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Chapter 16-WAN Concepts WAN Technologies IP Services for Internet Access

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1. Which of the following best describes the function of demodulation by a modem? a. Encoding an incoming analog signal from the PC as a digital signal for transmissio into the PSTN b. Decoding an incoming digital signal from the PSTN into an analog signal c. Encoding a set of binary digits as an analog electrical signal d. Decoding an incoming analog electrical signal from the PSTN into a digita signal e. Encoding a set of binary digits as a digital electrical signal

2. Which of the following standards has a limit of 18,000 feet for the length of the local loop? a. ADSL b. Analog modems c. ISDN d. Cable Internet service

3. Which of the following is true regarding the location and purpose of a DSLAM? a. Typically used at a home or small office to connect the phone line to a DSL router b. Typically used at a home or small office instead of a DSL router c. Typically used inside the telco’s CO to prevent any voice traffic from reaching the ISP’s router d. Typically used inside the telco’s CO to separate the voice traffic from the data traffic

4. Which of the following remote-access technologies support specifications that allow both symmetric speeds and asymmetric speeds? a. Analog modems b. WWW c. DSL d. Cable modems

5. Which of the following remote-access technologies, when used to connect to an ISP, is considered to be an “always on” Internet service? a. Analog modems b. DSL c. Cable modems d. All of these answers are correct.

6. For a typical Internet access router, using either cable or DSL, which of the following does the router typically do on the router interface connected to the LAN with the PCs in the small or home office? a. Acts as a DHCP server b. Acts as a DHCP client c. Performs NAT/PAT for the source address of packets that exit the interface d. Acts as DNS server

7. For a typical Internet access router, using either cable or DSL, which of the following does the router typically do on the router interface connected toward the Internet? a. Acts as a DHCP server b. Acts as a DHCP client c. Performs NAT/PAT for the source address of packets that exit the interface d. Acts as DNS server

8. This question examines a home-based network with a PC, a DSL router, and a DSL line. The DSL router uses typical default settings and functions. The PC connected to the router has IP address 10.1.1.1. This PC opens a browser and connects to the www.cisco.com web server. Which of the following are true in this case? a. The web server can tell it is communicating with a host at IP address 10.1.1.1. b. The PC learns the IP address of the www.cisco.com web server as a public IP address. c. The 10.1.1.1 address would be considered an inside local IP address. d. The 10.1.1.1 address would be considered an inside global IP address.

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WAN Connection DSL, Modems, Cable, ATM PSTN CO Designed for Voice-Analog Can support data now DSL, Modems CO Convert to Digital Pg 516

Modem Modulator/Demodulator Modem dials ISP router and gets an IP Convert the digital signal from computer to analog signal for local loop Modem dials ISP router and gets an IP Pg 518

DSL Just connectivity-utility service Can’t dial any number like modem DSL allows analog voice signals and digital data signals to be sent over the same local loop wiring at the same time. The local loop must be connected to something besides a traditional voice switch at the local CO, in this case a device called a DSL access multiplexer (DSLAM). DSL allows for a concurrent voice call to be up at the same time as the data connection. Unlike modems, DSL’s data component is always on; in other words, you do not have to signal or dial a phone number to set up a data circuit. Pg 519

DSL At CO DSLAM separates out DSL signal on higher frequencies The distance between the CO and the consumer (the longer the distance, the slower the speed) The quality of the local loop cabling (the worse the wiring, the slower the speed) The type of DSL (each standard has different maximum theoretical speeds) The DSLAM used in the CO (older equipment may not have recent improvements that allow for faster speeds on lower-grade local loops) Pg 519-522

Cable Internet Similar idea to DSL Higher overall speed Shared access on head-end

Summary Pg 525

Asynchronous Transfer Mode Packet Switching (like frame relay) Or Switching technology in core network ATM connection like Frame VCs 48 byte payload and 5 byte header 53 byte CELL

Packet vs. Circuit Switching Pg 527

Ethernet as WAN Metro Ethernet Limited to contract speed

Address assignment on Internet routers The Internet-facing interface needs one public IP address so that the routers in the Internet know how to route packets to the access router. The ISP typically assigns that public (and globally routable) IP address dynamically, using DHCP. The local PCs typically need to dynamically learn IP addresses with DHCP, so the access router will act as a DHCP server for the local hosts. The router needs a statically configured IP address on the local subnet, using a private network number. The local LAN subnet will use addresses in a private network number. Pg 529

Routers and DHCP Pg 530

Network Address Translation Hides the Private addresses behind the public address of the router NAT/PAT server keeps track of TCP/UDP port numbers of outgoing requests and maps them to a port on the outgoing public address

NAT

What PAT does More generally, the PAT feature causes the router to translate the source IP address and port for packets leaving the local LAN, and to translate the destination IP address and port on packets returning to the local LAN. The end result is that, as far as hosts in the Internet are concerned, all the packets coming from this one customer are from one host (64.100.1.1 in Figure 16-10), for which all the routers in the Internet should have a matching route. This allows the ISP to conserve public IPv4 addresses. Pg 534

PAT Terminology Inside host: Refers to a host in the enterprise network, like PC1 and PC2 in the last few figures. Inside local: Refers to an IP address in an IP header, with that address representing a local host as the packet passes over the local enterprise network (not the Internet). In this case, 192.168.1.101 and .102 are inside local IP addresses, and the packets at steps 1 and 4 in Figure 16-10 show inside local IP addresses. Inside global: Refers to an IP address in an IP header, with that address representing a local host as the packet passes over the global Internet (not the enterprise). In this case, 64.100.1.1 is the one inside global IP address, and the packets at steps 2 and 3 in Figure 16-10 show the inside global IP address. Inside interface: The router interface connected to the same LAN as the inside hosts. Outside interface: The router interface connected to the Internet.

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