Lecture 05: WAN Concepts, HDLC and PPP
HKUSPACE CCIT ENA
Syllabus inspired by Cisco Networking Academy CCNA v7.0 (ENSA)
Module Objectives
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Topic Title
Topic Objective
Purpose of WANs
Explain the purpose of a WAN.
WAN Operations
Explain how WANs operate.
Traditional WAN Connectivity
Compare traditional WAN connectivity options.
Modern WAN Connectivity
Compare modern WAN connectivity options.
Internet-Based Connectivity
Compare internet-based connectivity options.
HDLC connection
Configuring HDLC
PPP connection
Configuring PPP and security
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Purpose of WANs LANs and WANs
A WAN is a telecommunications network that spans over a relatively large geographical area and is required to connect beyond the boundary of the LAN.
Local Area Networks (LANs)
Wide Area Networks (WANs)
LANs provide networking services within a small geographic area.
WANs provide networking services over large geographical areas.
LANs are used to interconnect local computers, peripherals, and other devices.
WANs are used to interconnect remote users, networks, and sites.
A LAN is owned and managed by an organization or home user.
WANs are owned and managed by internet service, telephone, cable, and satellite providers.
Other than the network infrastructure costs, there is no fee to use a LAN.
WAN services are provided for a fee.
LANs provide high bandwidth speeds using wired Ethernet and Wi-Fi services.
WANs providers offer low to high bandwidth speeds, over long distances.
Purpose of WANs
Private and Public WANs
A private WAN is a connection that is dedicated to a single customer.
Private WANs provide the following:
• Guaranteed service level
• Consistent bandwidth • Security
A public WAN connection is typically provided by an ISP or telecommunications service provider using the internet. In this case, the service levels and bandwidth may vary, and the shared connections do not guarantee security.
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Purpose of WANs WAN Topologies
WANs are implemented using the following logical topology designs:
• Point-to-Point Topology
• Hub-and-Spoke Topology
• Dual-homed Topology
• Fully Meshed Topology
• Partially Meshed Topology
Note: Large networks usually deploy a combination of these topologies.
WAN Operations
WANs in the OSI Model
Most WAN standards focus on the physical layer and the data link layer.
Layer 1 Protocols
• Synchronous Digital Hierarchy (SDH)
• Synchronous Optical Networking (SONET)
• Dense Wavelength Division Multiplexing (DWDM) Layer 2 Protocols
• Broadband (i.e., DSL and Cable)
• Wireless
• Ethernet WAN (Metro Ethernet)
• Multiprotocol Label Switching (MPLS)
• Point-to-Point Protocol (PPP) (less used)
• High-Level Data Link Control (HDLC) (less used)
• Frame Relay (legacy)
• Asynchronous Transfer Mode (ATM) (legacy)
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WAN Operations
Common WAN Terminology
There are specific terms used to describe WAN connections between the subscriber (i.e., the company / client) and the WAN service provider.
Description
Data Terminal Equipment (DTE)
Connects the subscriber LANs to the WAN communication device
Data Communications Equipment (DCE)
Device used to communicate with the provider
Customer Premises Equipment (CPE)
This is the DTE and DCE devices located on the enterprise edge
Point-of-Presence (POP)
The point where the subscriber connects to the service provider network
Demarcation Point
The physical location in a building or complex that officially separates the CPE from service provider equipment.
WAN Operations
Common WAN Terminology (Cont.)
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Description
Local Loop (last mile)
The copper or fiber cable that connects the CPE to the CO of the service provider
Central office (CO)
The local service provider facility or building that connects the CPE to the provider network
Toll network
Includes backhaul, long-haul, all-digital, fiber-optic communications lines, switches, routers, and other equipment inside the WAN provider network
Backhaul network
Connects multiple access nodes of the service provider network
Backbone network
Large, high-capacity networks used to interconnect service provider networks and to create a redundant network.
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WAN Operations WAN Devices
There are many types of devices that are specific to WAN environments.
WAN Device
Description
Voiceband Modem
Dial-up modem – uses telephone lines Legacy device
DSL Modem / Cable Modem
Collectively known as broadband modems, these high-speed digital modems connect to the DTE router using Ethernet.
Digital-leased lines require a CSU and a DSU. It connects a digital device to a digital line.
Optical Converter
Connect fiber-optic media to copper media and convert optical signals to electronic pulses.
Wireless Router / Access Point
Devices are used to wirelessly connect to a WAN provider.
WAN Core devices
WAN backbone consists of multiple high-speed routers and Layer 3 switches.
WAN Operations
Serial Communication
• Almost all network communications occur using a serial communication delivery. Serial communication transmits bits sequentially over a single channel.
• In contrast, parallel communications simultaneously transmit several bits using multiple wires.
• As the cable length increases, the synchronization timing between multiple channels becomes more sensitive to distance. For this reason, parallel communication is limited to very short distances
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WAN Operations
Circuit-Switched Communication
A circuit-switched network establishes a dedicated circuit (or channel) between endpoints before the users can communicate.
• Establishes a dedicated virtual connection through the service provider network before communication can start.
• All communication uses the same path.
• The two most common types of circuit- switched WAN technologies are the public switched telephone network (PSTN) and the legacy Integrated Services Digital Network (ISDN).
WAN Operations
Packet-Switched Communication
Network communication is most commonly implemented using packet-switched communication.
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Segments traffic data into packets that are routed over a shared network.
Much less expensive and more flexible than circuit switching.
Common types of packet-switched WAN technologies are:
Ethernet WAN (Metro Ethernet), Multiprotocol Label Switching (MPLS) Frame Relay
Asynchronous Transfer Mode (ATM).
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WAN Operations
SDH, SONET, and DWDM
Service provider networks use fiber-optic infrastructures to transport user data between destinations. Fiber-optic cable is far superior to copper cable for long distance transmissions due to its much lower attenuation and interference.
There are two optical fiber OSI layer 1 standards available to service providers:
• SDH – Synchronous Digital Hierarchy (SDH) is a global standard for transporting data over fiber-optic cable.
• SONET – Synchronous Optical Networking (SONET) is the North American standard that provides the same services as SDH.
SDH/SONET define how to transfer multiple data, voice, and video communications over optical fiber using lasers or light-emitting diodes (LEDs) over great distances.
Dense Wavelength Division Multiplexing (DWDM) is a newer technology that increases the data-carrying capacity of SDH and SONET by simultaneously sending multiple streams of data (multiplexing) using different wavelengths of light.
Traditional WAN Connectivity
Traditional WAN Connectivity Options
To understand the WANs of today, it helps to know where they started.
• When LANs appeared in the 1980s, organizations began to see the need to interconnect with other locations.
• To do so, they needed their networks to connect to the local loop of a service provider.
• This was accomplished by using dedicated lines, or by using switched services from a service provider.
HDLC / PPP
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Traditional WAN Connectivity Common WAN Terminology
Point-to-point lines could be leased from a service provider and were called “leased lines”. The term refers to the fact that the organization pays a monthly lease fee to a service provider to use the line.
Leased lines are available in different fixed capacities and are generally priced based on the bandwidth required and the distance between the two connected points.
There are two systems used to define the digital capacity of a copper media serial link:
T-carrier – Used in North America, T-carrier provides T1 links supporting bandwidth up to 1.544 Mbps and T3 links supporting bandwidth up to 43.7 Mbps.
E-carrier – Used in Europe, E-carrier provides E1 links supporting bandwidth up to 2.048 Mbps and E3 links supporting bandwidth up to 34.368 Mbps.
Traditional WAN Connectivity
Common WAN Terminology (Cont.)
The table summarizes the advantages and disadvantages of leased lines.
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Advantages
Simplicity
Point-to-point communication links require minimal expertise to install and maintain.
Point-to-point communication links usually offer high quality service, if they have adequate bandwidth.
Availability
Constant availability is essential for some applications, such as e-commerce. Point-to-point communication links provide permanent, dedicated capacity which is required for VoIP or Video over IP.
Disadvantages
Point-to-point links are generally the most expensive type of WAN access. The cost of leased line solutions can become significant when they are used to connect many sites over increasing distances.
Limited flexibility
WAN traffic is often variable, and leased lines have a fixed capacity, so that the bandwidth of the line seldom matches the need exactly.
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Traditional WAN Connectivity Circuit-Switch Options
Circuit-switched connections are provided by Public Service Telephone Network (PSTN) carriers. The local loop connecting the CPE to the CO is copper media.
There are two traditional circuit-switched options:
Public Service Telephone Network (PSTN)
• Dialup WAN access uses the PSTN as its WAN connection. Traditional local loops can transport binary computer data through the voice telephone network using a voiceband modem.
• The physical characteristics of the local loop and its connection to the PSTN limit the rate of the signal to less than 56 kbps.
Integrated Services Digital Network (ISDN)
• ISDN is a circuit-switching technology that enables the PSTN local loop to carry digital signals. This provided higher capacity switched connections than dialup access. ISDN provides for data rates from 45 Kbps to 2.048 Mbps.
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Traditional WAN Connectivity Packet-Switch Options
Packet switching segments data into packets that are routed over a shared network. It allows many pairs of nodes to communicate over the same channel.
There are two traditional (legacy) circuit-switched options:
Frame Relay
• Frame Relay is a simple Layer 2 non-broadcast multi-access (NBMA) WAN technology that is used to interconnect enterprise LANs.
• Frame Relay creates PVCs which are uniquely identified by a data-link connection identifier (DLCI).
Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM) technology is capable of transferring voice, video, and data through private and public networks.
ATM is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes.
Note: Frame relay and ATM networks have been largely replaced by faster Metro Ethernet and internet-based solutions. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 18
Modern WAN Connectivity Modern WANs
Modern WANS have more connectivity options than traditional WANs.
• Enterprises now require faster and more flexible WAN connectivity options.
• Traditional WAN connectivity options have rapidly declined in use because they are either no longer available, too expensive, or have limited bandwidth.
Modern WAN Connectivity
Modern WAN Connectivity Options
New technologies are continually emerging. The figure summarizes the modern WAN connectivity options.
Dedicated broadband
• Fiber can be installed independently by an organization to connect remote locations directly together.
• Darkfibercanbeleasedorpurchasedfroma supplier.
Packet-switched
• MetroEthernet–ReplacingmanytraditionalWAN options.
• MPLS – Enables sites to connect to the provider regardless of its access technologies.
Internet-based broadband
• Organizations are now commonly using the global internet infrastructure for WAN connectivity.
The figure displays the local loop connections most likely encountered today.
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Modern WAN Connectivity Ethernet WAN
Service providers now offer Ethernet WAN service using fiber-optic cabling.
The Ethernet WAN service can go by many names, including the following:
• Metropolitan Ethernet (Metro E)
• Ethernet over MPLS (EoMPLS)
• Virtual Private LAN Service (VPLS)
There are several benefits to an Ethernet WAN:
• Reduced expenses and administration
• Easy integration with existing networks
• Enhanced business productivity
Note: Ethernet WANs have gained in popularity and are now commonly being used to replace the traditional serial point-to-point, Frame Relay and ATM WAN links.
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Modern WAN Connectivity MPLS
Multiprotocol Label Switching (MPLS) is a high-performance service provider WAN routing technology to interconnect clients without regard to access method or payload.
• MPLS supports a variety of client access methods (e.g., Ethernet, DSL, Cable, Frame Relay).
• MPLS can encapsulate all types of protocols including IPv4 and IPv6 traffic.
• An MPLS router can be a customer edge (CE) router, a provider edge (PE) router, or an internal provider (P) router.
• MPLS routers are label switched routers (LSRs). They attach labels to packets that are then used by other MPLS routers to forward traffic.
• MPLS also provides services for QoS support, traffic engineering, redundancy, and VPNs.
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Internet-Based Connectivity
Internet-Based Connectivity Options
Internet-based broadband connectivity is an alternative to using dedicated WAN options.
Internet-based connectivity can be divided into wired and wireless options.
Wired Options
• Wired options use permanent cabling (e.g., copper or fiber) to provide consistent bandwidth, and reduce error rates and latency. Examples: DSL, cable connections, and optical fiber networks.
Wireless Options
• Wireless options are less expensive to implement compared to other WAN connectivity options because they use radio waves instead of wired media to transmit data. Examples: cellular 3G/4G/5G or satellite internet services.
• Wireless signals can be negatively affected by factors such as distance from radio towers, interference from other sources and weather.
Internet-Based Connectivity DSL Technology
Digital Subscriber Line (DSL) is a high-speed, always-on, connection technology that uses existing twisted-pair telephone lines to provide IP services to users.
DSL are categorized as either Asymmetric
DSL (ADSL) or Symmetric DSL (SDSL).
• ADSL and ADSL2+ provide higher downstream bandwidth to the user than upload bandwidth.
• SDSL provides the same capacity in both directions.
DSL transfer rates are dependent on the actual length of the local loop, and the type and condition of the cabling.
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Internet-Based Connectivity DSL Connections
Service providers deploy DSL connections in the local loop. The connection is set up between the DSL modem and the DSL access multiplexer (DSLAM).
• The DSL modem converts the Ethernet signals from the teleworker device to a DSL signal, which is transmitted to a DSL access multiplexer (DSLAM) at the provider location.
• A DSLAM is located at the Central Office (CO) of the provider and concentrates connections from multiple DSL subscribers.
• DSL is not a shared medium. Each user has a separate direct connection to the DSLAM. Adding users does not impede performance.
Internet-Based Connectivity DSL and PPP
ISPs use PPP as the Layer 2 protocol for broadband DSL connections.
• PPP can be used to authenticate the subscriber.
• PPP can assign a public IPv4 address to the subscriber.
• PPP provides link-quality management features.
There are two ways PPP over Ethernet (PPPoE) can be deployed:
• Host with PPoE Client – The PPPoE client software communicates with the DSL modem using PPPoE and the modem communicates with the ISP using PPP.
• Router PPPoE Client – The router is the PPPoE client and obtains its configuration from the provider.
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Internet-Based Connectivity Cable Technology
Cable technology is a high-speed always-on connection technology that uses a coaxial cable from the cable company to provide IP services to users.
The Data over Cable Service Interface Specification (DOCSIS) is the international standard for
adding high-bandwidth data to an existing cable system.
• The optical node converts RF signals to light pulses over fiber-optic cable.
• The fiber media enables the signals to travel over long distances to the provider headend where a Cable Modem Termination System (CMTS) is located.
• The headend contains the databases needed to provide internet access while the CMTS is responsible for communicating with the cable modems.
Note: All the local subscribers share the same cable bandwidth. As more users join the service, available bandwidth may drop below the expected rate.
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Internet-Based Connectivity Optical Fiber
Many municipalities, cities, and providers install fiber-optic cable to the user location. This is commonly referred to as Fiber to the x (FTTx) and includes the following:
• Fiber to the Home (FTTH) – Fiber reaches the boundary of the residence.
• Fiber to the Building (FTTB) – Fiber reaches the boundary of the building with the final connection to the individual living space being made via alternative means.
• Fiber to the Node/Neighborhood (FTTN) – Optical cabling reaches an optical node that converts optical signals to a format acceptable for twisted pair or coaxial cable to the premise.
Note: FTTx can deliver the highest bandwidth of all broadband options.
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