Computer Networks and WAN Technologies Overview

ITEC 275

 Computer Networks – Switching,

Routing, and WANs

Week 10

Robert D’Andrea

Some slides provide by Priscilla

Oppenheimer and used with permission

Agenda

•

Learning Activities

–

PPP

–

Cable Modems and DSL

–

Leased Lines, SONET, Frame Relay, Metro

Ethernet, ATM

–

Selecting a WAN Service Provider

Enterprise Technologies and Devices

•

Remote access networks

•

Wide area networks (WANs)

•

Devices

–

End user remote access devices

–

Central site remote access devices

–

VPN concentrators

–

Routers

Selection Criteria

•

Business requirements and constraints

•

Cost

•

Technical goals

•

Bandwidth requirements

•

QoS requirements

•

Network topology

•

Traffic flow and load

•

Etc.

Remote Access Technologies

•

The Point-to-Point Protocol (PPP)

•

Integrated Services Digital Network (ISDN)

•

Cable modems

•

Digital Subscriber Line (DSL)

Point-to-Point Protocol (PPP)

•

PPP is used with synchronous, asynchronous,

dial-up, and ISDN links

•

Defines encapsulation scheme for transport of

different network-layer protocols

•

Supports authentication:

–

Password Authentication Protocol (PAP)

–

Challenge Handshake Authentication Protocol

(CHAP). CHAP more secure than PAP

Point-to-Point Protocol (PPP)

•

The Internet Engineering Task Force (IETF)

developed PPP as a standard data link layer

protocol for transporting various protocols

across serial, point-to-point links.

•

PPP can be used to create point-to-point links

between different vendor’s equipment.

•

PPP uses a Network Control Protocol field in

the Data Link header to identify the Network

layer protocol.

Point-to-Point Protocol (PPP)

•

PPP can be used to connect a single remote

user to a central office, or to connect a remote

office with many users to a central office.

•

PPP is a data-link protocol that can be used

over either asynchronous serial (dial-up) or

synchronous serial (ISDN) media and that uses

the LCP (Link Control Protocol) to build and

maintain data-link connections.

PPP Layers

Network Control Protocol (NCP)

Link Control Protocol (LCP)

Encapsulation based on

High-Level Data-Link Control Protocol (HDLC)

Physical Layer

PPP

Point-to-Point (PPP) is a data link protocol

commonly used in establishing a direct

connection between two networking nodes. It

provides connection authentication,

transmission authentication, and compression.

PPP is used over many types of physical

networks including serial cable, phone line,

trunk line, cellular telephone, and fiber optic

links such as SONET.  PPP is also used over the

Internet access connections (broadband).

Multilink MPPP

An ordinary dial-up modem connection to the Internet

through an Internet service provider (ISP) usually uses

PPP as its wide area network (WAN) data-link protocol,

but there are times when the 56-Kbps speed provided by

V.90 modems is insufficient. MPPP allows multiple

physical dial-up links to be inverse multiplexed together

to form a single high-bandwidth logical PPP connection

between the dial-up client and the ISP. MPPP works by

ordering the data frames from the client across the

multiple PPP channels and recombining them at the

ISP’s termination point, and vice versa.

Multilink MPPP

MPPP defines protocols for splitting the data stream

into PPP packets, sequencing the packets, transmitting

them over separate logical data links, and then

recombining them at the receiving station.

Multilink MPPP

Inverse multiplexing speeds up data transmission by

dividing a data stream into multiple concurrent streams

that are transmitted at the same time across separate

channels (such as a T-1 or E-1 lines) and are then

reconstructed at the other end back into the original

data stream. Just the reverse of ordinary multiplexing ,

which combines multiple signals into a single signal,

inverse multiplexing is a technique commonly used

where data in a high-speed local area network ( LAN )

flows back and forth into a wide area network ( WAN )

across the "bottleneck" of a slower line such as a T-1

(1.544 Mbps ).

Multilink MPPP

Various multiplexing methods are possible in terms of

the channel bandwidth and time, and the signal, in

particular the frequency, phase or time. The two basic

methods are:

•

Frequency Division Multiplexing (FDM) 

is derived

from AM techniques in which the signals occupy the

same physical ‘line’ but in different frequency bands.

Each signal occupies its own specific band of

frequencies all the time, 

i.e.

 the messages share the

channel bandwidth.

Multilink MPPP

•

Time Division Multiplexing (TDM ) is derived

from sampling techniques in which messages occupy

all the channel bandwidth but for short time

intervals of time, 

i.e.

 the messages share the channel

time.

Multilink MPPP

Multilink MPPP

•

Add support for channel aggregation to PPP.

Channel aggregation can be used for load

sharing and providing extra bandwidth. With

channel aggregation, a device can

automatically bring up additional channels as

bandwidth requirements increase.

•

MPPP ensures that packets arrive in order at

the receiving device.

Multi-chassis MPPP

•

Cisco enhancement to MPPP.

•

Allows WAN administrator to group multiple

access servers into a single stack group. User

traffic can be split and reassembled across

multiple access servers in the stack group.

Multi-chassis Multilink PPP

Stack group

Offload

server

CHAP

•

CHAP provides a three-way hand-shake.

•

Provides protection by verifying a remote

node with a three-way hand shake and a

variable challenge value that is unique and

unpredictable.

CHAP

Name: 760_1

Password: sfy45

Access Server

Connect

Challenge

Hashed Response

Accept or Deny

Database of

Users and

Passwords

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ISDN

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Digital data-transport service offered by regional

telephone carriers (telcos)

•

Circuit-switched service that carries voice and data.

ISDN is a set of digital services that transmit voice

and data over existing phone lines.

•

Cost-effective remote-access solution for

telecommuters and remote offices

–

Cost of an ISDN circuit is usually based on a

monthly fee plus usage time

•

Good choice as a backup link for another type of link,

for example, Frame Relay

•

Channel aggregation is popular with ISDN links.

ISDN Interfaces

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1.544 Mbps in U.S.

2.048 Mbps in

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Cable Modem Service

•

Operates over the coax cable used by cable TV

•

Much faster than analog modems, and usually

much faster than ISDN (depending on how

many users share the cable)

–

25 to 50 Mbps downstream from the head end

–

2 to 3 Mbps upstream from end users

•

Standard = Data Over Cable Service Interface

Specification (DOCSIS)

Cable Modem Service

•

Coax Cable does not require dial-up.

•

Cable modem operates more like a LAN.

•

Cable-network providers off hybrid fiber/coax

(HFC) systems that connect CATV networks to

the service provider’s high-speed fiber-optic

network.

•

HFC systems allow connections of home PCs

and small LANs to high-speed access to the

Internet or to a private network using VPN.

Cable Modem Service

•

CATV (Cable Modem Termination System)

provides high-speed connectivity for cable

modems.

•

Cable modem solution for remote users or

remote offices is the sharing a single cable and

the types of applications they use.

DSL

•

High-speed digital data traffic over ordinary

telephone wires

•

Sophisticated modulation schemes mean

higher speeds than ISDN

–

Speeds range from 1.544 to 9 Mbps

•

Actual bandwidth depends on type of DSL

service, DSL modem, and many physical-layer

factors

•

Symmetric communication (SDSL) traffic flow

travels at the same speed up to 1.544 Mbps.

•

Asymmetric DSL (ADSL) very popular

–

Downstream faster than upstream

PPP and ADSL

Asymmetric DSL (ADSL) uses two popular

PPP implementations.

PPP and ATM (PPPoA) the CPE acts

as an Ethernet-to-WAN router and the

PPP session is established between the

CPE and Layer 3 access concentrator in

the service provider’s network.

PPP and Ethernet (PPPoE) the CPE

acts as an Ethernet-to-WAN bridge.

PPP and ADSL

PPP and Ethernet (PPPoE) the CPE acts

as an Ethernet-to-WAN bridge. The client

initiates a PPP session by encapsulating PPP

frames in MAC frames and then bridging the

frames over ATM/DSL to a gateway router at

the service provider. From that point, the PPP

session can be established, authenticated,

and achieved. The client receives its IP

address from the service provider, using PPP

negotiation.

PPP and ADSL

Point-To-Point Protocol (PPP) Vedio:

https://www.youtube.com/watch?v=Oq0Si0WWHdM

Provisioning WAN Bandwidth

•

A critical network design is considering

capacity requirements. Selecting the right

amount of capacity for current and future

needs.

•

Provisioning requires an analysis of traffic

flows, and analysis of scalability goals.

WAN Technologies

•

Leased lines

•

Synchronous Optical Network (SONET)

•

Frame Relay

•

Asynchronous Transfer Mode (ATM)

Leased Lines

•

Dedicated digital, copper circuits that a

customer leases from a carrier for a

predetermined amount of time, usually for

months or years

•

Speeds range from 64 Kbps to 45 Mbps

•

Enterprises use leased lines for both voice

and data traffic

Leased Lines

•

Dedicated connection or Point-to-Point

connection.

•

Pre-established WAN communications path

from the CPE, through the DCE switch, to

the CPE of the remote site, allowing DTE

networks to communicate at any time with

no setup procedure before transmitting

data.

Digital Signal (DS)

•

A channel in the NADH  (North American

Digital Hierarchy) is called a digital signal

(DS). Digital signals are multiplexed

together to form high-speed WAN circuits.

DS-1 and DS-3 are the most commonly

used capacities.

The North American Digital

Hierarchy

Synchronous Optical Network (SONET)

•

Physical-layer specification for high-speed

synchronous transmission of packets or cells

over fiber-optic cabling

•

Service providers and carriers make wide use

of SONET in their internal networks

•

Gaining popularity within private networks

Synchronous Optical Network (SONET)

•

Goals of SONET and SDH

- Define higher speeds than the ones used

   by the NADH.

- Support efficient multiplexing and

  de-multiplexing of individual signals. With

  SONET, it is easy to isolate one channel

  from a multiplexed circuit. With

  plesiochronous systems, like NADH and

  European E system, isolating one channel

  is more difficult.

Synchronous Optical Network (SONET)

•

Terminating multiplexers (implemented in

switches and routers) provide user access to

the SONET network. Terminating multiplexers

convert electrical interfaces into optical signals

and multiplex multiple payloads into STS-N

signals required for optical transport.

SONET Optical Carrier (OC) Levels

aka Synchronous Transport Signal (STS) Levels

STS Rate

OC Level

Speed

STS-1

OC-1

51.84 Mbps

STS-3

OC-3

155.52 Mbps

STS-12

OC-12

622.08 Mbps

STS-24

OC-24

1.244 Gbps

STS-48

OC-48

2.488 Gbps

STS-96

OC-96

4.976 Gbps

STS-192

OC-192

9.952 Gbps

Working Pair

Backup Pair

Typical SONET Topology

SONET Multiplexer

Frame Relay

•

Industry-standard data-link-layer protocol

for transporting traffic across wide-area

virtual circuits

•

Optimized for efficiency on circuits with low

error rates

•

Attractively-priced in most parts of the

world

•

Carriers agree to forward traffic at a

Committed Information Rate (CIR)

Frame Relay and X.25

•

X.25 was optimized for excellent reliability

on physical circuits with high error rates.

•

X.25 was more complex to implement than

Frame Relay.

•

X.25 works at the physical, data link, and

network layers.

•

X.25 allows computers on different public

networks (CompuServe, TCP/IP) to

communicate through an intermediary

computer at the network layer level.

Frame Relay (continued)

Router A

Router B

To Router B: DLCI

100

To Router A: DLCI

200

Virtual Circuit (VC)

Virtual Circuit

What is a virtual circuit?

OSI model used in example. TCP takes large blocks

of information from an application and breaks them into

segments.  It numbers and sequences each segment so

that the destination ‘s TCP protocol can put the segments

back into the order the application intended. After the

segments are sent,  TCP (trans host) waits for an

acknowledgment of the receiving end’s TCP virtual circuit

session, retransmitting those that aren’t acknowledged.

Before a transmission occurs, a host sends

segments down the OSI model, the sender’s TC protocol

contacts the destination’s TCP protocol to establish a

connection. This type of connection is considered to be

connection-oriented. UDP is connectionless connection.

Frame Relay Hub-and-Spoke Uses

Subinterfaces

Central-Site Router

hostname central site

interface serial 0

encapsulation frame-relay

interface serial 0.1

ip address 10.0.1.1 255.255.255.0

frame-relay interface-dlci 100

interface serial 0.2

ip address 10.0.2.1 255.255.255.0

frame-relay interface-dlci 200

DLCI 100

DLCI 200

Frame Relay

Frame Relay Video:

https://www.youtube.com/watch?v=GCCpEM2CoGY

Split Horizon

A routing technique in which information about

routes is prevented from exiting the router interface

through which that information was received. Split

horizon updates are useful in preventing routing loops.

Use a sub-interfaces. This is a logical interface

that is associated with a physical interface. The central

site could have five PPP sub-interfaces defined, each

communicating with one of the remotes sites. With this

solution, the central site router applies the split horizon

rule based on logical sub-interfaces, instead of the

physical interface,  and includes remote sites in the

routing updates it sends out the WAN interface.

Split Horizon

Split horizon can be eliminated using full

mesh design with physical circuits between each

site.

Asynchronous Transfer Mode (ATM)

•

Used in service provider internal networks

•

Gaining popularity within private networks,

both WANs and sometimes LANs

•

Supports very high bandwidth requirements

–

Copper cabling: 45 Mbps (T3) or more

–

Fiber-optic cabling: OC-192 (9.952

Gbps) and beyond, especially if

technologies such as wavelength-division

multiplexing (WDM) are used

ATM

•

Provides efficient sharing of bandwidth among

applications with various Quality of Service

(QoS) requirements

–

Cell-based system inherently better for QoS than

frame-based system, because frame-based system,

large frames can monopolize

    bandwidth

•

ATM is with a connection-oriented technology

•

Application can specify upon connection

establishment the QoS it requires

•

Peak and minimum cell rates, cell-loss ratio,

and cell-transfer delay

ATM

•

A disadvantages of ATM is that ATM

interfaces for routers and switches are

expensive.

ATM Video:

https://www.youtube.com/watch?v=3VAmcN8VmIU

Ethernet over ATM

•

ATM router interfaces are expensive

•

Some providers allow a customer to use an

Ethernet interface to access the provider’s

ATM WAN

•

May require a converter

•

Expected to gain popularity because it has

the advantages of both worlds

–

Easy-to-use LAN

–

QoS-aware WAN

Metro Ethernet

•

A service offered by providers and carriers

that traditionally only offered WAN services

•

Carriers offer Metro Ethernet to customers

who are looking for cost-effective method

to interconnect campus networks and to

access the Internet.

•

Metro Ethernet allow users to continue

using 10/100 Mbps Ethernet interfaces.

•

Supports copper and fiber optics interfaces.

Metro Ethernet

•

Allows providers to offer bandwidth in 1-

Mbps increments.

Selection Criteria for Remote Access

Devices

•

Support for VPN features

•

Support for NAT

•

Reliability

•

Cost

•

Ease of configuration and management

•

Support for one or more high-speed

Ethernet interfaces

•

If desired, wireless support

Selection Criteria for VPN

Concentrators

•

Support for:

–

Tunneling protocols such as IPsec, PPTP, and L2TP

–

Encryption algorithms such as 168-bit Triple DES,

Microsoft Encryption (MPPE), RC4, AES

–

Authentication algorithms, including MD5, SHA-1, HMAC

–

Network system protocols, such as DNS, RADIUS,

Kerberos, LDAP

–

Routing protocols

–

Certificate authorities

–

Network management using SSH or HTTP with SSL

Selection Criteria for Enterprise

Routers

•

Number of ports

•

Processing speed

•

Media and technologies supported

•

MTTR and MTBF

•

Throughput

•

Optimization features

Selection Criteria for a WAN Service

Provider

•

Extent of services and technologies

•

Geographical areas covered

•

Reliability and performance characteristics of

the provider’s internal network

•

The level of security offered by the provider

•

The level of technical support offered by the

provider

•

The likelihood that the provider will continue

to stay in business

Selecting a Provider

•

The provider’s willingness to work with you to

meet your needs

•

The physical routing of network links

•

Redundancy within the network

•

The extent to which the provider relies on other

providers for redundancy

•

The level of oversubscription on the network

•

QoS support

Summary

•

A major task during the physical design phase is

selecting technologies and devices for enterprise

networks

–

Remote access networks

–

WANs

–

Service providers

–

Devices

•

End user remote access devices

•

Central site remote access devices

•

VPN concentrators

•

Routers

Review Questions

•

Compare and contrast technologies for

supporting remote users.

•

Compare and contrast WAN technologies.

•

What selection criteria can you use when

purchasing internetworking devices for

enterprise network customers?

•

What criteria can you use when selecting a

WAN service provider?

This Week’s Outcomes

•

PPP

•

Cable Modems and DSL

•

Leased Lines, SONET, Frame Relay, Metro

Ethernet, ATM

•

Selecting a WAN Service Provider

Due this week

•

11-1 – Concept questions 8

Next week

•

Read Chapters 12 and 13 in 

Top-Down

Network Design

•

12-1 – Concept questions 9

•

1-5-3 – Network design project

–

New office network

•

Franklin

Live 

session 13

Q & A

•

Questions, comments, concerns?