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EE 543

EE 543 . Packet Switched Networks Fall 2005. Data base. Network mgmt and control. Circuit switching. switch. switch. switch. switch. switch. Route through the network is reserved end-to-end for exclusive use over duration of call

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EE 543

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  1. EE 543 Packet Switched Networks Fall 2005

  2. Data base Network mgmt and control Circuit switching switch switch switch switch switch • Route through the network is reserved end-to-end for exclusive use over duration of call • Call set up: establish route, reserve links, set switches • Call tear down: release resources Where is the intelligence?

  3. Routing table Routing table Routing table Routing table Packet switching router router router router • Packets have source and destination addresses in headers • Packets stored in buffer at each node • Node reads headers routes packet to next node How do routers know where to send the packet?

  4. Circuit Well suited for continuous flows (CBR) “easy” to implement Network has “state” Utilization can be low Multiplex traffic to increase efficiency Packet Well suited for bursty flows – datagrams Storage needed at each node No “state” – each node functions independently High utilization Queuing delays Packet loss due to congestion Switching: circuit versus packet

  5. Packet – circuit hybrid: virtual circuits ATM switch ATM switch ATM switch ATM switch ATM switch • Path through the network is the same for all packets in a connection • Capacity along a virtual circuit is not reserved for a connection

  6. Delays in networks • Transmission: (packet size)/(bit rate) • How long does it take to get the packet on to the link? • Propagation: (path length)/(signal speed) • How long does it take to get from the sending node to the receiving node? • Queuing (buffers) • How long does the packet sit in a buffer? • Processing • How long does it take the switch/router to move packet from input buffer to output buffer?

  7. Delays in networks: examples • Transmission: (packet size)/(bit rate) • 10kbit packet, 10Mb/s link speed • Propagation: (path length)/(signal speed) • 100km path, fiber link (3.3 msec/km) • Queuing (buffers) • Typically controlled to 4x Ttrans • Processing delay • Typically negligible

  8. Delay-bandwidth product • How many bits are enroute from source to destination? • Prop delay * bandwidth Network as a “pipe” bandwidth Bits in delay

  9. Network as a “pipe” bandwidth Bits in delay Very large buffers needed at source and destination! Delay-bandwidth product • Example- long distance, high speed network: • Distance = 5000 km, delay = 5000*5*10-6 = 25 msec • Bit rate = 10 gbs = 1010 bps • Bw*delay = 1010 * 25 * 10 –3 = 25 * 107 = 250 MILLION BITS!

  10. Delay-bandwidth product: buffer management The prevalent delay-bandwidth product rule of thumb is that a router port should have a buffer capacity B equal to the average Round Trip Time (RTT) of the TCP sessions flowing through the link times the link bandwidth (BWlink in bps). B = RTT x BWlink Since the propagation delays for transcontinental links and transoceanic links are large, routers and switches with high bandwidth WAN interfaces need large buffers. For example, according to the delay-bandwidth rule of thumb, a switch/router with 10 GbE WAN interface linking two research campuses 10,000 miles apart would require approximately 200 ms x 10 Gbps = 250 MB of buffering available to that interface. Ref: http://www.force10networks.com/applications/buffermgmt.asp?content=1

  11. Error, flow and congestion control • Interrelated functions: • Depend on delay-bandwidth product • Several alternatives for error control: • Detect errors • Detect and correct errors • “Best” approach depends on application requirements and available bandwidth • Detect/retransmit: fine when latency is not a concern • Detect/correct: Better where timing is critical (but adds overhead to bandwidth)

  12. Several retransmission protocols available to correct errors • Alternating bit protocol (ABP): • Receiver sends ack for each correct packet • Sender waits for ack before sending next packet • Sender resends packet if ack not received before a time out Time out interval Corruptedsend Corrupted ack

  13. Error control and flow control protocols • Some special cases: • Satellite communications links: • High bandwidth (100 Mbps) • long delays (250 msec) • Deep space communications links: • Low bandwidth (1-10 kbps) • Very long delays (days, weeks, months…) • High loss paths: • Non-line of sight radio (error rate high and variable) • Meteor propagation (low bandwidth, high error rates)

  14. Principles of layering • Layer should be created where a different level of abstraction is needed • Each layer should perform a well-defined function • Layer boundaries should be chosen to minimize information flow across interfaces • Each layer provides a service to the layer above, receives a service from the layer below Separation of concerns

  15. FTP HTTP NV TFTP UDP TCP IP NET NET NET ■ ■ ■ 1 2 n Internet Architecture • Defined by Internet Engineering Task Force (IETF) • Hourglass Design • Application vs Application Protocol (FTP, HTTP)

  16. ISO Architecture

  17. Applications application presentation UDP Connection- less session TCP Connection Oriented transport IP datagrams network Network bitways link physical Layering: the OSI model compared to the Internet OSI = open system interconnect OSI Internet OSI precedes commercial Internet

  18. Protocols • Building blocks of a network architecture • Each protocol object has two different interfaces • service interface: operations on this protocol • peer-to-peer interface: messages exchanged with peer • Term “protocol” is overloaded • specification of peer-to-peer interface • module that implements this interface

  19. Protocol interfaces

  20. Host 1 Host 2 Digital Digital Video Video File File library library application application application application application application Protocol machinery • Protocol Graph • most peer-to-peer communication is indirect • peer-to-peer is direct only at hardware level

  21. Host Host Application Application Application Application program program program program Data Data RRP RRP RRP Data RRP Data HHP HHP HHP RRP Data Protocol machinery (cont) • Multiplexing and Demultiplexing (demux key) • Encapsulation (header/body)

  22. OSI: Physical Layer • “Bitways” in the Internet nomenclature • Hardware on which the network is built- carries the signals • Fiber, copper, radio, etc. • No assumptions of reliable delivery of bits • Physical layer standards – specify • Modulation scheme • Characteristics of interfaces between xmtr, rcvr and medium • Example protocol: 802.3/10BASEx (x=10, 100, 100, etc)

  23. OSI: Physical Layer • Phy layer: • converts bits into electrical or optical signals • May add synch bits to synchronize rcvr with xmtr.

  24. OSI: Data link layer • Transmitter and receiver execute a protocol to retransmit corrupted packets • Transmitter adds error detection bits (and may add sequence numbers) to packet • Issue: end-to-end error control versus link-layer error control • Link-layer control adds overhead • May be unnecessary if links are reliable • Link-layer control preferable for high-loss links: • Wireless! • Satellite (long delays) • Example protocol: Ethernet frames, ATM

  25. OSI: Data link layer • Data link layer: • Adds sequence number • Adds error detection bits (CRC)

  26. SAR MAC OSI: Data Link Layer • Sometimes divided into two sub-layers • SAR: segmentation and reassembly (or LLC) • MAC: media access control Segments data into smaller blocks, Adds tags for error detection, sequencing, etc. Controls media sharing- multiplexes Data from several sources onto common link

  27. Media access control (MAC) sublayer • Required for a shared link: • MAC address – designates destination for packet • MAC protocol: rules for sharing the link resources

  28. Logical link control (LLC) sublayer • LLC: • Implements error detection/sequencing for shared link • Same as data link protocol for point-to-point link • LLC + MAC = data link layer

  29. Routing OSI: Network layer • Performs delivery of packets between source and destination • Source-destination path may consist of multiple links Network layer adds source and destination addresses Example protocol: IP

  30. OSI: Transport layer • Delivers messages between ports in different computers • Port numbers used to differentiate between different processes running on the computers • Transport layer can be either connection oriented or connectionless • connection oriented: delvers error free messages in the correct order • Connectionless: Delivers messages one-by-one, no guarantee on order or correctness • Breaks messages into packets for transmission by network layer • Reassembles packets from network layer into messages • Example protocols: TCP, UDP

  31. OSI: Session Layer • Concern: supervision of dialog between end computers • Sets up connections prior to exchange of user information • Controls dialog during an information exchange • May assist in synchronization of flow of large messages, insert checkpoints • Example: SS7 (signaling system 7), MPLS

  32. OSI: Presentation Layer • Concern: Syntax and semantics of information transmitted • Encoding of data into standard application formats (e.g, ASCII) • May include functions such as data compression, encryption • Examples: ASCII, DPCM speech coding, MPEG

  33. OSI: Application Layer • Concern: Provide commonly used functions for users • Examples: file transfer protocols, terminal emulation, remote login, directory services • Examples: FTP, Telnet • User applications run on top of this layer: e-mail, www, etc.

  34. OSI: Summary of layers 4-7 functions

  35. Summary of services provided by layers in the OSI model

  36. application presentation session transport network link physical The end-to-end principle: Minimize the functions at the lower layers; do as much as possible On an end-to-end basis

  37. The end-to-end principle: • Examples of functions best done end-to-end: • Encryption • Error detection/correction • Packet ordering • Deletion of duplicate packets • ……………

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