Dcn lecture 3
- 2. • Two fundamental approaches to moving data through a network of links and switches: circuit switching and packet switching. The network core
- 3. • In circuit-switched networks, the resources needed along a path (buffers, link transmission rate) to provide for communication between the end systems are reserved for the duration of the communication session between the end- systems. The network core
- 4. • In packet-switched networks, these resources are not reserved; a session 's messages use the resources on demand, and as a consequence, may have to wait (that is, queue) for access to a communication link. The network core
- 5. • In this network, the four circuit switches are interconnected by four links. • Each of these links has n circuits, so that each link can support n simultaneous connections • The hosts (for example, PCs and workstations) are each directly connected to one of the switches. • When two hosts want to communicate, the network establishes a dedicated end- to-end connection between the two hosts. Circuit Switching
- 6. • Thus, in order for Host A to send messages to Host B, the network must first reserve one circuit on each of two links. • Because each link has n circuits, for each link used by the end-to-end connection, the connection gets a fraction 1/n of the link's bandwidth for the duration of the connection Circuit Switching
- 7. • With FDM, the frequency spectrum of a link is divided up among the connections established across the link. Specifically, the link dedicates a frequency band to each connection for the duration of the connection. • In telephone networks, this frequency band typically has a width of 4 kHz (that is, 4,000 hertz or 4,000 cycles per second). Circuit Switching
- 8. • For a TDM link, time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots . When the network establishes a connection across a link, the network dedicates one time slot in every frame to this connection. These slots are dedicated for the sole use of that connection, with one time slot available for use (in every frame ) to transmit the connection's data. Circuit Switching
- 9. Circuit switching: FDM versus TDM FDM frequency time TDM frequency time 4 users Example:
- 10. • In modern computer networks, the source breaks long messages into smaller chunks of data known as packets. • Between source and destination, each of these packets travels through communication links and packet switches Packet Switching
- 11. • mesh of interconnected routers • packet-switching: hosts break application-layer messages into packets – forward packets from one router to the next, across links on path from source to destination – each packet transmitted at full link capacity Packet Switching
- 12. Packet-switching: store-and-forward • takes L/R seconds to transmit (push out) L-bit packet into link at R bps • store and forward: entire packet must arrive at router before it can be transmitted on next link one-hop numerical example: L = 7.5 Mbits R = 1.5 Mbps one-hop transmission delay = 5 sec more on delay shortly … source R bps destination 123 L bits per packet R bps end-end delay = 2L/R (assuming
- 13. Packet Switching: queueing delay, loss A B CR = 100 Mb/s R = 1.5 Mb/s D Equeue of packets waiting for output link queuing and loss: If arrival rate (in bits) to link exceeds transmission rate of link for a period of time: packets will queue, wait to be transmitted on link packets can be dropped (lost) if memory (buffer) fills up
- 14. Packet Switching A B CR = 100 Mb/s R = 1.5 Mb/s D Equeue of packets waiting for output link If the arrival rate of packets to the switch exceeds the rate at which the switch can forward packets across the 1.5 Mbps output link, congestion will occur as packets queue in the link' s output buffer before being transmitted onto the link.
- 15. Packet switching versus circuit switching • packet switching is not suitable for real-time services (for example, telephone calls and video conference calls) because of its variable and unpredictable end-to-end delays (due primarily to variable and unpredictable queuing delays) • Proponents of packet switching argue that (I) it offers better sharing of bandwidth than circuit switching and (2) it is simpler, more efficient, and less costly to implement than circuit switching.
- 16. Packet switching versus circuit switching example: 1 Mb/s link each user: • 100 kb/s when “active” • active 10% of time • circuit-switching: – 10 users • packet switching: – with 35 users, probability > 10 active at same time is less than .0004 * packet switching allows more users to use network! N users 1 Mbps link
- 17. – resource sharing – simpler, no call setup • excessive congestion possible: packet delay and loss – protocols needed for reliable data transfer, congestion control is packet switching a “slam dunk winner?” Packet switching versus circuit switching
- 18. Four sources of packet delay • router A has an outbound link leading to router B . • This link is preceded by a queue (also known as a buffer) . • When the packet arrives at route A from the upstream node, router A examines the packet' s header to determine the appropriate outbound link for the packet and then directs the packet to this link.
- 19. Four sources of packet delay The most important of these delays are the nodal processing delay, queuing delay, transmission delay, and propagation delay; together, these delays accumulate to give a total nodal delay. A B propagation transmission nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop
- 20. Four sources of packet delay Processing Delay • The time required to examine the packet's header and determine where to direct the packet is part of the processing delay. • The processing delay can also include other factors, such as the time needed to check for bit-level errors in the packet that occurred in transmitting the packet' s bits from the upstream node to router A • After this nodal processing, the router directs the packet to the queue that precedes the link to router B
- 21. Four sources of packet delay Queuing Delay • the packet experiences a queuing delay as it waits to be transmitted onto the link. • The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission across the link. • If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. • If the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long
- 22. Four sources of packet delay Transmission Delay • Assuming that packets are transmitted in a first-come- first-served manner, as is common in packet-switched networks, our packet can be transmitted only after all the packets that have arrived before it have been transmitted. • Denote the length of the packet by L bits, and denote the transmission rate of the link from router A to router B by R bits/sec. • The transmission delay (also called the store-and- forward delay is L/R. • This is the amount of time required to push (that is, transmit) all of the packet's bits into the link.
- 23. Four sources of packet delay Propagation Delay • Once a bit is pushed into the link, it needs to propagate to router B. The time required to propagate from the beginning of the link to router B is the propagation delay. • The bit propagates at the propagation speed of the link. • The propagation speed depends on the physical medium of the link (that is, fiber optics, twisted-pair, copper wire, and so on) and is in the range of which is equal to, or a little less than, the speed of light 2 ∗ 108 𝑚 𝑠 𝑡𝑜 3 ∗ 108 𝑚 𝑠 • The propagation delay is distance between two routers divided by the propagation speed. • propagation delay is d/s, where d is the distance between router A and router B and s is the propagation speed of the link. • Once the last bit of the packet propagates to node B, it and all the preceding bits of the packet are stored in router B.
- 24. Four sources of packet delay dproc: nodal processing check bit errors determine output link typically < msec A B propagation transmission nodal processing queueing dqueue: queueing delay time waiting at output link for transmission depends on congestion level of router dnodal = dproc + dqueue + dtrans + dprop
- 25. How do loss and delay occur? packets queue in router buffers • packet arrival rate to link (temporarily) exceeds output link capacity • packets queue, wait for turn A B packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers
- 26. Four sources of packet delay • The transmission delay is the amount of time required for the router to push out the packet; it is a function of the packet's length and the transmission rate of the link, but has nothing to do with the distance between the two routers. • The propagation delay, on the other hand, is the time it takes a bit to propagate from one router to the next; it is a function of the distance between the two routers, but has nothing to do with the packet's length or the transmission rate of the link.
- 27. dtrans: transmission delay: L: packet length (bits) R: link bandwidth (bps) dtrans = L/R dprop: propagation delay: d: length of physical link s: propagation speed in medium (~2x108 m/sec) dprop = d/sdtrans and dprop very different Four sources of packet delay propagation nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop A B transmission