Chapter Seven WANs and Remote Connectivity
Objectives Identify network applications that require WAN technology Describe a variety of WAN transmission and connection methods Identify criteria for selecting an appropriate WAN topology, transmission method, and operating system Understand the hardware and software requirements for connecting to a network via modem Install and configure simple remote connectivity for a telecommunicating client
WAN Essentials WAN link n n Connection between one WAN site and another site A WAN link is typically described as point-topoint Dedicated line n Continuously available link that is leased through another carrier
WAN Essentials Figure 7 -1: Differences in LAN and WAN connectivity
PSTN Public Switched Telephone Network Refers to the network of typical telephone lines and carrier equipment that service most homes Also called plain old telephone service (POTS)
PSTN A dial-up connection uses a PSTN or other line to access remote servers via modems at both the source and destination The Federal Communications Commission (FCC) sets standards and policy for telecommunications transmission equipment in the United States The place where two telephone systems meet is the point of presence (POP)
PSTN Figure 7 -2: A typical PSTN connection to the Internet
X. 25 and Frame Relay X. 25 n Analog, packet-switched LAN technology optimized for longdistance data transmission Frame Relay n Updated, digital version of X. 25 that also relies on packet switching Figure 7 -3: A WAN using frame relay
X. 25 and Frame Relay SVCs (switched virtual circuits) n Connections established when parties need to transmit, then dismantled once the transmission is complete PVCs (private virtual circuits) n Connections established before data needs to be transmitted and maintained after transmission is complete CIR (committed information rate) n Guaranteed minimum amount of bandwidth selected when leasing a frame relay circuit
ISDN (Integrated Services Digital Network) International standard for transmitting data over digital lines Established by the ITU All ISDN connections are based on two types of channels: n The B channel is the “bearer” channel n The D channel is the “data” channel
BRI (Basic Rate Interface) A variety of ISDN using two 64 -Kbps bearer (B) channels and one 16 -Kbps data (D) channel, as indicated by the following notation: n 2 B+D Through bonding, the two 64 -Kbps channels can be combined to achieve an effective throughput of 128 -Kbps
BRI (Basic Rate Interface) The Network Termination 1 (NT 1) device connects twisted-pair wiring at customer’s building with ISDN terminal equipment (TE) via RJ-11 or RJ-45 data jacks A terminal adapter (TA) converts digital signals into analog signals for use with ISDN phones and other analog devices Figure 7 -4: A BRI link
PRI (Primary Rate Interface) A variety of ISDN using 23 B channels and one 64 -Kbps D channel, as represented by the following notation: n 23 B+D PRI links use same kind of equipment as BRI links, but require the services of an extra network termination device—called a Network Termination 2 (NT 2)—to handle multiple ISDN lines
PRI (Primary Rate Interface) Figure 7 -5: A PRI link It is only feasible to use ISDN for the local loop portion of a WAN link
T-Carriers Broadband n Group of network connection types or transmission technologies generally capable of exceeding 1. 544 Mbps throughput T-carriers n Term for any kind of leased line that follows the standards for T 1 s, fractional T 1 s, T 1 Cs, T 2 s, T 3 s, or T 4 s
Types of T-Carriers The most common T-carrier implementations are T 1 and T 3 Signal level n ANSI standard for T-carrier technology that refers to its Physical layer electrical signaling characteristics DSO (digital signal, level 0) n Equivalent of one data or voice channel Fractional T 1 n Arrangement allowing an organization to use only some channels on a T 1 line, paying for what they use
Types of T-Carriers Figure 7 -1: Carrier specifications
T-Carrier Connectivity Wiring n Can use unshielded or shielded twisted-pair copper wiring CSU/DSU (Channel Service Unit/Data Service Unit) n n CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring DSU converts the digital signal used by bridges, routers, and multiplexers into the digital signal sent via the cabling Figure 7 -6: A CSU/DSU connecting a T 1
T-Carrier Connectivity Multiplexer n Device that combines multiple voice or data channels on one line Figure 7 -7: Typical use of a multiplexer on a T 1 -connected data network
T-Carrier Connectivity Routers and bridges n On a typical T 1 -connected data network, terminal equipment will consist of bridges, routers or a combination of the two Figure 7 -8: A router on a T 1 -connected network
DSL Digital Subscriber Lines Uses advanced data modulation techniques to achieve extraordinary throughput over regular phone lines Like ISDN, DSL can span only limited distances without the help of repeaters
Types of DSL Term x. DSL refers to all DSL varieties, of which at least eight currently exist DSL types can be divided into two categories: n n Asymmetrical Symmetrical To understand the difference between these two categories, you must understand the concept of downstream and upstream data transmission
Types of DSL Table 7 -2: Comparison of DSL types
DSL Connectivity DSL connectivity, like ISDN, depends on the PSTN Inside carrier’s POP, a device called a DSL access multiplexer (DSLAM) aggregates multiple DSL subscriber lines and connects them to a larger carrier or to the Internet backbone Figure 7 -9: A DSL connection
DSL Connectivity Once inside the customer’s home or office, the DSL line must pass through a DSL modem Figure 7 -10: A DSL modem
Cable connections require that the customer use a special cable modem, a device that modulates and demodulates signals for transmission and reception via cable wiring Figure 7 -11: A cable modem
Cable Hybrid fiber-coax (HFC) n n Very expensive fiber-optic link that can support high frequencies HFC upgrades to existing cable wiring are required before current TV cable systems can serve as WAN links Cable drop n Fiber-optic or coaxial cable connecting a neighborhood cable node to a customer’s house Head-end n Cable company’s central office, which connects cable wiring to many nodes before it reaches customers’ sites
Cable Figure 7 -12: Cable infrastructure
SONET (Synchronous Optical Network) Can provide data transfer rates from 64 Kbps to 39. 8 Gbps using the same TDM technique used by T-carriers Known internationally as SDH (Synchronous Digital Hierarchy) SONET is self- healing Figure 7 -13: SONET technology on a long-distance WAN
SONET (Synchronous Optical Network) Table 7 -3: SONET OC levels
WAN Implementation: Speed Table 7 -4 a: A comparison of WAN technology transmission speeds
WAN Implementation: Speed Table 7 -4 b: A comparison of WAN technology transmission speeds
WAN Implementation: Reliability WAN implementations can roughly be divided as follows: n Not very reliable, suited to individual or unimportant transmissions: PSTN dial-up n Sufficiently reliable, suited for day-to-day transmissions: ISDN, T 1, fractional T 1, T 3, DSL, cable, X. 25, and frame relay n Very reliable, suited to mission-critical applications: SONET
WAN Implementation: Security Among other things, consider the following issues: n n WAN security depends in part on the encryption measures each carrier provides for its lines Enforce password-based authorization for LAN and WAN access and teach users how to choose difficult-to-decrypt passwords Take the time to develop, publish, and enforce a security policy for users in your organization Maintain restricted access to network equipment rooms and data centers
WAN Implementation: Virtual Private Networks (VPNs) VPNs are wide area networks logically defined over public transmission systems that serve an organization’s users, but isolate that organization’s traffic from other users on the same public lines Figure 7 -14: An example of a VPN
Remote Connectivity Remote access methods: n Direct dial to the LAN The computer dialing into the LAN becomes a remote node on the network n Direct dial to a workstation Software running on both remote user’s computer and LAN computer allows remote user to “take over” the LAN workstation, a solution known as remote control n Internet/Web interface Through a browser, a user at home or on the road connects to a LAN whose files are made visible to the Web through Web server software
Remote Connectivity ICA (Independent Computing Architecture) client n n Remote access client developed by Citrix Systems, Inc. Enables remote users to use virtually any LAN application over any type of connection, public or private Remote Access Service (RAS) n n One of the simplest dial-in servers This software is included with Windows 2000 Server
Dial-Up Networking Refers to the process of dialing into a LAN’s (private) access server or to an ISP’s (public) access server to log onto a network Figure 7 -15: Choosing a network connection type
SLIP and PPP Serial Line Internet Protocol (SLIP) n Communications protocol enabling a workstation to connect to a server using a serial connection Can carry only IP packets n Supports only asynchronous transmission n Point-to-Point Protocol n n n Communications protocol enabling a workstation to connect to a server using a serial connection Can carry many different types of Network layer packets Supports both asynchronous and synchronous transmission
Chapter Summary WANs are distinguished from LANs by the fact that the former networks traverse a wider geographical area One WAN transmission method, PSTN, relies on the network of telephone lines that typically service homes X. 25 is an analog packet-switched technology optimized for long-distance data transmission Frame Relay is an updated, digital version of X. 25
Chapter Summary Another WAN transmission method, ISDN, is an international standard established by the ITU for transmitting data over digital lines Two types of ISDN connections are commonly used in North America are BRI and PRI Another WAN transmission method is digital subscriber line (DSL) Cable is another option for high bandwidth local loop WAN transmission
Chapter Summary T-carrier technology uses time division multiplexing (TDM) to divide a single channel into multiple channels for carrying voice, data, video, or other signals SONET can provide data transfer rates from 64 Kbps to 39. 8 Gbps using the same TDM technique employed by T-carriers When installing or upgrading a WAN, consider its ability to integrate with your present LAN or WAN equipment, transmission speed required, security needed, geographical distance the WAN must span, growth, and cost VPNs represent one way to construct a WAN from existing public transmission systems
Chapter Summary Three ways remote users connect to LANs: n n n Direct dial to the LAN Direct dial to a workstation An Internet connection with a Web interface SLIP and PPP are communications protocols enabling a workstation to connect to a server using a serial connection
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