1/28/2010
1/28/2010
Unit 4 - Section 1

Internet connections may be Wired or Wireless. In this section we will look at wired connections using either the Telephone or
Cable networks.

• PSTN (Public Switched Telephone Network)
– Network of lines, carrier equipment providing telephone service
– POTS (plain old telephone service)
– Encompasses entire telephone system
– Originally: analog traffic
– Today: digital data, computer controlled switching
• Dial-up connection
– Modem connects computer to distant network
– Works from almost anywhere
• DSL
– Provides high speed, continuous Internet connect where available
– Multiple types of DSL connections
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Figure 7-2 A bus topology WAN
Figure 7-3 A ring topology WAN
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Figure 7-5 Full-mesh and partial-mesh WANs
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• DSL (digital subscriber line)
– Operates over PSTN at physical layer
– Directly competes with other PSTN services such as
ISDN and T1
– Shares voice and data over same line
• Uses high frequency range, inaudible telephone line frequencies
– Voices uses only 300 – 3000 Hz
– Uses advanced data modulation techniques
• Amplitude or phase modulation
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• xDSL refers to all DSL varieties
– ADSL, G.Lite, HDSL, SDSL, VDSL, SHDSL
• Two DSL categories
– Asymmetrical and symmetrical
• Downstream
– Data travels from carrier’s switching facility to customer
• Upstream
– Data travels from customer to carrier’s switching facility
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Table 7-2 Comparison of DSL types
• How DSL types vary
– Data modulation techniques
– Capacity
– Distance limitations
– PSTN use
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Figure 7-17 A DSL connection
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• Cable companies connectivity option
• Based on TV signals coaxial cable wiring
– Theoretically transmission
• 150 Mbps downstream, 10 Mbps upstream
– Real transmission
• 10 Mbps downstream, 2 Mbps upstream
• Transmission limited ( throttled)
• Shared physical connections
• Best use
– Web surfing
– Network data download
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Figure 7-18 A cable modem
• Requires cable modem
– Modulates, demodulates transmission, reception signals via cable wiring
– Operates at Physical and Data Link layer
– May connect to connectivity device
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• WAN technologies are used to provide high bandwidth connection between major data centers.
• In this section we will look at some older as well as current WAN technologies.

• X.25 ITU standard
– Analog, packet-switching technology from 1970s
• Mainframe to remote computers: 64 Kbps throughput
– Update: 1992
• 2.048 Mbps throughput
• Client, servers over WANs
– Operates at Physical, Data Link, and Network layers.
– Verifies transmission at every node
• Excellent flow control, ensures data reliability
• Slow for time-sensitive applications
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• Frame relay
– Updated X.25: digital, packet-switching
– Protocols operate at Data Link layer
• Supports multiple Network, Transport layer protocols
– No data delivery guarantee
– Customer chooses data speed
– Use Virtual Circuits
• PVC – Permanent Virtual Circuit
• SVC – Switched Virtual Circuit
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Figure 7-9 A WAN using frame relay
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• Digital data transmitted over PSTN
• Gained popularity: 1990s
– Connecting WAN locations
• Exchanges data, voice signals
• Protocols at Physical, Data Link, Transport layers
• Relies on PSTN for transmission medium
• Dial-up or dedicated connections
– Dial-up relies exclusively on digital transmission
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• Two channel types
– B channel: “bearer”
• Circuit switching for voice, video, audio: 64 Kbps
– D channel: “data”
• Packet-switching for call information: 16 or 64 Kbps
• BRI (Basic Rate Interface) connection
• PRI (Primary Rate Interface) connection
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Figure 7-10 A BRI link
• BRI: two B channels, one D channel (2B+D)
– B channels treated as separate connections
• Carry voice and data
• Bonding
– Two 64-Kbps B channels combined
• Achieve 128 Kbps
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Figure 7-11 A PRI link
• PRI: 23 B channels, one 64-Kbps D channel
(23B+D)
– Separate B channels independently carry voice, data
– Maximum throughput: 1.544 Mbps
• PRI and BRI may interconnect
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• Functions in Data Link layer
• Asynchronous communications method
– Nodes do not conform to predetermined schemes
• Specifying data transmissions timing
– Each character transmitted
• Start and stop bits
• Specifies Data Link layer framing techniques
• Fixed packet size sets ATM apart from Ethernet
– Packet (cell)
• 48 data bytes plus 5-byte header = 53 byts
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• Smaller packet size requires more overhead
– Decrease potential throughput
– Cell efficiency compensates for loss
• ATM relies on virtual circuits
– ATM considered packet-switching technology
– Virtual circuits provide circuit switching advantage
• Circuit path setup by switches in advance
– Reliable connection
• Allows specific QoS (quality of service) guarantee
– Important for time-sensitive applications
• Often used on SONET rings
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• T1s, fractional T1s, T3s
• Physical layer operation
• Single channel divided into multiple channels
– Using TDM (time division multiplexing) over two wire pairs
• Medium
– Telephone wire, fiber-optic cable, wireless links
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Table 7-1 Carrier specifications
• Many available
– Most common: T1 and T3
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Figure 7-12 A T1 smart jack
• Smart Jack
– Terminate T-carrier wire pairs
• Customer’s demarc (demarcation point)
• Inside or outside building
– Connection monitoring point
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• CSU/DSU (Channel Service Unit/Data Service Unit)
– Two separate devices
– Combined into single stand-alone device
• Interface card
– T1 line connection point
• At customer’s site
• CSU
– Provides digital signal termination
– Ensures connection integrity
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Figure 7-14 A point-to-point T-carrier connection
• Incoming T-carrier line
– Multiplexer separates combined channels
• Outgoing T-carrier line
– Multiplexer combines multiple LAN signals
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• Four key strengths
– WAN technology integration
– Fast data transfer rates
– Simple link additions, removals
– High degree of fault tolerance
• Synchronous
– Data transmitted, received by nodes conforms to timing scheme
• Advantage
– Interoperability
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• Fault tolerance
– Double-ring topology over fiber-optic cable
• SONET Ring
– Begins, ends at telecommunications carrier’s facility
– Connects organization’s multiple WAN sites in ring fashion
– Connect with multiple carrier facilities
• Additional fault tolerance
– Terminates at multiplexer
• Easy SONET ring connection additions, removals
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Figure 7-21 SONET connectivity
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Figure 7-20 A SONET ring
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Table 7-3 SONET OC levels
• Data rate
– Indicated by OC (Optical Carrier) level
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Table 7-4 A comparison of WAN technology throughputs
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• Remote access is used to allow users to connect to a network or single computer from a remote location.

Figure 7-22 Clients connecting with a remote access server
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• SLIP and PPP
– Workstations connect using dial-up connection
• Encapsulate higher-layer networking protocols, in lower-layer data frames
– SLIP carries IP packets only
• Harder to set up
• Supports only asynchronous data
– PPP carries many different Network layer packets
• Automatic set up
• Performs error correction, data compression, supports encryption
• Supports asynchronous and synchronous transmission
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Figure 7-23 Protocols used in a remote access Internet connection
• PPPoE (PPP over Ethernet) standard
– Connects home computers to ISP
• Via DSL, broadband cable
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• Remote desktop
– Windows client and server operating systems
– Relies on RDP (Remote Desktop Protocol)
• Application layer protocol
• Uses TCP/IP to transmit graphics, text quickly
• Carries session, licensing, encryption information
• Exists for other operating systems
– Not included in Windows home editions
– Uses Port 3389 by default
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Figure 7-24 Remote tab in the Windows XP System Properties window
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• Uses virtual machines running on a server
– One server can host many desktop OS environments
• Uses Thin client for users
– Remote virtual computing software requires little bandwidth
• Useful in BYOD environments

• Uses tunnel to Isolate traffic from other public line traffic
• Software
– Inexpensive
– Sometimes included with other widely used software
• Tailored to customer’s distance, bandwidth needs
• Two important design considerations
– Interoperability and security
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Figure 7-26 An example of a VPN
• Tunneling
– Ensures VPN carries all data types privately
• Tunnel
– Virtual connection between two VPN nodes
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• Types of tunneling
– PPTP (Point-to-Point Tunneling Protocol)
• Microsoft
• Authentication and minimal encryption
– L2TP (Layer 2 Tunneling Protocol)
• Cisco
• Uses IPSec encryption at Network Layer to provide strong security
– SSL VPN
• Uses SSL encryption at Presentation level
• Establishes connection at Browser level
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• An SSL VPN (Secure Sockets Layer virtual private network) is a form of VPN that can be used with a standard Web browser.
• In contrast to the traditional Internet Protocol
Security (IPsec) VPN, an SSL VPN does not require the installation of specialized client software on the end user's computer.
• Used to give remote users with access to Web applications, client/server applications and internal network connections.

There are three major families of VPN implementations in wide usage today: SSL, IPSec, and PPTP. OpenVPN is an SSL VPN and as such is not compatible with IPSec, L2TP, or PPTP.
– The IPSec protocol is designed to be implemented as a modification to the IP stack in kernel space, and therefore each operating system requires its own independent implementation of IPSec.
– By contrast, OpenVPN's user-space implementation allows portability across operating systems and processor architectures, firewall and NAT-friendly operation, dynamic address support, and multiple protocol support including protocol bridging.
– There are advantages and disadvantages to both approaches .
The principal advantages of OpenVPN's approach are portability, ease of configuration, and compatibility with NAT and dynamic addresses. The learning curve for installing and using OpenVPN is on par with that of other security-related daemon software such as SSH.

Network+ Guide to Networks, 5 th Edition