Chapter 6: Topologies and Access Methods

1. Chapter 6: Topologies and Access Methods

2. Objectives • Describe the basic and hybrid LAN physical topologies, and their uses, advantages, and disadvantages • Describe the backbone structures that form the foundation for most LANs • Compare the different types of switching used in data transmission

3. Objectives (continued) • Understand the transmission methods underlying Ethernet, LocalTalk, Token Ring, FDDI, and ATM networks • Describe the characteristics of different wireless network technologies, including the three IEEE 802.11 standards

4. Simple Physical Topologies • Physical topology is the physical layout, or pattern, of the nodes on a network • Physical topologies are divided into three fundamental geometric shapes: bus, ring, and star

5. Simple Physical Topologies (continued) • Bus • A bus topology consists of a single cable connecting all nodes on a network without intervening connectivity devices • The single cable is called the bus and can support only one channel for communication • Most bus networks use coaxial cable as their physical medium • At the ends of each bus network are 50-ohm resistors known as terminators

6. Simple Physical Topologies (continued) • Ring • In a ring topology, each node is connected to the two nearest nodes so that the entire network forms a circle • Data is transmitted clockwise, in one direction (unidirectional), around the ring • The fact that all workstations participate in delivery makes the ring topology an active topology • A ring topology also differs in that it has no “ends” and data stops at its destination and, twisted-pair or fiber-optic cabling is used as the physical medium22

7. Simple Physical Topologies (continued) • In a star topology, every node on the network is connected through a central device, such as a hub or switch • Star topologies are usually built with twisted-pair or fiber-optic cabling • Star topologies require more cabling than ring or bus networks • Each node is separately connected to a central connectivity device, they are more fault-tolerant

8. Hybrid Physical Topologies • Star-Wired Ring • The star-wired ring topology uses the physical layout of a star in conjunction with the ring topology’s data transmission method • Data is sent around the star in a circular pattern • This hybrid topology benefits from the fault tolerance of the star topology

9. Hybrid Physical Topologies (continued) • In a star-wired bus topology, groups of workstations are star-connected to hubs and then networked via a single bus • With this design, you can cover longer distances and easily interconnect or isolate different network segments

10. Hybrid Physical Topologies (continued) • More expensive than using either the star or, especially, the bus topology alone because it requires more cabling and potentially more connectivity devices • The star-wired bus topology forms the basis for modern Ethernet and Fast Ethernet networks

11. Backbone Networks • A network backbone is the cabling that connects the hubs, switches, and routers on a network • Backbones usually are capable of more throughput than the cabling that connects workstations to hubs

12. Backbone Networks (continued) • In networking, the term enterprise refers to an entire organization, including its local and remote offices, a mixture of computer systems, and a number of departments • The backbone is the most significant building block of enterprise-wide networks

13. Backbone Networks (continued) • Serial Backbone • The simplest kind of backbone • It consists of two or more internetworking devices connected to each other by a single cable in a daisy-chain fashion • In networking, a daisy chain is simply a linked series of devices • Hubs and switches are often connected in a daisy chain to extend a network

14. Backbone Networks (continued) • Distributed Backbone • Consists of a number of connectivity devices connected to a series of central connectivity devices such as hubs, switches, or routers, in a hierarchy • This kind of topology allows for simple expansion and limited capital outlay for growth, because more layers of devices can be added to existing layers

15. Backbone Networks (continued) • A more complicated distributed backbone connects multiple LANs or LAN segments using routers • Provides network administrators with the ability to segregate workgroups and therefore manage them more easily

16. Backbone Networks (continued) • Collapsed Backbone • Uses a router or switch as the single central connection point for multiple subnetworks • A single router or switch is the highest layer of the backbone • The router or switch that makes up the collapsed backbone must contain multiprocessors to handle the heavy traffic going through it • This arrangement allows you to interconnect different types of subnetworks

17. Backbone Networks (continued) • Parallel Backbone • The most robust type of network backbone • The most significant advantage of using a parallel backbone is that its redundant (duplicate) links ensure network connectivity to any area of the enterprise • Parallel backbones are more expensive than other enterprise-wide topologies • They make up for the additional cost by offering increased performance and better fault tolerance

18. Logical Topologies • Logical topology refers to the way in which data is transmitted between nodes • The most common logical topologies are bus and ring • In a bus logical topology, signals travel from one network device to all other devices on the network • In a ring logical topology signals follow a circular path between sender and receiver • Logical topologies is useful when troubleshooting and designing networks

19. Switching • A component of a network’s logical topology that determines how connections are created between nodes • There are three methods for switching: circuit switching, message switching, and packet switching

20. Switching (continued) • Circuit Switching • A connection is established between two network nodes before they begin transmitting data • Bandwidth is dedicated to this connection and remains available until the users terminate communication between the two nodes • While the nodes remain connected, all data follows the same path initially selected by the switch

21. Switching (continued) • Message Switching • Establishes a connection between two devices, transfers the information to the second device, and then breaks the connection • The information is stored and forwarded from the second device once a connection between that device and a third device on the path is established

22. Switching (continued) • This “store and forward” routine continues until the message reaches its destination • Message switching requires that each device in the data’s path have sufficient memory and processing power to accept and store the information before passing it to the next node

23. Switching (continued) • Packet Switching is the most popular method for connecting nodes on a network • Breaks data into packets before they are transported • Packets can travel any path on the network to their destination • When packets reach their destination node, the node reassembles them based on their control information • Does not waste bandwidth by holding a connection open until a message reaches its destination

24. Ethernet • Carrier Sense Multiple Access with Collision Detection (CSMA/CD) • The access method used in Ethernet • The term “Carrier Sense” refers to the fact that Ethernet NICs listen on the network and wait until they detect (or sense) that no other nodes are transmitting data over the signal (or carrier) on the communications channel before they begin to transmit

25. Ethernet (continued) • The term “Multiple Access” refers to the fact that several Ethernet nodes can be connected to a network and can monitor traffic, or access the media, simultaneously • The last part of the term CSMA/CD, “collision detection,” refers to the way nodes respond to a collision • When two transmissions interfere with each other; this is known as a collision

26. Ethernet (continued) • The NIC will issue a special 32-bit sequence that indicates to the rest of the network nodes that the its previous transmission was faulty and that those data frames are invalid which is called jamming • A collision domain is the portion of a network in which collisions occur if two nodes transmit data at the same time • A data propagation delay is the length of time data takes to travel from one point on the segment to another point

27. Ethernet (continued) • Switched Ethernet • Traditional Ethernet LANs, called shared Ethernet, supply a fixed amount of bandwidth that must be shared by all devices on a segment, and all nodes on that segment belong to the same collision domain • Switched Ethernet enables multiple nodes to simultaneously transmit and receive data over different logical network segments • Using switched Ethernet increases the effective bandwidth of a network segment because fewer workstations must vie for the same time on the wire

28. Ethernet (continued) • Ethernet Frames • Ethernet networks may use one (or a combination) of four kinds of data frames: Ethernet_802.2 (“Raw”), Ethernet_802.3 (“Novell proprietary”), Ethernet_II (“DIX”), and Ethernet_SNAP • Each frame type differs slightly in the way it codes and decodes packets of data traveling from one device to another

29. Ethernet (continued) • Using and Configuring Frames • You can use multiple frame types on a network, but you cannot expect interoperability between the frame types • Frame types are typically specified through a device’s NIC configuration software • Most NICs can automatically sense what types of frames are running on a network and adjust themselves to that specification which is a feature is called autodetect, or autosense

30. Ethernet (continued) • The preamble signals to the receiving node that data is incoming and indicates when the data flow is about to begin • The start-of-frame delimiter (SFD) identifies where the data field begins

31. Ethernet (continued) • Each Ethernet frame also contains a 14-byte header, which includes a destination address, a source address, and an additional field that varies in function and size, depending on the frame type • The extra bytes are known as padding and have no significance other than to fill out the frame • Ethernet_II (“DIX”) and Ethernet_SNAP • An Ethernet frame type developed by DEC, Intel, and Xerox (abbreviated as DIX) before the IEEE began to standardize Ethernet

32. Ethernet (continued) • Ethernet_II frame type contains a 2-byte type field. This type field identifies the Network layer protocol (such as IP,ARP, RARP, or IPX) contained in the frame • The Ethernet_SNAP standard calls for additional control fields • Ethernet_SNAP frames allow less room for data

33. Ethernet (continued) • Power over Ethernet • Recently, IEEE has finalized a new standard, 802.3af, that specifies a method for supplying electrical power over Ethernet connections, also known as Power over Ethernet (PoE) • The PoE standard specifies two types of devices: power sourcing equipment (PSE) and powered devices (PDs) • Power sourcing equipment (PSE) • Powered devices (PDs)

34. LocalTalk • LocalTalk is a network access method designed by Apple Computer, Inc. specifically for networking Macintosh computers • It provided a simple, cost-effective way of interconnecting Macintosh devices • LocalTalk uses a transmission method called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • LocalTalk relies on the AppleTalk protocol, but it may also support the Macintosh version of TCP/IP called MacTCP

35. Token Ring • A network technology first developed by IBM in the 1980s • Token Ring networks have traditionally been more expensive to implement than Ethernet networks • The 100-Mbps Token Ring standard, finalized in 1999, is known as High-Speed Token Ring (HSTR) • In token passing, a 3-byte packet, called a token, is transmitted from one node to another in a circular fashion around the ring • The active monitor maintains the timing for ring passing

36. Token Ring (continued) • Token Ring Switching • Token Ring networks can take advantage of switching to better utilize limited bandwidth • A Token Ring switch can subdivide a large network ring into several smaller network rings • Token Ring technology does not allow collisions

37. Token Ring (continued) • Token Ring Frames • Token Ring networks may use one of two types of frames: the IEEE 802.5 or the IBM Token Ring frame • Every Token Ring frame includes Start Delimiter (SD), Access Control (AC), and End Delimiter (ED) fields • Token Ring frames use a Frame Status (FS) to provide low-level acknowledgment that the frame was received whole

38. Fiber Distributed Data Interface (FDDI) • A network technology whose standard was originally specified by ANSI in the mid-1980s and later refined by ISO • FDDI (pronounced “fiddy”) uses a double ring of multimode or single mode fiber to transmit data at speeds of 100 Mbps • FDDI is more reliable and more secure than transmission methods that depend on copper wiring • FDDI works well with Ethernet 100BaseTX technology • FDDI technology has a high cost relative to Fast Ethernet

39. Asynchronous Transfer Mode (ATM) • An ITU networking standard describing Data Link layer protocols for both network access and signal multiplexing • ATM may run over fiber-optic or CAT 5 or higher UTP or STP cable • In ATM, a packet is called a cell and always consists of 48 bytes of data plus a 5-byte header • ATM technology is that it relies on virtual circuits • ATM a connection-oriented technology using virtual circuits

40. ATM (continued) • Establishing a reliable connection allows ATM to guarantee a specific Quality of Service (QoS) for certain transmissions • QoS is a standard that specifies that data will be delivered within a certain period of time after it is sent • ATM networks can be integrated with Ethernet or Token Ring networks through the use of LAN Emulation (LANE)

41. Wireless Networks • Each wireless technology is defined by a standard that describes unique functions at both the Physical and the Data Link layers of the OSI Model • These standards differ in their specified signaling methods, geographic ranges, and frequency usages, among other things. • The most popular wireless standards used on contemporary LANs are those developed by IEEE’s 802.11 committee

42. Wireless Networks (continued) • 802.11 Another name for Wireless Local Area Networks (WLAN) standards committee • Access Method • 802.11 standards specify the use of Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to access a shared medium • Use of ACK packets to verify every transmission • RTS/CTS enables a source node to issue an RTS signal to an access point requesting the exclusive opportunity to transmit

43. Wireless Networks (continued) • Association • In the context of wireless networking, communication that occurs between a station and an access point to enable the station to connect to the network via that access point • As long as a station is on and has its wireless protocols running, it periodically surveys its surroundings for evidence of an access point, a task known as scanning

44. Wireless Networks (continued) • There are two types of scanning: active and passive • In active scanning, the station transmits a special frame, known as a probe, on all available channels within its frequency range • In passive scanning, a wireless station listens on all channels within its frequency range for a special signal, known as a beacon frame, issued from an access point • Service Set Identifier (SSID), a unique character string used to identify an access point • A station might choose a different access point through a process called re-association

45. Wireless Networks (continued) • Frames • For each function, the 802.11 standard specifies a frame type at the MAC sublayer • These multiple frame types are divided into three groups: management, control and data • Management frames are those involved in association and re-association, such as the probe and beacon frames • Control frames are those related to medium access and data delivery, such as the ACK and RTS/CTS frames • Data frames are those that carry the data sent between stations

46. Wireless Networks (continued) • 802.11b • Also known as “Wi-Fi,” for Wireless Fidelity • Uses direct sequence spread spectrum (DSSS) signaling • 802.11b was the first to take hold and remains the most popular • It is also the least expensive of all the 802.11 WLAN technologies

47. Wireless Networks (continued) • 802.11a • 802.11a’s high throughput is attributable to its use of higher frequencies, its unique method of encoding data, and more available bandwidth • Higher frequency signals require more power to transmit and travel shorter distances than lower frequency signals • The additional access points, as well as the nature of 802.11a equipment, make this standard more expensive than either 802.11b or 802.11g

48. Wireless Networks (continued) • 802.11g • 802.11g benefits from being compatible with 802.11b networks • 802.11g has high throughput • 802.11g’s compatibility with the more established 802.11b has caused many network managers to choose it over 802.11a, despite 802.11a’s comparative advantages • Laptops could roam between the ranges of the 802.11b and 802.11g access points without an interruption in service

49. Wireless Networks (continued) • Bluetooth • Bluetooth is a mobile wireless networking standard that uses DSSS signaling in the 2.4-GHz band to achieve a maximum theoretical throughput of 1 Mbps • Bluetooth was designed to be used on small networks composed of personal communications devices, also known as personal area networks (PANs) • Bluetooth’s low throughput and short range makes it impractical for business LANs.

50. Wireless Networks (continued) • HomeRF • HomeRF is a wireless networking specification developed by the HomeRF Working Group • The most unique aspect of the HomeRF standard is that it was designed to allow both traditional telephone signals and data signals to be exchanged over the same wireless network • Its working group was disbanded in January 2003