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# Network Topologies

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### Network Topologies

1. 1. Topology <ul><li>Mathematical term Roughly interpreted as &quot;geometry for curved surface&quot; </li></ul>
2. 2. Network Topologies <ul><li>A network &quot;topology&quot; is the structure or organization of communications that links between hosts or devices on a network. </li></ul><ul><li>LAN topology </li></ul><ul><ul><li>A LAN is a shared medium that serves many DTEs( data terminal equipment) located in close proximity such as in one building. </li></ul></ul><ul><ul><li>Three basic topologies associated with LANs: bus, ring, and star </li></ul></ul><ul><li>WAN topology </li></ul><ul><ul><li>A WAN links networks that are geographically separated by long distance through switches, routers, and/or bridges. </li></ul></ul><ul><ul><li>Two topologies: mesh and tree </li></ul></ul>
3. 3. LAN Topologies <ul><li>Bus topology: All hosts (DTEs) are connected to a common cable or medium. </li></ul>
4. 4. LAN Topologies (cont’d) <ul><li>Tree topology: Transmission medium is a branching cable with no closed loops. Generalization of bus topology. </li></ul>Hub Host
5. 5. LAN Topologies (Cont’d) <ul><li>Ring topology: Each device (DTE) is connected to another in sequence to form a &quot;ring” </li></ul>
6. 6. LAN Topologies (Cont’d) <ul><li>Star Topology: All of the devices on the network are connected to a central &quot;hub&quot; or concentrator </li></ul>
7. 7. Advantages and Disadvantages of Bus Topology <ul><li>Advantages of bus topologies: </li></ul><ul><ul><li>Inexpensive to install (uses less cable) </li></ul></ul><ul><ul><li>Easy to add new devices onto the bus or onto the network </li></ul></ul><ul><li>Disadvantages of bus topologies: </li></ul><ul><ul><li>Can be expensive to maintain and troubleshoot </li></ul></ul><ul><ul><li>A naive user can easily &quot;bring down&quot; the entire bus </li></ul></ul><ul><ul><li>Overall maximum length of the bus is limited (for example, in a 10-Base-2 ethernet, 200m maximum from one terminator to the other along the cable) </li></ul></ul>
8. 8. Advantages and Disadvantages of Ring Topology <ul><li>Advantages of ring topologies: </li></ul><ul><ul><li>Very predictable network performance </li></ul></ul><ul><ul><li>May be slightly more secure than other topologies </li></ul></ul><ul><li>Disadvantages of ring topologies: </li></ul><ul><ul><li>Expensive as compared to bus/star topologies </li></ul></ul><ul><ul><ul><li>Hardware for ring topologies is less available and therefore more expensive </li></ul></ul></ul><ul><ul><ul><li>Many systems lack good support for networking in ring environments </li></ul></ul></ul><ul><ul><li>Unique wiring requirements </li></ul></ul><ul><ul><li>More complex networking and operational protocol </li></ul></ul>
9. 9. Advantages and Disadvantages of Star Topology <ul><li>Advantages of star topologies: </li></ul><ul><ul><li>Each node has a dedicated connection to the network --disconnecting a single node does not bring down the rest of the nodes on the network </li></ul></ul><ul><ul><li>Network and cable administration are centralized </li></ul></ul><ul><li>Disadvantages of star topologies: </li></ul><ul><ul><li>More expensive to install -- require more cable and the additional cost of a hub </li></ul></ul><ul><ul><li>Maximum length of each spoke of the hub is limited to the allowed maximum length of the medium (for example, on a 10-Base-T network using UTP cable, the maximum distance from the hub to a host is 100m) </li></ul></ul><ul><ul><li>Breakdown of the hub causes breakdown of the entire system </li></ul></ul>
10. 10. WAN Topologies <ul><li>Mesh Topology: </li></ul><ul><ul><li>provides multiple paths between nodes or networks (N) </li></ul></ul><ul><ul><li>usually implemented with switches and routers </li></ul></ul>N1 N2 N3 N4 N6 N5
11. 11. WAN Topologies (Cont’d) <ul><li>Tree Topology: A hierarchical architecture starts with header node and branches out to other nodes. Simpler to implement than mesh topology. </li></ul>
12. 12. Data Link Layer <ul><li>Specifies how two devices or hosts communicate with each other when they are connected to the same medium (e.g., connected via a common bus or a common hub). </li></ul><ul><li>Major functions of the layer: </li></ul><ul><ul><li>Flow control: prevents receiver’s buffer overflow </li></ul></ul><ul><ul><li>Error detection: uses error-detecting code and algorithm </li></ul></ul><ul><ul><li>Error control : retransmits damaged frames upon request or if no acknowledgement received from the receiver </li></ul></ul>
13. 13. Terminology <ul><li>Subnet </li></ul><ul><ul><li>The devices which are linked together by a common medium are collectively known as a subnet . </li></ul></ul><ul><li>Frame </li></ul><ul><ul><li>Data are sent in blocks called frames . A frame, in addition to data, contains some header information such as source and destination addresses, control data bits, error-checking bits, etc. Frame size, the number of bits, depends on the underlying protocol. Actual frame format depends on the protocol. </li></ul></ul>
14. 14. Data Link Layer: Sharing Medium In A LAN <ul><ul><li>Shared medium used for all transmissions </li></ul></ul><ul><ul><li>Only one station transmits at any time </li></ul></ul><ul><ul><li>Stations &quot;take turns&quot; using medium </li></ul></ul><ul><ul><li>Media Access Control (MAC) policy ensures fairness (MAC protocol) </li></ul></ul>
15. 15. Media Access Control Protocols <ul><li>Media Access Control </li></ul><ul><ul><li>determines the rules about when hosts on a subnet are allowed to transmit data onto the physical medium </li></ul></ul><ul><li>Two broad control schemes: </li></ul><ul><ul><li>Centralized control </li></ul></ul><ul><ul><ul><li>greater control through priorities, overrides, and guaranteed capacity </li></ul></ul></ul><ul><ul><ul><li>simple </li></ul></ul></ul><ul><ul><ul><li>but creates a bottleneck and a single point of failure </li></ul></ul></ul><ul><ul><li>Distributed control </li></ul></ul>
16. 16. MAC Control Techniques <ul><ul><li>Round Robin </li></ul></ul><ul><ul><ul><li>Each station takes turn to transmit </li></ul></ul></ul><ul><ul><ul><li>May be centralized (polling) or distributed (token passing) </li></ul></ul></ul><ul><ul><ul><li>Efficient when many stations transmit </li></ul></ul></ul><ul><ul><ul><li>High overhead when only few stations transmit </li></ul></ul></ul><ul><ul><li>Reservation </li></ul></ul><ul><ul><ul><li>Transmitting station reserves slots (stream traffic) </li></ul></ul></ul><ul><ul><ul><li>May be centralized or distributed </li></ul></ul></ul>
17. 17. MAC Control Techniques (cont’d) <ul><ul><li>Contention </li></ul></ul><ul><ul><ul><li>Appropriate for bursty traffic </li></ul></ul></ul><ul><ul><ul><li>Distributed by nature </li></ul></ul></ul><ul><ul><ul><li>Simple to implement </li></ul></ul></ul><ul><ul><ul><li>Efficient for light to moderate load </li></ul></ul></ul><ul><ul><ul><li>Performance tends to collapse under heavy load </li></ul></ul></ul><ul><ul><ul><li>Examples: </li></ul></ul></ul><ul><ul><ul><ul><li>ALOHA </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Slotted ALOHA </li></ul></ul></ul></ul><ul><ul><ul><ul><li>CSMA </li></ul></ul></ul></ul><ul><ul><ul><ul><li>CSMA/CD </li></ul></ul></ul></ul>
18. 18. ALOHA <ul><li>Developed for packet radio networks </li></ul><ul><li>Transmits whenever a station has a frame to send </li></ul><ul><li>Wait and listens for an acknowledgement </li></ul><ul><li>Wait time = maximum possible roundtrip propagation delay plus a small fixed time increment </li></ul><ul><li>Resends the frame if no acknowledgement is received. </li></ul><ul><li>Gives up after many repeated, failed transmissions </li></ul><ul><li>Simple but poor utilization. Maximum utilization is only about 18% </li></ul>
19. 19. Slotted Aloha <ul><ul><li>Similar to ALOHA but stations are allowed to transmit during a time slot </li></ul></ul><ul><ul><li>Channel is organized into uniform time slots </li></ul></ul><ul><ul><li>Size of the time slot = Frame transmission time </li></ul></ul><ul><ul><li>Some central clock synchronizes all stations </li></ul></ul><ul><ul><li>If a frame collides with other one, it collides completely </li></ul></ul><ul><ul><li>Maximum utilization is about 37% </li></ul></ul>
20. 20. An Important Note <ul><li>Both ALOHA and slotted ALOHA exhibit poor utilization and fail to take advantage of the fact that propagation delay is usually very small compared to frame transmission time for both packet radio and LANs. </li></ul>
21. 21. CSMA (Carrier Sense Multiple Access) <ul><ul><li>Advantageous over slotted Aloha when propagation time is small compared to frame transmission time . </li></ul></ul><ul><ul><li>Listens if another transmission is in progress (carrier sense) </li></ul></ul><ul><ul><li>Transmits if medium is idle </li></ul></ul><ul><ul><li>Wait for acknowledgement </li></ul></ul><ul><ul><li>Wait time = maximum roundtrip propagation time + medium access time for the receiver </li></ul></ul><ul><ul><li>Retransmits if no acknowledgement is received </li></ul></ul><ul><ul><li>Disadvantage: medium remains unusable for the duration of transmission of damaged frames after collision </li></ul></ul>
22. 22. CSMA/CD Carrier Sense Multiple Access/Collision Detection) <ul><li>A protocol for Ethernet </li></ul><ul><li>1. If medium is idle, transmit and go to step 3; otherwise, go to step 2. </li></ul><ul><li>2. If medium is busy, continue to listen; transmit immediately if idle. </li></ul><ul><li>3. If collision detected, transmit a brief jamming signal to inform other stations. </li></ul><ul><li>4. Wait a random amount of time after transmitting jamming signal (backoff), then go to step 1. </li></ul><ul><li>Note:With CSMA/CD, the amount of wasted capacity is reduced to the time it takes to detect a collision. </li></ul>
23. 23. Collision Detection Mechanisms <ul><li>Baseband Ethernet : Higher voltage swings than those produced by a single transmitter are detected. Cable length is limited to 500 meters. </li></ul><ul><li>Broadband Ethernet : RF Carrier is detected. Bit-by-bit comparison is done between transmitted and received data. </li></ul><ul><li>Twisted-pair star topology : If a hub detects presence of more than one input signal at its ports, it assumes a collision and sends out collision presence signal. </li></ul>
24. 24. Backoff After Collision (Wait Time Calculation) <ul><li>When collision occurs </li></ul><ul><ul><li>Wait random time t 1 , 0 < t 1 < d </li></ul></ul><ul><ul><li>Use CSMA and try again </li></ul></ul><ul><ul><li>If second collision occurs </li></ul></ul><ul><ul><ul><li>Wait random time t 2 , 0 < t 2 < 2d </li></ul></ul></ul><ul><ul><li>Double range for each successive collision </li></ul></ul><ul><ul><li>Called exponential backoff </li></ul></ul>
25. 25. CSMA/CA <ul><li>Used on wireless networks </li></ul><ul><li>Both sides send small message followed by data transmission </li></ul><ul><li>&quot;X is about to send to Y” </li></ul><ul><li>&quot;Y is about to receive from X” </li></ul><ul><li>Data frame sent from X to Y </li></ul><ul><li>Purpose: inform all stations in range of X or Y before transmission </li></ul><ul><li>Known as Collision Avoidance (CA) </li></ul>
26. 26. Example Bus Network: Ethernet <ul><ul><li>Most popular LAN </li></ul></ul><ul><ul><li>Widely used </li></ul></ul><ul><ul><li>IEEE standard 802.3 </li></ul></ul><ul><ul><li>Several generations </li></ul></ul><ul><ul><ul><li>Same frame format </li></ul></ul></ul><ul><ul><ul><li>Different data rates </li></ul></ul></ul><ul><ul><ul><li>Different wiring schemes </li></ul></ul></ul>
27. 27. Identifying A Destination <ul><li>All stations on shared-media LAN receive all transmissions </li></ul><ul><li>To allow sender to specify destination </li></ul><ul><li>Each station assigned unique number </li></ul><ul><li>Known as station’s address </li></ul><ul><li>Each frame contains address of intended recipient </li></ul>
28. 28. Ethernet Addressing <ul><ul><li>Standardized by IEEE </li></ul></ul><ul><ul><li>Each station is assigned with a unique 48-bit address </li></ul></ul><ul><ul><li>Address is assigned when network interface card is (NIC) manufactured </li></ul></ul><ul><ul><li>Each address is a physical address </li></ul></ul>
30. 30. Possible Ways to Direct Frames <ul><li>Frames can be sent to: </li></ul><ul><ul><li>Single destination (unicast) </li></ul></ul><ul><ul><li>All stations on network (broadcast) </li></ul></ul><ul><ul><li>Subset of stations (multicast) </li></ul></ul><ul><li>Some feature of destination address is used to distinguish type (unicast, broadcast, or multicast) </li></ul>
32. 32. Multicast on Ethernet <ul><li>Half of addresses reserved for multicast </li></ul><ul><li>Network interface card </li></ul><ul><ul><li>Always accepts unicast and broadcast </li></ul></ul><ul><ul><li>Can accept zero or more multicast addresses </li></ul></ul><ul><li>Software </li></ul><ul><ul><li>Determines multicast address to accept </li></ul></ul><ul><ul><li>Informs network interface card </li></ul></ul>
33. 33. Promiscuous Mode <ul><ul><ul><li>Designed to testing/debugging </li></ul></ul></ul><ul><ul><ul><li>Allows interface to accept all packets </li></ul></ul></ul><ul><ul><ul><li>Available on most interface hardware </li></ul></ul></ul>
34. 34. Network Analyzer <ul><ul><li>Device used for testing and maintenance </li></ul></ul><ul><ul><li>Listens in promiscuous mode </li></ul></ul><ul><ul><li>Produces </li></ul></ul><ul><ul><ul><li>Summaries (e.g., % of broadcast frames) </li></ul></ul></ul><ul><ul><ul><li>Specific items (e.g., frames from a given address) </li></ul></ul></ul>
35. 35. Identifying Frame Contents <ul><li>Integer type field tells recipient the type of data being carried </li></ul><ul><li>Two possibilities </li></ul><ul><ul><li>Self-identifying or explicit type (hardware records type) </li></ul></ul><ul><ul><li>Implicit type (application sending data must handle type) </li></ul></ul>
37. 37. Example Ethernet Frame Format 8 6 6 2 46 - 1500 4 Preamble Dest. Addr. Src. Addr. Data In Frame CRC Frame Type Preamble: Alternating 1s and 0s. Used by receiver synchronization
38. 38. Example Frame Types
39. 39. When Network Hardware Does Not include Types <ul><ul><ul><li>Sending and receiving computers must agree </li></ul></ul></ul><ul><ul><ul><ul><li>To send one type of data </li></ul></ul></ul></ul><ul><ul><ul><ul><li>To put type information in first few octets of payload </li></ul></ul></ul></ul><ul><ul><ul><li>Most systems need type information </li></ul></ul></ul>
40. 40. Handling Frames of Many Types <ul><ul><li>Network interface hardware </li></ul></ul><ul><ul><ul><li>Receives copy of each transmitted frame </li></ul></ul></ul><ul><ul><ul><li>Examines address and either discards or accepts </li></ul></ul></ul><ul><ul><ul><li>Passes accepted frame to system software </li></ul></ul></ul><ul><ul><li>Network device software </li></ul></ul><ul><ul><ul><li>Examines frame type </li></ul></ul></ul><ul><ul><ul><li>Passes frame to correct software module </li></ul></ul></ul>
41. 41. Network Analyzer <ul><li>Device used for testing and maintenance </li></ul><ul><li>Listens in promiscuous mode </li></ul><ul><li>Produces </li></ul><ul><ul><ul><li>Summaries (e.g., % of broadcast frames) </li></ul></ul></ul><ul><ul><ul><li>Specific items (e.g., frames from a given address) </li></ul></ul></ul><ul><li>Note: Check web for free network analyzer/packet sniffer. </li></ul>
42. 42. 10Base2 Ethernet Wiring (Thinnet) <ul><li>Use coax cables (10Base2), NICs, BNC connectors, terminators </li></ul>
43. 43. Twisted Pair (10Base-T) Ethernet Wiring <ul><li>Use 10Base-T wire, hubs, NICs, and RJ-45 connectors </li></ul>Hub Twisted pair wiring RJ-45 connectors
44. 44. IEEE 802.3 10-Mbps Ethernet Specifications <ul><ul><ul><li>Notation: <Mbps><signaling><length in 100m> </li></ul></ul></ul><ul><ul><ul><li>10BASE5 </li></ul></ul></ul><ul><ul><ul><ul><li>50-ohm coax cable </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Topology: bus </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Maximum segment length: 500 meters </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Nodes per segment: 100 </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Data rate: 10Mbps </li></ul></ul></ul></ul><ul><ul><ul><ul><li>4 repeaters maximum (2.5 km) </li></ul></ul></ul></ul>
45. 45. IEEE 802.3 10-Mbps Ethernet Specifications (cont’d) <ul><ul><ul><li>10BASE2 </li></ul></ul></ul><ul><ul><ul><ul><li>50-ohm coax cable (thinner brand) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Maximum Segment length: 185 meters </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Nodes per segment: 30 </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Topology: bus </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Data rate: 10Mbps </li></ul></ul></ul></ul>
46. 46. IEEE 802.3 10-Mbps Ethernet Specifications (cont’d) <ul><ul><ul><li>10BASE-T (Twisted pair) </li></ul></ul></ul><ul><ul><ul><ul><li>Maximum segment length: 100 meters </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Topology: star </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Data rate: 10Mbps </li></ul></ul></ul></ul><ul><ul><ul><li>10BROAD36 </li></ul></ul></ul><ul><ul><ul><ul><li>75-ohm CATV coaxial cable </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Maximum individual segment length is 1800 meters. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Maximum end-to-end span 3600 meters. </li></ul></ul></ul></ul>
47. 47. IEEE 802.3 100-Mbps Fast Ethernet Specifications <ul><li>100BASE-TX </li></ul><ul><ul><li>Data rate: 100Mbps </li></ul></ul><ul><ul><li>2 Shielded twisted pair(STP) or high-quality Category 5 unshielded twisted pair(UTP) </li></ul></ul><ul><ul><li>Maximum segment length: 100 meters </li></ul></ul><ul><ul><li>Network span: 200 meters </li></ul></ul><ul><li>100BASE-FX </li></ul><ul><ul><li>2 Optical fibers </li></ul></ul><ul><ul><li>Data rate: 100 Mbps </li></ul></ul><ul><ul><li>Maximum segment length: 200 meters </li></ul></ul><ul><ul><li>Network span: 400 meters </li></ul></ul>
48. 48. Token Ring <ul><li>Token Ring </li></ul><ul><ul><li>Most commonly used MAC protocol for rings </li></ul></ul><ul><ul><li>IEEE 802.5 standard </li></ul></ul><ul><ul><li>Token - a small frame </li></ul></ul>
49. 49. Token passing mechanism <ul><li>A station seizes a token by changing one bit </li></ul><ul><li>Changed token is a start-of-frame sequence </li></ul><ul><li>Transmitted frame is absorbed by the transmitting station after a round-trip </li></ul><ul><li>The station will insert a new token </li></ul><ul><ul><li>at the end of transmission and </li></ul></ul><ul><ul><li>at the detection of leading edge of transmitted frame after circulation </li></ul></ul>