ITEC 275 Computer Networks – Switching, Routing, and

WANs

Week 10Robert D’Andrea

2015

Agenda• The course is 2/3 completed• Learning Activities

• PPP• Cable Modems and DSL• Leased Lines, SONET, Frame Relay,

Metro Ethernet, ATM• Selecting a WAN Service Provider• Metro Ethernet

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

Enterprise Selection Criteria• Business requirements and constraints• Cost • Technical goals• Bandwidth requirements• QoS requirements• Network topology• Traffic flow and load

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• PPP defines an encapsulation scheme for

transporting different network-layer protocols• PPP 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 onHigh-Level Data-Link Control Protocol (HDLC)

Physical Layer

PPPPoint-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).

PPPLeased Line

PPP, HDLC

Packet Switching Frame Relay, X.25, ATM

Circuit SwitchingPPP, HDLC

PPP

Ethernet

BroadbandPPPoE, PPPoA,

Ethernet

Metro Ethernet

PPPLeased lines: ISP is the service provider. PPP is the protocol. The user does not want to share the wire or the expense of leasing a wire with another user.

Packet Switching: Telco is the service provider. Frame Relay protocol. The user shares the service with other users to save money. Frame Relay is an alternate technology to PPP.

Circuit Switching: POTS or ISDN are the service providers. PPP is the protocol.

Data is sent encapsulated over the public telephone network system.

PPPMetro Ethernet: Telco is the service provider. Ethernet protocol.

The user wants the data sent at a very high speed.

Broadband: Telco is the service provider. PPPoE, PPPoA, Ethernet protocols.

PPPPPP encapsulation includes:

1. Line set up using LCP. The link is established during this phase.

2. Authentication, encryption and compression (optional).

2.1 Password Authentication Protocol (PAP) Avoid using PAP because it sends the password in ASCII format (clear text).

2.2 Challenge Handshake Authentication Protocol (CHAP) never sends the password, but sends a challenged handshake processes.

PPP3. PPP carries packets from many different

protocol suites using NCP. The NCP sends packets to negotiate the needed settings.

Is all data encrypted?

No

Encryption is used to simply verify the passwords.

Multilink MPPPAn 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 MPPPMPPP 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 MPPPInverse 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 MPPPVarious 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

• Adds support for channel aggregation of 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’s enhancement to PPP is MPPP. MPPP allows WAN administrator to group multiple access servers into a single stack group. The user’s traffic can be split and reassembled across multiple access servers in the stack group.

Multi-chassis Multilink PPP

Stack group

ISDN

Analog

Offloadserver

CHAPCHAP 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_1Password: sfy45

Remote Node Access Server

Connect

Challenge

Hashed Response

Accept or Deny

Database of Users and Passwords

Name: 760_1Password: sfy45

Name: 760_2Password: kingsford

ISDN• 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 transmits 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

23B or 30B

D

1.544 Mbps in U.S.

2.048 Mbps in Europe

Primary Rate Interface (PRI)

64 Kbps

64 Kbps

}

64 Kbps64 Kbps

16 Kbps

144 Kbps

2B

D}

Basic Rate Interface (BRI)

ISDN Components

ISDN device (TE1) with built-in NT1

To ISDNservice

U

NT1

ISDNdevice (TE1)

S/T UNT1

To ISDNservice

Non-ISDNdevice (TE2)

S/T UTA NT1

R

4-wirecircuit

2-wirecircuit

To ISDNservice

ISDNdevice (TE1)

T UNT1

To ISDNserviceNT2

S

Cable TV Service

CATV (originally "community antenna television," now often "community access television") is more commonly known as "cable TV." Television programs were brought to millions of people throughout the world who were connected to a community antenna, cable TV.

  Today, CATV has become an increasingly popular way to interact with the World Wide Web and other new forms of multimedia information and entertainment services.

Cable TV Service

• Operates over the coax cable used by cable TV providers

• 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 TV Service

• Coax Cable does not require dial-up.• Cable modem operates more like a LAN.• Cable-network providers offer 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 TV 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 ADSLAsymmetric DSL (ADSL) uses two popular PPP implementations.

1. 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.

2. PPP and Ethernet (PPPoE) the CPE acts as an Ethernet-to-WAN bridge.

PPP and ADSLPPP 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.

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 Customer Premise Equipment (CPE) , through the Data Communications Equipment (DCE) switch, to the CPE of the remote site, then allowing Data Terminal Equipment (DTE) networks to communicate at anytime with no set up procedure before transmitting data.

DTE and DCEThe end instrument is a piece of

equipment connected to the wires at the end of a telecommunication link. End instruments that relate to data terminal equipment (DTE) include printers, computers, barcode readers, automated teller machines (ATMs) and the console ports of routers.

DTE and DCEThe data communication(s) equipment

(DCE) is a device that sits between the DTE and a data transmission circuit.

The DCE performs functions such as signal conversion, line clocking, and coding.

The DTE/DCE classification was introduced by IBM.

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)

Synchronous Optical Network (SONET) is the American National Standards Institute standard for synchronous data transmission on optical

SDH (Synchronous Digital Hierarchy) is a standard technology for synchronous data transmission on optical media.

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

1. Define higher speeds than the ones used by the NADH.

2. Support efficient multiplexing and de-multiplexing of individual signals.

With SONET, it is easy to isolate one channel from a multiplexed circuit. In some instances where there are many different parts of the system are almost , but not quite, perfectly synchronised. Like the 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 (Synchronous Transport Signal) signals required for optical transport.

SONET Optical Carrier (OC) Levelsaka Synchronous Transport Signal (STS) Levels

STS Rate OC Level Speed

STS-1 OC-1 51.84 Mbps STS-3 OC-3 155.52 MbpsSTS-12 OC-12 622.08 MbpsSTS-24 OC-24 1.244 GbpsSTS-48 OC-48 2.488 GbpsSTS-96 OC-96 4.976 GbpsSTS-192 OC-192 9.952 Gbps

Working Pair

Backup Pair

Typical SONET Topology

SONET Multiplexer

Frame RelayFrame relay is a telecommunication service designed

for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN).

Frame relay puts data in a variable-size unit called a frame and leaves any necessary error correction (retransmission of data) up to the end-points, which speeds up overall data transmission. For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continuous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination and can charge based on usage. 

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

Router A Router B

To Router B: DLCI 100

To Router A: DLCI 200

Virtual Circuit (VC)

Virtual CircuitWhat 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 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 TCP 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 Routerhostname 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

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.

Split HorizonA 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-interface. 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 HorizonSplit 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 EthernetWhat is Metro Ethernet?

Metro Ethernet makes use of Carrier Ethernet technology in metropolitan area networks (MANs).

Carrier Ethernet is the use of high-bandwidth Ethernet technology for Internet access and for communication among business, academic and government local area networks (LANs).

Metro EthernetWidely available in larger cities, Metro

Ethernet offers efficient and scalable network services within a specific metropolitan region. These regions are frequently meshed in fiber optic networks, making Ethernet more widely available in the specified metropolitan area.

Comcast Metro Ethernet offers a simple solution for users that have a complex network.

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• Frame Relay• 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• FranklinLive session 13

Q & A

• Questions, comments, concerns?