Chapter 6: Topologies and Access Methods

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Chapter 6 Topologies and Access Methods
Network Guide to Networks Third Edition
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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

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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

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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

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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

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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

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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

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Hybrid Physical Topologies

• Star-Wired Ring
• The star-wired ring topology uses the physical
  layout of a star in conjunction with the ring
  topologys 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

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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

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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

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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

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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

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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

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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

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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

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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 backbone6
• 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

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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

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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

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Switching

• A component of a networks logical topology that
  determines how connections are created between
  nodes
• There are three methods for switching circuit
  switching, message switching, and packet switching

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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

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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

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Switching (continued)

• This store and forward routine continues until
  the message reaches its destination
• Message switching requires that each device in
  the datas path have sufficient memory and
  processing power to accept and store the
  information before passing it to the next node

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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

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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

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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

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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

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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

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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

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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
  devices 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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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)

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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 IEEEs
  802.11 committee

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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

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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

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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

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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

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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

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Wireless Networks (continued)

• 802.11a
• 802.11as 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

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Wireless Networks (continued)

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

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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)
• Bluetooths low throughput and short range makes
  it impractical for business LANs.

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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

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Summary

• Basic and hybrid LAN physical topologies, and
  their uses, advantages, and disadvantages
• Describe the backbone structures
• Compared the different types of switching used in
  data transmission

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Summary (continued)

• Transmission methods underlying Ethernet,
  LocalTalk, Token Ring, FDDI, and ATM networks
• Characteristics of different wireless network
  technologies, including the three IEEE 802.11
  standards