

Telco can install repeaters for a price

Selecting Devices for Remote

Users using for less than 2 hours a day can
use an analog modem

Before selecting a modem read reviews for
reliability, interoperability with other brands
of modems, interoperability with typical
services, speed and throughput, latency,
ease of setup, support for advanced features
and cost

Selecting Devices for Remote
Users (Cont’d)

Faster access requires cable modems, DSL
or a small router that has an ISDN or other
type of WAN port

for a router look at: protocol support,
support for a remote user or remote LAN,
channel aggregation, ease of configuration,
security, reliability, interoperability and cost

Selecting Devices for the Central

Generally includes remote access servers.
Five types of services

Remote node services

various devices

Terminal services

Telnet, rlongin X.25, etc

Protocol translation services

access hosts

Asynchronous routing services

layer 3
routing functionality

Dialout services

share access server modems

Selecting Devices for the Central
Site (Cont’d)

Additional criteria

number of ports and type

support for services

configuration flexibility and modularity

support for network address translation (NAT)
or port address translation (PAT) for hosts on
remote networks, Dynamic Host Configuration
Protocol and multimedia features and protocols

WAN Technologies

Variety of common and established WAN

Newer technologies are growing such as
Wireless WAN

orbit satellite, cellular and radio
frequency wireless will probably become
popular option of voice, pager and data

Systems for Provisioning WAN

Must select correct bandwidth
required.Varies depending on location.
Europe uses 30 B
channels and 1 d
for admin purposes, the U.S. uses 24 b
channels and 1 d
channel for admin

the number of digital streams multiplexed
together determine network speed

Leased Lines

A dedicated circuit that a customer leases
from a carrier for a predetermined amount
to time usually for months or years

line is dedicated to that customer

Mature and are proven technology

Dedicated to a single customer

Synchronous Optical Network

Physical layer specification for high
synchronous transmission of packets or cells
over fiber
optic cabling

can be ATM or packet
based networks

provides more sophisticated multiplexing and
demultiplexing of individual signals

4 layer protocol stack

photonic layer, section
layer, line layer and path layer

Usually connected in a ring topology

Switched Multimegabit Data

SMDS is a physical and data link layer WAN
technology alternative to leased lines which
runs on fiber or copper media

usually positioned as a high
speed, low
latency, cost effective solution for connecting
LANs and MANs over a cell relay WAN

can have just one physical connection to the
service provider’s network

Frame Relay

performance WAN protocol that
operates at the physical and data
link layers
of OSI

offers a cost
effective method for
connecting remote sites typically at speeds
from 64Kbps to 1.544 Mbps

it is efficient, flexible and has low latency

Frame Relay Hub
Topologies and Subinterfaces

Frame relay is often designed in hub and spoke

A central site router can have many logical
connections to remote sites with only one
physical connection to the WAN

One problem is that split horizon can limit
routing. Distance vector routing protocols do
not repeat information out the interface it was
received on

Frame Relay Hub
Spoke Topologies
and Subinterfaces (Cont’d)

A solution to split
horizon is to use a full
design with physical circuits between each site

use subinterface which is a logical interface that
is associated with a physical interface. The
router then applies the split horizon rule based
on logical interfaces instead of physical ones

Downside is that router configurations are more

Frame Relay Congestion Control

A frame relay device has the ability to
transmit data at its physical access rate for
extended periods of time

Includes congestion control mechanisms to
ensure fair bandwidth allocation and
feedback mechanisms

explicit congestion notification

explicit congestion notification

Frame Relay Bandwidth

Most frame relay networks provides some
guarantee of band width availability

Committed information rate (CIR) is the

At or below CIR will accept it

Leaky bucket

some providers offer zero CIR (inexpensive)
only make a best effort

Frame Relay/ATM Interworking

As ATM gains popularity WANs that use both
ATM and Frame Relay technologies are
becoming more popular

Frame Relay/ATM Interworking is used to
describe the protocols and processes for
connecting ATM and Frame Relay WANs

With network interworking 2 or more frame relay
networks are connected via an ATM core network

With service interworking an ATM network
connects to a Frame Relay network

ATM Wide Area Networks

Support ver high bandwidth requirements

copper cabling T3 or above

Fiber optic

speed up to 9.952 Gbps and beyond

Facilitates efficient sharing of bandwidth among
applications with various QoS requirements

Save money spent on monthly tariffs for WAN

Selecting Devices and Service Providers
for an Enterprise WAN Design

An enterprise WAN design is based on
performance routers and WAN

Selecting Routers for an
Enterprise WAN Design

High throughput, high availability and
advanced features to optimize the utilization of
expensive WAN circuits

select to support bandwidth requirements,
appropriate packets
second level and
adequate memory and processing power to
forward data and handle routing protocols

Also provide advanced features such as
advanced switching and queuing techniques,
traffic shaping express forwarding

Selecting WAN Switches for an
Enterprise WAN Design

Multiservice WAN switches that handle ATM,
Frame Relay and remote
access technologies
are gaining in popularity

support a variety of data types, interfaces and
services and optimize bandwidth utilization

Intelligent queue
handling algorithms that take
into account the behavior of different types of

Reallocate buffers as needed

Selecting a WAN Service

Cost of service but may not be the most

extent of services and technologies offered

geographical areas covered

reliability and performance characteristics of
providers internal network

level of security offered

level of technical support

Selecting a WAN Service
Provider (Cont’d)

Characteristics of provider’s network:

physical routing of network links


extend relies on other providers for redundancy

level of over subscription

bandwidth allocation mechanisms for QoS

Selecting a WAN Service
Provider (Cont’d)

types of switches and optimization features

frequency and typical causes of outages

security methods from intrusion

security methods for privacy

disaster recovery plans

Selecting a WAN Service
Provider (Cont’d)

Service Level Support Agreements (SLAs)

experience level of installation and support staff

experience with your particular protocols and

dedicated single
point of contact

received ISO 9002 certification

How difficult to provision and price new services

Offer a customer training program


There are many options of LANs and WANs

No single technology or device is the right
answer for all circumstances

The physical design depends on business
objectives, technical requirements, traffic
characteristics, and traffic flows

The physical design builds on the logical