FAQ: Network Topologies and Architecture

Transcription

1 Question 1: In a wide area network (WAN) environment, what topologies are used, and how are they structured? Answer 1: A local area network (LAN) structure is configured inside a campus or facility to provide local connectivity using bus, ring, star, mesh, or hierarchal topology. Similar layouts are used within a wide area network (WAN) topology. The difference is the technology used to connect them and the sites themselves. Inside each site, the typical topology is maintained; the site connections are based on the technology applied. The WAN bus can use leased or subscribed media to send data. When an organization has only a few sites, this topology usually is adequate. In city-to-city connection (pointto-point), T-carriers usually provide access from a local telephone exchange; for intercity connection (metropolitan area network or MAN), sites are supported by digital subscriber lines (DSL). Wide area network ring topology is similar to simple ring topology except that it connects sites in a physical ring instead of nodes in a logical ring. Again, it uses common carriers (telephone companies) to provide the distance connection (Davis & Rajkumar, 2004). For example, imagine the sites 1, 2, 3, and 4. Site 1 connects with dedicated T1 (Full) to Site 2; Site 2 connects to Site 3 with T-1 lines 3 to 4 with a DSL, and Site 4 to Site 1 with T1, creating a true ring. Modifying the ring would be difficult, and the cost involved would be high, so this WAN usually has a limited number of sites (normally not more than four or five) (Dean, 2006). 1

2 Wide area network star topology uses a central site as a distribution center (like a hub) connecting through dedicated lines to each subordinate site. This does create a single point of failure from the hubbed site stopping all communication within the WAN; however, if a subordinate site fails, the remaining site will still maintain connectivity. Extending the WAN star is relatively easy. It simply connects to the central site with the dedicated connection. This is why it is used by most organizations in a WAN environment (Dean, 2006). Wide area network mesh topology is just like the LAN mesh topology. All sites are directly connected through dedicated lines to all other sites. These are the most fault-tolerant WAN layouts because they provide multiple paths to send and receive data. If a route goes down, data can bypass that route and still reach their destination. There is a configuration within the mesh topology that permits separate branch connections from a primary site to a connected site 2

3 that is not in direct connection with all sites. This configuration is called a partial mesh WAN (Dean, 2006). Question 2: What is meant by a high-speed Ethernet network? Answer 2: A high-speed network usually refers to a network where the throughput exceeds 100 Mbps and the network connects to outside communication system configuring for a MAN or WAN environment (Dean, 2006). The standard throughput in local area networks (LANs) is between 10 Mbps and 100 Mbps; although, in some cases, depending on the topology and media, higher speeds are achieved. High-speed Ethernet, IEEE standard 802.3z, achieves transfer in the gigabit range. There are several types of gigabit Ethernet standards used today in networking (Dean, 2006): The standard 1000BASE-T can use twisted-pair CAT 5 or higher media with a range of 100 meters. The standard 1000BASE-CX, the standard uses twinaxial (dual core coaxial) with a range of 25 meters. The standard 1000BASE-LX relies on multimode fiber-optic cable (MMF) with a range of up to 550 meters; if single-mode fiber-optic (SMF) cable is used, the range is 5000 meters. 3

4 The standard 1000BASE-SX uses MMF and has a range of up to 500 meters (Dean, 2006). Additionally, there is media that supports 10 Gbps throughput: The standard 10GBASE-SR uses MMF and has a range of 300 meters. The standard 10GBASE-LR uses SMF and has a range of 10,000 meters. The standard 10GBASE-ER uses SMF and has a range of 40,000 meters. In all cases, the topology is star configuration (Dean, 2006). By definition, the term high-speed is based on network throughput. Extending the definition to systems outside the LAN (MAN/WAN), the standards available are 1000BASE-LX with SMF, 10GBASE-LR, and 10GBASE-ER. The maximum is only about 25 miles. The use of fiber-optic repeaters (concentrators) can extend the range creating a solely corporate owned high-speed network. These fiber-optic systems are expensive and require additional connectivity components. More often than not, corporations use leased services to extend their network using the telecommunication providers to outside the MAN configuration (Dean, 2006). Question 3: What is the T-carrier service, and how is it used? Answer 3: T-carrier is a leased WAN transmission method that uses digital transmission over high-speed public switched telephone network (PTSN). This service is 4

5 usually contracted through a network service provider (NSP). This high-speed network does not have the same throughput as the high-speed Ethernet fiber-optic network. The T-carrier method uses time division multiplexing (TDM), a system of streaming synchronized compressed data to provide a maximum of 64 Kbps over each channel. T1 contains 24 channels providing a throughput of Mbps. T1 can be dedicated to the full throughput or provide individual channels, called fractionalt1, to the users (Davis & Rajkumar, 2004). Each channel is equivalent to one voice circuit through a normal phone line (Dean, 2006). T1 can provide data packet switching and circuit switching as a transmission format. Similar to the T-carrier, Europe uses an E-carrier system. Europe's E1 provides 30 channels instead of 24 like in the United States. E1 has Mbps throughput. For the purpose of this discussion, the T-series will be addressed. T-carrier common designations are T1 and fractionalt1, both having total throughput of Mbps, and T3, which equates to 672 voice channels 45 Mbps (28 T-1s). T3 also has a fractional designation (Dean, 2006). With the T-carrier, full-dedicated channel allocation can approach high-speed Ethernet levels over wire cable system, but to meet true high-speed networking, users need to access SONET systems, a fiber-optic leased system (Dean, 2006). The following table displays the carrier specifications (Dean, 2006): FractionalT1 service provides a specified number of dedicated channels to the leaser. The contract provides the services based on actual use. It might be just one channel at 64 Kbps or bundled channels each providing 64 Kbps. For example, if a user needed 500 Kbps throughput available, then the lease 5

6 would be for 8 channels (7.8 channels required). If a user only needed access to T1 at night, then the lease would specify that as well. One pays for what one uses. Question 4: What are the various types of DSL, and what are their capabilities? Answer 4: Digital subscriber line (DSL) services are a WAN connection method designed in Bell Labs. DSL has a unique frequency. It works in bandwidths above 3300 Hz. It is limited to the distance from the service provider (telephone company-telco). The range from the source provider is between 1,000 ft and 26,000 ft depending on the technology implemented. Downstream refers to traffic from the Telco switching facility to the subscriber's throughput, and upstream traffic from the subscriber to the Telco throughput is also dependent on the implementation. The technology that offers more throughputs in one direction over the other is described as asymmetrical; balanced throughput is deemed symmetrical. The following DSL categories are available (Dean, 2006): ADSL (asymmetrical DSL) G.Lite (a kind of ADSL) HDSL (High Bit-Rate DSL) SDSL (Symmetric or Single-Line DSL) VDSL (Very High Bit-Rate DSL) SHDSL (Single-Line High Bit-Rate DSL) The following table is a comparison of DSL types (Dean, 2006): 6

7 Digital subscriber line requires unique equipment to establish its connection. It should be noted that the further away from the switching source (Telco), the less throughput that can be achieved. Connection is established using a DSL modem that manages the modulation of the signal. Knowing the difference between upstream and downstream will explain why. When using a DSL modem connected to the Internet at ADSL (full rate) mode, one can receive data from the connection faster than sending data to it (Dean, 2006). The connection is asymmetrical. When connected to the switching facility (Telco), the transmitted signal is sent to a DSL access multiplexer (DSLAM), which combines multiple DSL lines and streams them into a larger carrier or even the Internet. Note that DSL provides faster throughput than the T- carrier, but the range is severely constrained; therefore, in business, companies use DSL to the Telco and T-carrier within the service network (Dean, 2006). All implementation of DSL is established in the OSI physical layer. Question 5: How does a network manage remote access to the corporate network? Answer 5: Remote users access the corporate network using authentication protocols usually through a dial-up interface, DSL (digital subscriber line), VPN (virtual private network), or PBX (private branch exchange, which is a private telephone system) supported by an application gateway or proxy server [COEG1]. The proxy server acts as a filter between the public network (WWW and Internet) and the corporate network. Passage through the system is usually encrypted using IPSec (Internet Protocol Security a Network layer security mechanism), PPTP (Point-to-Point Tunneling Protocol a TCP connection-oriented Transport layer protocol that provides internal validation and acknowledgment), or L2TP Transport protocol (Layer 2 Tunneling Protocol transmitted in UDP a connectionless protocol. Connectionless means that no acknowledgment is given from the receiver, also known as best effort (Dean, 2006). Access may be granted using the digital signature (a record comparison of one's user ID and password), hashed password, user certificate, or using the RADIUS server (Remote Authentication Dial-in User Service) and log-on messages that are compared to the security server's database (Palmer, 2005). Remote access using the tunneling protocols (PPTP and L2TP) are implemented through specified ports established for these protocol 1723 and 1701 respectively. Accesses through other ports are denied. Internal authentication is applied through call back, certificate, or 7

8 private key. CHAP (challenge handshake authentication protocol) may be used or password authentication protocol (PAP) verifying the user. Question 6: What packet-switching technologies were designed for use for long-distance data transmission? Answer 6: Two prominent technologies were designed for long-distance transmission: X.25, an analog signal format, and frame relay, a digital signal format. X.25 was established by ITU as a standard for packet switching across the Internet at a maximum rate of 64 Kbps based on the standard voice rate. Later, the technology was updated and provided Mbps throughput. X.25 standard specifies protocols used with the physical, data link, and network layers in the OSI model. This protocol set supported excellent flow control but lost efficiency with transmitting digital-based information like audio and video data. X.25 was a dominant packet switching WAN technology for a long time (Dean, 2006). Frame Relay supplanted X.25 protocols as the standard technology because of its digital format. Frame Relay works in the Data Link layer of the OSI model and supports multiple Transport and Network layer protocols (TCP/IP and IPX/SPX). In digital format data, errors are minimized. Though Frame Relay can identify errors, it lets the higher layers correct them. Errors are identified, and a corrected packet is then resent. Frame Relay supports more throughput than X.25, ranging from 64 Kbps to 45 Mbps depending on the bandwidth the customer chooses. An issue with Frame Relay is that depending on the traffic on the network (WAN), the throughput may be established at one rate (the CIR-committed information rate) and fall below the guaranteed level. Midnight data would flow faster than midday when more traffic is on the network (Dean, 2006). Question 7: How is wireless technology applied to WAN? Answer 7: Wireless is becoming a new implementation in WANs (Dean, 2006). IEEE standard (Wi-Fi) and (WiMax) are being applied all over the world. Users are granted access to this standard by subscription; corporations are implementing mobile wireless networking as remote utilization or wireless broadband. All the processes applied to normal remote access are still required for wireless access. Most wireless transmission format is DSSS 8

9 (Phase 2). The most current technology for wireless WAN is wireless broadband designed for high throughput and distances-data exchange (Dean, 2006). IEEE standard is the base guideline supporting this technology. Connectivity is available at airports and coffee shops around the world. Cyber cafes are in major business locations, airports, and many hotels. These are designated as hotspots. Most provide wireless access to the Internet and company Web services. An access point (AP) is connected to a server, then the local ISP or service provider. Authentication is required to access company intranets, but World Wide Web access is directly available at these hotspots. Internal network (corporate access) is not as common. Throughput varies depending on which standard is implemented and the available technology for its connection. AP to wire is much faster than AP to wireless network access provider (range 11 Mbps-54 Mbps). Hard-wired connection usually uses standard Fast Ethernet (100 Mbps). The latest standard being developed for wireless access is N, which may have throughput up to 500 Mbps, although the technology is still experimental. Even with Super G, the rate is maximized at 110 Mbps. Pure wireless networks also have a major issue with system security and secure data transfer. The supporting system that is used would require data to be encrypted before transfer (Dean, 2006). 9

10 Question 8: What is SONET, and how is it used? Answer 8: SONET technology refers to a timing-based optical fiber network configuration that provides high-speed data transfer over a long distance. SONET stands for synchronous optical network. Its implementation is similar to fiber distributed data interface (FDDI) by running dual optical fiber rings within a WAN ring. Another term for this technology is synchronous digital hierarchy (SDH). SONET is one of the best choices between international connections because it has a low fault/error rate with transfers. All connected sites must be synchronized to a system clock to ensure continuity of traffic transfers (Dean, 2006). Synchronization is established during the handshake (communication request) between sites by the sending node. SONET is a Telco service option available by lease and service agreement. It can be maintained as purely optical, crossover to T-carrier, or implemented with DSL. Access to the SONET connections and multiplexer can be achieved with any WAN topology. The following table identifies the available throughput using SONET/SDH technology (Dean, 2006): Because of its high cost, it seems that only large businesses can effectively implement this technology. It is more often used internationally or by an ISP guaranteeing QoS and reliability. It works well for streamed data types audio, video, and imaging (Dean, 2006). SONET is known as a self-healing network. Damage to the network causes an automatic reroute of the traffic, which makes it one of the most reliable networks around (Dean, 2006). 10

11 Question 9: What types of satellite orbits, targeting systems, and common satellite frequencies are used with a WAN? Answer 9: Though not as common as other technology, satellites accessing the Internet and corporate networks is an application of wireless connectivity. Recently, several satellite bandwidths have been offered from the government's control to commercial utilization. Microwaves directed at satellite receivers (transponders) have become a more viable solution to low-frequency shorthaul data transfers. Short-haul refers to line-of-sight terrestrial transmissions approximately miles. Satellite orbits are in three distinct distances and positions above the Earth. Satellites orbiting above the equator are in geosynchronous or geostationary earth orbit (GEO) at an altitude of 22,300 miles. This means that these satellites stay in the same location above the Earth. One of the main reasons for satellite link is data relay; GEO works to relay data in all directions to Earth's surface. Satellites that are in orbit between the equator and the upper latitudes (imaginary bands around Earth running parallel to the equator toward the poles) at an altitude between 10,350 and 10,390 kilometers are called medium/middle earth orbit (MEO). They provide coverage to a wide range of Earth's surface. Satellites in orbit toward the poles circle at an altitude of 700 1,400 km and cover a considerably smaller area of the Earth's surface. Data transfers to satellites require an upload from a ground station. Power from the ground base to the satellite is higher because the power to 11

12 retransmit from the satellite to Earth (downlink) has to be generated from on-board solar panels and batteries, but the power to uplink is from a commercial power generation. There are two targeting methods used to downlink data. Small aperture downlink is best correlated with point-to-point transfer. The receiving satellite stations usually have an array of dishes to receive downlinks tuned to certain frequencies. The frequencies are generated from the transponder (which is not the same frequency originally used to uplink). Data sent to a frequencytuned satellite dish (the dish is listening to a certain frequency) are connected in a line of sight logical circuit (the satellite sends data to a target receiver, the dish, which may only be a few feet across (diameter of the dish make it small aperture) [COEG2]. This is called small aperture targeting. When data are sent to a frequency tuned array, the military and government agencies usually needing a large downlink (amount of information) or large dish receivers (SETI- search for extra-terrestrial intelligence Institute), stream the information to terrestrial dishes simultaneously. Commercial use of the frequencies are leased from the government though the Federal Communications Commission (FCC). The first three bands are just slightly above microware channels ( GHZ). The upper frequency 12

13 bands were recently made available for use. The military channels are not much higher (60GHz-128GHz) (Dean, 2006). Question 10: Internet addressing works with domain designation. What is meant by domain, and how is it used? Answer 10: The Internet uses a number-base addressing system called Internet protocol (IP) addressing ( ). Additionally, it uses a hierarchal addressing designation that defines domains (a group of nodes that is managed as a single unit LAN, network cluster, Web server group.). All computers within an organization are identified as a grouping for example, OSNET or SOPRO own their computers. The computers are connected to the corporate network and managed by the system administrators that would identify those computers in the owner's domain-control. Domains can be managed locally or distributed across several LANs connected to a MAN or WAN. A university might have a single domain, but domains might have domains too. Consider the following: The University of Anywhere has student access and faculty access as well as and Web services access. Not everyone has permission to access all nodes. One would not necessarily want the students accessing the faculty work area where the exams are being developed or stored. These areas are segregated. Each would be a subdomain of the University of Anywhere's domain. As one can see, the hierarchal configuration exists. What about the university being connected to other university networks? Each university (campus) has its own domain. What are the universities connected to state networks, regional networks, or the Internet? Each layer provides a domain that controls a group of similar subdomains. A service that is available is the DNS (domain names service). DNS is a TCP/IP application that provides coordination to the network (domain) controllers. It is used to identify each node within the domain with a unique name and IP address. Each level has this ability as well. The local listing (database) names the node for a local name, and the domain name is added to the local name. Example: A node in a network is called mikesnode. If the node is part of the University of Anywhere network, the mikesnode belongs to UOA. The domain designation would be mikesnode.uoa. If the UoA network were part of a county network, Reed County, then an additional extension for that domain 13

14 would be added as well (mikesnode.uoa.reed). The concept continues if the Reed County domain was a domain in the state of California's network (mikesnode.uoa.reed.ca). Each layer adds a moniker telling the network the path needed to reach a specific node and no node can be exactly the same the node name must be unique. The domain identification tells the network application the way to an end user's computer. The same constructs support topic specified domains. Several commonly used domain identifiers have been traditionally used with the Internet. Most countries have a country domain. For example,.gb,.ru,.jp, and.us represent Great Britain, Russia, Japan, and the United States respectively; even Togo, Africa, uses.tg. Other domains used include the following:.mil for military,.gov for government,.com for U.S. commercial,.org for nonprofit organizational domains,.edu for U.S. educational, and.net for U.S. networks. Uses of the domain names are separated by a period (.) between domain levels. Domains are used by network control devices to map the layer and location of the node (Davis & Rajkumar, 2004). 14

15 References Davis, W. S., & Rajkumar, T. M. (2004). Operating systems: A systematic view (6th ed.). Boston, MA: Addison-Wesley. Dean, T. (2006). Network+ guide to networks (4th ed.). Boston, MA: Thomson. Palmer, M. (2004). Guide to operating system security. Boston, MA: Thomson. 15