Bài giảng Network+ Certification - Chapter 12, Remote Network Access

Chapter 12, Remote Network Access |1| Chapter Overview A. Using Remote Connections B. SLIP and PPP C. WAN Technologies Chapter 12, Lesson 1 Using Remote Connections 1. Remote Networking A. Connecting to a remote network using a modem or other device is not very different from connecting to a LAN. 1. From the network layer up, a remote connection is no different from a direct LAN connection, but the data-link and physical layers can take several different forms. B. Modems and other wide area networking devices can function as network interfaces, just as network interface adapters do. |2| C. Remote network connections can take several forms, including 1. Computers to Internet service providers (ISPs) 2. Computers to private networks 3. Computers to computers 4. Networks to networks 2. Connection Types |3| A. Public Switched Telephone Network (PSTN) 1. Technical name for the standard voice telephone system 2. Also known as the Plain Old Telephone Service (POTS) 3. Analog, circuit-switched network 4. Works with asynchronous modems to transmit data between computers at almost any location 5. Typically uses copper-based, twisted-pair cable with RJ-11 jacks a. RJ-11 connectors have four (or sometimes six) pins, rather than the eight pins in the RJ-45 connectors used by LANs. |4, 5| 6. Modems a. A modem (modulator/demodulator) is required to convert a computer’s digital signals to the analog signals used by PSTN. b. At the other end of the connection, another modem converts the analog signals back to digital. c. Early modems used proprietary communication protocols, requiring the same manufacturer’s modems at each end of the connection. d. Today, organizations such as the International Telecommunication Union (ITU) develop specifications for the communication, compression, and error-detection protocols that modems use when generating and interpreting analog signals. 2 Outline, Chapter 12 Network+ Certification, Second Edition (1) The ITU was formerly known as the Comité Consultatif International Télégraphique et Téléphonique (CCITT). e. V.90 is the current industry-standard modem communication protocol. (1) V.90 defines the 56-Kbps data transfer mode that most modem connections use today. 7. PSTN advantages a. No special service installation is required. b. No special equipment is required except modems and a PSTN line. c. Portable systems can dial into a network from any location. 8. Disadvantage: using PSTN frequently over long distances can be expensive. 9. PSTN connections are relatively slow. a. The maximum speed for a connection with two modems is 33.6 Kbps. b. 56-Kbps connections require one of the devices to have a digital connection to the PSTN. c. PSTN connections can vary in quality, depending on the age and condition of the equipment and the location of the modems. 10. Leased lines are permanent, dedicated PSTN connections between two locations. a. Advantages of leased lines (1) Higher speeds (2) Available in analog or digital form (3) More consistent quality of service b. Disadvantages (1) Less flexible than standard PSTN modem connections (2) More expensive than standard PSTN connections |6| 11. Configuring a modem a. Most modems today support the Plug and Play standard. b. Modems typically need an interrupt request (IRQ) and an input/output (I/O) port to communicate with the computer. (1) For external modems, the IRQ and I/O port are assigned to the serial port used to connect the modem to the computer. (2) Internal modems plug into a bus slot, and you configure the modem itself to use a specific COM port. c. Serial ports use a chip called a universal asynchronous receiver- transmitter (UART) to manage the communications of the device connected to the port. (1) For today’s high-speed modems, you should always use a 16550 UART. B. Virtual private networks 1. A VPN is a connection between a remote computer and a server on a private network that uses the Internet as its network medium. |7| 2. VPN communications Outline, Chapter 12 3 Network+ Certification, Second Edition a. The remote user connects to the Internet by using a modem to dial in to a local ISP. b. The network is permanently connected to the Internet and has a server that is configured to receive incoming VPN connections. c. The remote computer and the network server establish a secured connection across the Internet. (1) This technique is called tunneling, because the connection runs across the Internet inside a secure conduit. 3. Point-to-Point Tunneling Protocol (PPTP) a. Makes VPNs possible by tunneling through the Internet b. To establish a VPN connection, a client (1) Uses a standard PPP connection to dial in to the local ISP (2) Establishes a control connection with the VPN server on the target network, using the Transmission Control Protocol (TCP) c. The control connection is the PPTP tunnel through which the computers transmit and receive all subsequent data. |8| 4. PPTP encapsulation a. The computers involved in a VPN connection encapsulate their PPP data within IP datagrams. b. PPTP encapsulation violates the rules of the OSI model because a data-link layer frame is carried within a network layer datagram. c. The PPP frames are encapsulated by IP, but at the same time, they can also contain other IP datagrams that contain the user data that one computer is sending to the other. d. The messages transmitted through the TCP connection that forms the tunnel are IP datagrams that contain PPP frames, with the PPP frames containing messages generated by IP or any network layer protocol. e. Because the PPP user data is secured within the IP datagrams, that data can be another IP datagram or an Internetwork Packet Exchange (IPX) or NetBIOS Extended User Interface (NetBEUI) message. f. Because the tunnel is encrypted and secured using an authentication protocol, the data is protected from interception. g. After the IP datagrams pass through the tunnel to the other computer, the receiver extracts and processes the PPP frames in the normal manner. |9| C. Integrated Services Digital Network (ISDN) 1. Digital dial-up service that uses the PSTN infrastructure to create digital connections at higher speeds 2. Designed to be a digital replacement for the analog telephone network 3. More popular in Europe than the United States 4. Used primarily for Internet connections, but also supports special ISDN telephones, fax machines, etc. 5. ISDN is a dial-up service, so you can connect to different networks by dialing a different number. 6. ISDN Basic Rate Interface (BRI) a. Provides two 64-Kbps B channels and one 16-Kbps D channel 4 Outline, Chapter 12 Network+ Certification, Second Edition b. B channels carry application data and D channels carry control data. c. The BRI service is also called 2B+D. d. The B channels can be combined into one 128-Kbps channel. 7. ISDN Primary Rate Interface (PRI) is a higher grade of service than BRI. a. Twenty-three 64-Kbps B channels plus one 64-Kbps D channel b. Same bandwidth as a T-1 leased line c. Not often used in the United States 8. Requires additional equipment at the terminal location, so it cannot be used in mobile devices a. The telephone company provides a U interface. b. A Network Terminator 1 (NT-1), usually supplied by the customer, connects to the U interface and provides a four-wire connection, called an S/T interface. c. The S/T interface supports up to seven devices, called terminal equipment (TE). (1) TE1 devices are designed for use with ISDN and connect directly to the S/T interface. (2) TE2 devices, such as computers and standard analog telephones, require a terminal adapter. |10| d. The terminal adapter connects to the S/T interface and provides a jack for the TE2 device. e. The NT-1 and the terminal adapter can be separate devices or one unit, called an ISDN adapter. 9. The length of an ISDN connection is limited. a. The U interface must be within 18,000 feet of the telephone company’s nearest central office. 10. ISDN providers typically charge a monthly fee plus a per-minute rate. a. To connect to the Internet, there is a separate monthly fee for an ISP. 11. Disadvantages of ISDN a. As an Internet access solution, ISDN tends to be slower and more expensive than Digital Subscriber Line (DSL) or cable television (CATV) access. b. ISDN has a reputation in the United States for difficult installations and problematic service. D. Digital Subscriber Line (DSL) 1. A blanket term for a variety of digital communication services that a. Use standard telephone lines b. Provide data transfer speeds much greater than the PSTN or even ISDN |11, 12| 2. Each DSL service type has a different descriptive word added to its name, which is why some sources use the generic abbreviation xDSL. a. High-bit-rate Digital Subscriber Line (HDSL) is used by telephone companies and large corporations for wide area network (WAN) links. Outline, Chapter 12 5 Network+ Certification, Second Edition b. Asymmetrical Digital Subscriber Line (ADSL) is commonly used for end-user Internet access. 3. Many DSL services are asymmetrical, meaning that they run faster downstream than upstream. a. Standard Internet access generates more downstream than upstream traffic, making an asymmetrical connection feasible. b. Running Internet servers requires a symmetrical connection, or one with sufficient upstream bandwidth. 4. DSL connections are subject to distance restrictions, like ISDN. 5. DSL uses the higher frequencies of the PSTN infrastructure, making it possible for DSL to share a telephone line with voice traffic. |13| 6. The hardware needed for an ADSL connection is called an ADSL Termination Unit-Remote (ATU-R), or sometimes a DSL transceiver or a DSL modem. a. The ATU-R connects to your computer using either a standard Ethernet network interface adapter or a universal serial bus (USB) port. (1) You also need to install a line splitter if you will use the line for voice traffic. b. At the other end of the link at the ISP’s site is a more complicated device called a Digital Subscriber Line Access Multiplexer (DSLAM). 7. DSL connections are direct, permanent links between two sites that remain connected at all times. a. If you use DSL to connect to the Internet, the telephone company installs the DSL connection between your home or office and the ISP’s site. b. If you want to change your ISP, the phone company must install a new link. |14| E. CATV 1. Cable television companies have their own private networks that provide Internet access through the same cable used for television signals. 2. CATV networks are broadband, meaning that the cable carries multiple signals simultaneously. 3. CATV connections can run at speeds of 512 Kbps or more. 4. As with DSL, you connect a computer to a CATV network using a standard Ethernet network interface adapter. 5. CATV connections are not dedicated links, as with ISDN and DSL. 6. A CATV network is essentially a metropolitan area network (MAN), on which you and your neighbors share bandwidth. a. Slowdowns can occur during peak performance periods. b. As with any shared network connection, you must be aware of security and protect your system against unauthorized access. 7. CATV networks are asymmetrical, like DSL. |15| 8. The main limitation of a CATV connection is that it connects only to the Internet. 6 Outline, Chapter 12 Network+ Certification, Second Edition a. You can only connect to a private network using a CATV connection by establishing a VPN connection through the Internet. b. You can get DSL and ISDN connections between any two points. |16| F. Satellite connections 1. Some ISPs provide Internet access using geosynchronous satellites and dishes like those used for television connections. 2. In most cases, satellite connections are downstream only. a. For upstream traffic, you must maintain a dial-up connection to the ISP’s network. 3. Not suitable for connections to a private network, as with CATV |17| G. Terminal connections and thin client computing 1. Thin client computing is a type of remote connection used within a site, instead of between sites. 2. Thin client computing consists of a terminal client program running on a low-end computer or a dedicated network client device that communicates with a terminal server elsewhere on the network. a. The client provides the interface to the operating system and nothing more. b. The actual operating system and all applications run on the terminal server. 3. The client and the server communicate using a specialized protocol, such as ICA, developed by Cyrix Systems, Inc. a. ICA carries keystrokes, mouse actions, and screen updates between the client and the server. b. ICA lets a user at the client side function as though the applications are running locally, when they are actually running at the server. 4. Advantages of thin client computing a. Enables a network to use inexpensive machines for its clients b. Leaves most of the computing environment on the server, where administrators can easily monitor and maintain it |18| 3. Remote Connection Requirements A. Common protocols 1. The two computers to be connected must share common protocols at the data-link layer and above. 2. You must configure both computers to use a data-link layer protocol suitable for point-to-point connections, such as PPP or SLIP. 3. There must also be network and transport layer protocols in common, such as TCP/IP, IPX, or NetBEUI. B. TCP/IP configuration 1. Must have IP address and other configuration parameters appropriate for that network 2. Most remote networking solutions enable the server to assign configuration parameters automatically using Dynamic Host Configuration Protocol (DHCP). C. Host and remote software Outline, Chapter 12 7 Network+ Certification, Second Edition 1. The remote (or client) computer needs a client program that can use the physical layer medium to establish a connection. 2. The host (or server) computer must have a program that can respond to a connection request from the remote computer and provide access to the network. D. Security 1. The host computer and the other systems on the network must have security mechanisms that a. Control access to network resources b. Permit access only to authorized users c. Restrict the access of authorized users to the resources they need Chapter 12, Lesson 2 SLIP and PPP |19| 1. SLIP and PPP Characteristics A. TCP/IP data-link layer protocols 1. SLIP and PPP operate at the data-link layer of the OSI model and the link layer of the TCP/IP model. B. SLIP and PPP are very different from Ethernet, Token Ring, and other LAN protocols. 1. They are end-to-end protocols, meaning they connect two systems only, using a dedicated connection, such as a telephone line. 2. Because there is no shared medium, there is no contention, no collisions, and no need for addresses or a Media Access Control (MAC) mechanism. C. SLIP and PPP do not include physical layer specifications. |20| 2. SLIP A. SLIP is the acronym for Serial Line Internet Protocol. B. Simple protocol used to transmit signals over a serial connection, such as a modem and telephone line C. Very low control overhead 1. SLIP adds one byte to each data packet, while Ethernet adds 18 bytes. D. Provides no special functions, such as error detection, network layer protocol identification, or security E. Transmits an IP datagram received from the network layer and follows it with a single framing byte called an End Delimiter 1. The End Delimiter informs the receiving system when it has finished receiving the data portion of the packet. 2. Some SLIP implementations use two End Delimiter fields to exclude any line noise between packets from the frame. F. SLIP is rarely used today because PPP has replaced it. |22| 3. PPP A. PPP is the acronym for Point-to-Point Protocol. 8 Outline, Chapter 12 Network+ Certification, Second Edition B. Used for dial-up Internet connections and many other WAN technologies C. More complex than SLIP D. Provides additional services that SLIP lacks, such as 1. IP address exchange 2. Multiplexing of network layer protocols 3. Support for authentication protocols E. Uses a 5-byte header |23| F. The PPP frame 1. Flag (1 byte). Indicates the transmission of a packet is about to begin 2. Address (1 byte). Contains a value indicating that the packet is addressed to all recipients 3. Control (1 byte). Contains a code indicating that the frame contains an unnumbered information packet 4. Protocol (2 bytes). Identifies the protocol that generated the information found in the Data field 5. Data and Pad (up to 1500 bytes). Contains information generated by the protocol identified in the Protocol field, plus padding if necessary 6. Frame Check Sequence (2 or 4 bytes). Contains a checksum value that the receiving system will use for error detection 7. Flag (1 byte). Indicates that the transmission of the packet has been completed G. Establishing a PPP connection 1. PPP does not use the frame to provide most of its advanced functions; it uses an elaborate connection establishment procedure instead. 2. The connection establishment procedure eliminates the need to include redundant information (such as IP addresses) in every packet. |24| H. The PPP connection establishment procedure consists of seven phases. 1. Link dead. The two computers begin with no communication, until one of the two initiates a physical layer connection, such as running a program that causes the modem to dial. 2. Link establishment. Once the physical layer connection is established, one computer generates a PPP frame containing a Link Control Protocol (LCP) request message. a. The computers use the LCP to negotiate the parameters they will use during the rest of the PPP session. b. The message contains a list of options, such as the use of a specific authentication protocol, link quality protocol, header compression, network layer protocols, and so on. c. The receiving system can then acknowledge the use of these options or deny them and propose a list of its own. (1) Eventually, the two systems agree on a list of options they have in common. Outline, Chapter 12 9 Network+ Certification, Second Edition 3. Authentication. If the two systems have agreed to use a particular authentication protocol during the link establishment phase, they then exchange PPP frames containing messages specific to that protocol in the Data field. a. PPP computers commonly use the Password Authentication Protocol (PAP) or the Challenge Handshake Authentication Protocol (CHAP), but they also use other authentication protocols. 4. Link quality monitoring. If the two computers have negotiated the use of a link quality monitoring protocol during the link establishment phase, the exchange of messages for that protocol occurs here. 5. Network layer protocol configuration. For each of the network layer protocols that the computers have agreed to use, a separate exchange of Network Control Protocol (NCP) messages occurs at this point. 6. Link open. Once the NCP negotiations are complete, the PPP connection is fully established and the exchange of packets containing network layer application data can begin. 7. Link termination. When the two computers have finished communicating, they sever the PPP connection by exchanging LCP termination messages. a. The systems return to the link dead phase. I. Point-to-Point Protocol over Ethernet (PPPoE) 1. TCP/IP standard that defines a methodology for using a broadband device, such as a cable or DSL modem, to create a. Individual PPP connections between computers on an Ethernet LAN b. External services connected to the LAN 2. Broadband remote network access devices can easily support multiple computers. a. Ethernet is the most common protocol used to network the computers together and connect them to the broadband device. b. Shared Ethernet LANs do not enable each computer to access remote services by using individual parameters for functions such as access control and billing. 3. Blends the following advantages: a. Simplicity of connecting multiple computers to a remote network by using an Ethernet LAN and broadband technology b. Ability to establish a separate PPP connection between each computer and a given remote service c. All of the PPP components, such as LCP negotiation, authentication, and NCP configuration Chapter 12, Lesson 3 WAN Technologies |25| 1. WAN Characteristics A. Any computer accessing the Internet with a modem and a PSTN connection is using a wide area network (WAN). 10 Outline, Chapter 12 Network+ Certification, Second Edition B. The term “WAN” more commonly refers to connections between networks at different locations. C. WAN connections are typically point-to-point links that do not use a shared medium, as a LAN does. D. WANs can use PSTN, ISDN, or DSL connections. E. To connect LANs at remote locations, install a router at each site and connect them with a WAN link. F. Most WAN connections use PPP at the data-link layer. G. WAN links nearly always involve a third-party service provider, such as a telephone company, that charges a fee based on the distance spanned and the bandwidth used. 1. This is the primary reason that even “high-speed” WANs are always slower than the average LAN. |26| 2. Leased Lines A. A leased line is a permanent analog or digital telephone connection between two locations that provides a predetermined amount of bandwidth at all times. B. The most common type of leased line in the United States is called a T-1, which runs at 1.544 Mbps. 1. The European equivalent of a T-1 is called an E-1, which runs at 2.048 Mbps. 2. Many organizations use T-1s to connect their networks to the Internet or to connect remote networks. C. Leased line services are split into 64-Kbps channels. 1. A T-1 consists of 24 channels. 2. When you lease a T-1 line, you can use each channel as an individual 64-Kbps link or combine them into a single data pipe. 3. Leasing part of a T-1 (in 64-Kbps increments) is called fractional T-1 service. D. For data transmission purposes, a leased line is typically left as a single channel using all of the available bandwidth. 1. T-1s and other leased lines are also used for telephone communications, with a private branch exchange (PBX) or switchboard allotting each 64- Kbps channel for one voice telephone line. E. A T-3 connection runs at 44.736 Mbps and an E-3 runs at 34.368 Mbps. 1. A T-3 connection is the equivalent of 672 channels of 64 Kbps each, or 28 T-1s. 2. T-3s are typically used only by ISPs and other service providers that need huge amounts of bandwidth. |27| F. To install a leased line, you contract with a telephone service provider to furnish a link between two specific sites, running at a specific bandwidth. 1. Leased line prices are based on the amount of bandwidth and the distance between the sites. a. A T-1 can easily cost $1000–$2000 per month. Outline, Chapter 12 11 Network+ Certification, Second Edition 2. Each end of a leased line must be connected to a device called a channel service unit/data service unit (CSU/DSU). a. The CSU/DSU functions as the terminus for the link and provides testing and diagnostic capabilities. 3. To use the line, you connect the CSU/DSU to your network by using a. A router, in the case of a data network b. A PBX, in the case of a telephone network G. Leased line disadvantages 1. You pay for the full bandwidth of the line around the clock, even when you are not using it. 2. The bandwidth is capped at a particular rate. a. If your needs grow to exceed the capacity of the line, you must install another line. |28| 3. Frame Relay A. Provides bandwidth similar to that of a leased line, but with greater flexibility B. Speeds range from 56 Kbps to the equivalent of a T-3. C. Frame relay links are not locked into a specific speed. 1. In the contract, you and the provider agree to a base speed for the link, called the committed information rate (CIR). 2. The link can also support bursts of bandwidth at higher speeds than the CIR. a. Burst bandwidth is borrowed from other lines not operating at full capacity. 3. The contract includes a committed burst information rate (CBIR), which is the maximum amount of bandwidth provided during burst periods. a. If you exceed the contracted bandwidth, there are extra charges. D. A frame relay connection is not a permanent link, like a leased line. 1. Each site is connected to the service provider’s network, called a cloud. 2. The links from each site to the cloud are usually short-distance leased lines. 3. The two sites are linked by a dynamic connection through the cloud. E. Each site requires a hardware device called a frame relay assembler/disassembler (FRAD). 1. The FRAD strips off the data-link layer protocol header from each packet and repackages it for transmission through the cloud. F. Frame relay can use a single connection from a site to a cloud to replace multiple leased lines. 1. If you have multiple sites to connect, you can install one frame relay connection at each site instead of installing a separate leased line for each pair of sites. 12 Outline, Chapter 12 Network+ Certification, Second Edition 4. SONET/Synchronous Digital Hierarchy A. The Synchronous Optical Network (SONET) is a physical layer standard that defines a method for building a synchronous telecommunications network based on fiber optic cables. 1. First ratified by the American National Standards Institute (ANSI) 2. SONET was adapted by the ITU, which called it the Synchronous Digital Hierarchy (SDH). B. Intended as a replacement for the T-carrier and E-carrier services |29| C. Provides connections at various optical carrier (OC) levels running at different speeds D. The purpose of SONET is to create a standardized series of transmission rates and formats, eliminating the problems that currently affect connections between different types of carrier networks. |30| 5. ATM A. ATM is the acronym for Asynchronous Transfer Mode. B. Originally designed to carry voice, data, and video traffic both on LANs and WANs 1. Today, ATM is sometimes used for network backbones, but it is more commonly found in WAN connections. C. Uses fixed-length, 53-byte frames called cells D. Provides a connection-oriented, full-duplex, point-to-point service between devices E. Uses no broadcast transmissions 1. Data is relayed between networks by switches, not routers. 2. Speeds range from a 25.6-Mbps service, intended for desktop LAN connections, to 2.46 Gbps. F. Physical media include 1. Standard multimode fiber optic and unshielded twisted-pair (UTP) cables on LANs 2. SONET or T-carrier services for WAN connections G. On an all-ATM internetwork, cells originating at a workstation can travel all the way to a destination at another site through switches without having to be reencapsulated in a different data-link layer protocol. H. ATM never gained popularity on the desktop, and has largely been relegated to use on WANs. 6. FDDI A. FDDI is the acronym for Fiber Distributed Data Interface. B. Essentially a LAN protocol, but sometimes grouped with WAN technologies 1. FDDI cannot provide truly long distance links, as leased lines and other WAN technologies can. 2. FDDI can connect LANs located in nearby buildings, forming a campus internetwork. Outline, Chapter 12 13 Network+ Certification, Second Edition |31| Chapter Summary A. Using remote connections 1. Stand-alone computers become remote clients when they connect to a distant network by using any one of several different technologies. 2. PSTN connections use modems and standard telephone lines to transmit data. They are relatively slow, but also flexible and universal. 3. VPNs are secure tunnels through the Internet that enable remote computers to communicate with their networks without using long- distance telephone connections. B. SLIP and PPP 1. SLIP is a simple protocol that enables two systems connected through their serial ports to exchange messages with very little control overhead. 2. PPP is a more complicated end-to-end protocol that enables two systems to negotiate the use of optional features such as authentication protocols and multiple network layer protocols. |32| C. WAN technologies 1. Leased lines are dedicated, permanent, point-to-point connections between two sites that are provided by telephone carriers. 2. Frame relay is a service that uses a cloud of virtual circuits to provide flexible amounts of bandwidth between sites. 3. SONET is a fiber optic telecommunications network standard consisting of a series of optical carrier levels. 4. ATM is a switched, connection-oriented service that was designed for use on both LANs and WANs.