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LAN topologies and access techniques ( with a focus on Ethernet)

LAN topologies and access techniques ( with a focus on Ethernet). BSAD 141 Dave Novak Chap 5 Network + Guide to Networks, Dean . Topics Covered. LAN Topologies Bus Star Ring Hierarchical Star Mesh Wireless. Topics Covered. Media access techniques (4) Ethernet – CSMA/CD

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LAN topologies and access techniques ( with a focus on Ethernet)

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  1. LAN topologies and access techniques (with a focus on Ethernet) BSAD 141 Dave Novak Chap 5 Network+ Guide to Networks, Dean

  2. Topics Covered • LAN Topologies • Bus • Star • Ring • Hierarchical Star • Mesh • Wireless

  3. Topics Covered • Media access techniques (4) • Ethernet – CSMA/CD • Priority access • Token passing • Wireless/Localtalk – CSMA/CA • Network backbone

  4. Network Topologies • Topology – physical / spatial arrangement or design used to connect devices to network medium • How is topology different from architecture?

  5. Network Topologies • Topology is typically related to the networking technology and the specific standards used • Cannot adopt every standard and technology to any topology • Can create separate LANs using different standards and topologies and connect them using bridges, switches, or routers

  6. Network Topologies • Three basic network topologies • 1) Bus • 2) Star • 3) Ring

  7. Bus • “Legacy” Ethernet standards support this topology • Each device is cabled directly to the device next to it • Early Ethernet used bus topology with coaxial cable: Thicknet and Thinnet

  8. Bus • Signal is sent along connecting media and all attached devices receive the signal • Signal travels in both directions along the bus • Ends of bus medium must be terminated • Devices must coordinate to ensure that only one device transmits a signal at a time

  9. Star • All devices attached to central point (hub or switch) • Separate cable from each device to hub

  10. Star • Most Ethernet LANs (as well as LANs using other access technologies and layer 2 protocols) employ some variation of star topology • Can use different types of cable including twisted pair and fiber • Functionally, star-wired Ethernet LAN with hub uses shared medium • Basic hub propagates signals it receives out all other ports

  11. Ring • In terms of signal transmission, ring is like bus – each device logically connected to the next • Two ends connected forming an endless loop

  12. Ring • The ring is a LOGICAL loop, not necessarily a physical ring layout • Ring topology LAN physically look like a star topology LAN • The “ring” is a logical function of the MAU to which all PCs are connected • Each device acts as a repeater • Signal regenerated and forwarded • Packets passed to devices one at a time (sequentially) • Packets flow in one direction

  13. Hierarchical Star / Star Bus • Variation of the base star • Once all ports on hub are used, new hub is added to the network • Hubs connected using standard cable - one end of cable plugged into the uplink port of one of the hubs

  14. Mesh • In terms of many LAN / WAN connections, more theoretical than actual solution • Not used in practice, but is a valid topology • Each device has dedicated connection to all other devices on network

  15. Mesh • Not practical for most wired LANs • Number of connections required: • Number of NICs required

  16. Mesh • May be used as a model for WAN wiring (internetworking) • Mesh internetwork has multiple paths between two destinations using redundant routers

  17. Wireless • Term “topology” also used to describe wireless communication patterns • 1) Ad hoc topology • 2) Infrastructure network

  18. Ad Hoc Topology • Group of wireless devices communicate directly with one another • Inside the communication range of the wireless technology • Free to communicate • Free to roam • Works for small number of devices in a small geographical setting

  19. Infrastructure Topology • Wireless devices communicate with wired network using wireless transceivers • Wireless devices communicate directly with transceivers (physical access points to the wired network) • Allows communication between both wired and wireless devices through transceivers • Typical in larger network with mix of wireless and wired devices

  20. Compare / Contrast Base Topologies

  21. Sharing medium on a LAN • LANs typically rely on shared media • Devices attached to network must coordinate use of the network • Coordination requires communication • Communication requires time • Time increases as distance between computers grows • Shared networks with long delay are inefficient • Spend more time coordinating actions than sending data

  22. Shared versus Dedicated? • Shared – media / channel capacity is used jointly by multiple users or applications as needed • Envision a toll road • Dedicated – media / channel capacity is used exclusively by a single user or application • Envision a toll road where users pay to reserve their own lane and no one else can use that lane while occupied

  23. Public versus Private? • Public – users pay fees to use a shared network • Often a “pay-as-you-go” approach • Parts of the physical infrastructure network are available to the general public for sharing • Private – users pay fees to obtain a dedicated portion of the network • Often a “flat fee” approach • Parts of the physical infrastructure network are dedicated just for the private user

  24. Point-to-Point and PSTN • In many cases we are talking about “classification” or a stratification of the same physical infrastructure network… • For example, AT&T infrastructure networks can provide users with both: • Dedicated / private service • Shared / public service

  25. Locality of Reference • LAN technology is inexpensive and widely available • Locality of reference principle: • 1) Communicate most frequently with computers nearby • 2) Communicate repeatedly and most frequently with the same subset of computers

  26. Access control techniques • Describe how devices share media • The process or method devices follow for sharing media • Data Link (2) Layer has 2 sublayers • 1) LLC • 2) MAC • Access control defined by MAC mechanism used

  27. Access control techniques • Access control defined by Data Link Layer protocols (Layer 2) • 1) CSMA/CD • 2) CSMA/CA • 3) Token passing • 4) Demand priority

  28. Ethernet (IEEE 802.3) • Most commonly used data link standard / protocols • Does NOT have a central control structure controlling when devices can transmit • Provides many Physical Layer (1) Specifications including (see Table 5.1 on page 121): • 100BaseTX – Cat 5 UTP, Star, 100 Mbps, 100 meters • 1000BaseSX (160 MHz) – 62.5/125 multimode fiber, Star, 1000 Mbps, 220 meters • 10Base2 – RG-58 coaxial, Bus, 10 Mbps, 185 meters

  29. Ethernet (IEEE 802.3) • Provides Data Link Layer (2) Specifications: • Encapsulates data received from Network Layer into Ethernet frame which specifies: • MAC address • Size and format of frame

  30. Ethernet (IEEE 802.3) • Carrier Sense Multiple Access (CSMA) • Uses activity on cable to determine status • Cable is free, no activity – used, activity • Device checks for activity before transmitting • If activity, the device waits • Checking for activity is called Carrier Sensing

  31. CSMA/CD • Sharing on legacy Ethernet (think about a bus topology for conceptual reasons) • One device has exclusive use of cable during transmission • After one is done, another can transmit

  32. CSMA/CD • Collision detection (CD) – most important phase of transmission • Electrical and fiber • Collisions do not harm hardware • Result in data being destroyed or corrupted • Devices detect collisions using collision detection • Once a collision is detected, any device currently transmitting stops and sends jamming signal • After collision occurs, devices wanting to transmit must wait for the wire to become idle again

  33. Binary exponential backoff • In the event of a collision how do devices using CSMA/CD know when to attempt to transmit again?

  34. CSMA/CD • Collisions are normal part of Ethernet operation • More devices and more activity result in more collisions • Collisions result in retransmission and delay • Ethernet does not perform well when heavily utilized • Install switch, bridge, router

  35. Wireless (IEEE 802.11) • Generally considered to be slower and less reliable than wired technologies • Provides Physical Layer (1) specifications: • 802.11 supports different modulation techniques at Physical Layer • 1) Direct Sequence Spread Spectrum (DSSS) • 2) Frequency Hopping Spread Spectrum (FHSS)

  36. Wireless (IEEE 802.11) • 1) DSSS • 2) FHSS

  37. Wireless (IEEE 802.11) • Provides Data Link (2) Layer specifications • Frame - use standard 802.3 frame • CSMA/CA (collision avoidance) as opposed to CD (collision detection) • CD requires full duplex – not available in wireless • Performs layer 2 error detection on incoming packets – if no errors, sends ACK indicating no collisions • If sender does not receive ACK, assumes a collision and retransmits

  38. Token Passing (IEEE 802.5) • Far less common than Ethernet • Hardware is generally more expensive • Single shared media • Logical ring, physical star • Passive MAU – packets are forwarded to single device at a time in order

  39. Token Passing (IEEE 802.5) • Provides NO Physical Layer (1) specifications • Original IBM Token Passing employed a variety of cable specifications • Modern 802.5 generally use Cat 5/5e/6 and RJ45 connectors

  40. Token Passing (IEEE 802.5) • Provides Data Link Layer (2) specifications • Token Ring Frames • 4 different frame formats used in communication (Ethernet employs only 1): 1) Data Frame, 2) Token Frame, 3) Command Frame, and 4) Abort Delimiter Frame

  41. Token Passing (IEEE 802.5) • Devices must wait for the token before transmitting • Device can only transmit if it has token • When sending device is finished, the token is passed to the next device on the ring

  42. Token Passing (IEEE 802.5) Logical operation of MAU

  43. Token Passing (IEEE 802.5) • Considered more efficient than CSMA/CD because access technique works well even under heavy load • Provides each device with equal opportunity to transmit • Collision free environment

  44. FDDI / CDDI • Prior to Fast Ethernet, FDDI was only data link protocol to offer 100 Mbps transmission • Provides redundancy to avoid failure • Two rings • Only one used when network is functioning properly • Counter rotating • Data flow in opposite directions on the two rings • Self healing • Hardware can detect failure and recover automatically • Failure is bypassed

  45. FDDI • Provides unique Layer 1 and 2 specifications - frame format different from token ring, although access method (token passing) the same

  46. Demand priority • Access method REQUIRES use of star topology • Each device sends request to active hub • Hub processes requests and assigns a communication order – one at a time (FCFS) • Fair access • Central hub controls traffic • Devices do not compete for use of media

  47. Comparison of Access Methods CSMA/CD CSMA/CA Token passing Demand priority

  48. Topology and Access methods • Three basic topologies • Bus • Star • Ring • Four basic access methods • CSMA/CD • CSMA/CA • Token passing • Demand priority

  49. LAN technology Topology Architecture CSMA/CD Star, bus Any 802.3 Ethernet Star, bus Ad hoc infrastructure 802.11 Wireless, Localtalk CSMA/CA Any Token Passing 802.5 HSTR 802.4 Token bus Any Any Demand Priority 802.12 Demand priority Star Any Mapping • Do topologies map to the access methods on a one-to-one basis?

  50. Backbone Networks • Serial • Distributed • Collapsed • Parallel

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