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Introduction to This Class

This class covers the fundamentals of data networking using the textbook "Computer Networking: A Top Down Approach" by Kurose and Ross. Topics include protocols, application layer, transport layer, network layer, datalink and MAC layer, and the internet.

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Introduction to This Class

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  1. Introduction to This Class • Instructor: Stephan Bohacek • bohacek@udel.edu, 302-831-4274 • http://www.eecis.udel.edu/~bohacek • Textbook: Kurose and Ross: Computer Networks (6th edition) • Web page has • syllabus • class notes • videos lectures of some topics (most, but not all topics are covered) • homework assignments • project assignments • announcements • Issues • Programming languages: C++ or java? • Computers: • >=2GB ram? • Windows or Linux?

  2. Introduction to Data Networking Kurose and Ross: Computer Networking: A Top Down Approach • Chapter 1: Overview and general principles • Protocol stack • Sharing • Statistic multiplexing • Packet switching • Circuit switching • Performance of packet switching networks • Chapter 2: Application layer • TCP and UDP, multiplexing and ports • Applications • http, ftp, email, DNS, P2P, DHT • Chapter 3: Transport layer • Tools for reliable transport • TCP • Chapter 4: Network layer • IP and IPv6 • NAT • Routing • Intra-network routing • Inter-network routing • Chapter 5: Datalink and MAC layer • Multiple access • ARP • Ethernet • Switches and hubs • Link layer routing

  3. The Internet • What is the longest time that you have gone without using “the Internet?” • What is the Internet? • How do you use the Internet? • Do you only watch TV over the Internet, or do you have cable? • Skype? • Facebook? • IM? • Twitter? • Smart phone? • Do you only have a data plan on your phone? What is the Internet

  4. Networking Basics • Core components of the Internet – the protocol stack • Multiplexing, circuit switching, and packet switching • Loss and delays • The structure of the Internet

  5. Networking Basics • Core components of the Internet – the protocol stack • Multiplexing, circuit switching, and packet switching • Loss and delays • The structure of the Internet

  6. The Internet • Objective: split an application across multiple machines • Web client and web server • Option 1: users get a DVD of the NY Times every day • Option 2: Split the NY Times application into 2 parts. The client part of the application runs on the users machine and the server part runs in NY Times headquarters. NY Times reporters can load articles on to the server and the client can get articles from the server and read them on their machine • Email • Option 1: Write a letter on paper and mail it to its destination. • Option 2: Write an email on one machine and allow this message to be sent to the destination machine • Buy something • Option 1: go to the store, see item, pick up item and pay for it • Option 2: The client machine can explore all items via a GUI. When an item is selected for purchase, the vendor is notified, credit card is charged, and the item is sent • Key derived requirement: send message between machines • These are distributed applications • The task of sending messages between machine is very complicated • To solve this task, we subdivide it into subtasks. Wht? • Each task is easier to solve • If we have multiple solutions for a subtask, perhaps we can use different solutions at different times • E.g., a solution for wired networks and a solution for wireless. It is easier to solved these specific tasks then a very general one (e.g., one that must for for wired and wireless networks)

  7. The Internet • Objective: split an application across multiple machines • Key derived requirement: send message between machines • These are distributed applications • The task of sending messages between machine is very complicated • To solve this task, we subdivide it into subtasks. Why • Each task is easier to solve • If we have multiple solutions for a subtask, perhaps we can use different solutions at different times • E.g., a solution for wired networks and a solution for wireless. It is easier to solved these specific tasks then a very general one (e.g., one that must for for wired and wireless networks) • Subtasks are called the protocol stack • Application layer • Transport layer • Network layer • Link layer • Physical Layer (Not covered in this course. Communication theory covers this topic)

  8. Application Layer – where the applications live Email: Rules/protocols for how an end-host gets mail from the mail server Web: Rules/protocols for how the end-hosts gets a web page from the web servers Question: How is a networking application different from a non-networking application (e.g., MS Word). That is, why, when talking about networking application, do we focus on protocols, but do not focus on protocols when discussing non-networked applications such as MS-Word? Answer: The networking applications must communicate, and rules are required to define the communication. Roles that end-hosts play: Client, server, and peer The client asks the server for a service. E.g., The client asks the server to send a mail for it. The client asks the server for a web page The client asks the server to translate a web address to an IP address. Peer: A host can act as both a client and a server. But usually in one transaction, the host takes only one role Application layer defined messages Protocol Stack 7. Application 4. Transport 3. Network 2. Data link 1. Physical The protocol stack

  9. Layers 2-4 Protocol Stack 7. Application 4. Transport 3. Network 2. Data link 1. Physical Definition: End-host Which are the end-host? client server Routers The protocol stack

  10. Layers 2-4 Goal: move messages from between end-hosts Approach: break the problem into little pieces. Each piece is a layer in the “protocol stack” • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols client server The protocol stack

  11. Layers 2-4 client server • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols Top down approach of breaking problems into small pieces • Transport layer • Reliability: The server must make sure that the client gets the data Congestion control (or lack there of) (http://www.youtube.com/watch?v=RjrEQaG5jPM) • Congestion Control: The server should send data as fast as possible, but not too fast • TCP provides these features (services), while UDP does not • Network layer (could be called the routing layer, but it isn’t) • The packets must find their way through the network. • Each packet has the IP address of the destination • By examining the IP address, routers decide where to send the packet next • Link Layer or MAC layer (link layer and MAC layer) • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include • congestion control, media access, error detection/correction The protocol stack

  12. Layers 2-4 • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols Top down approach of breaking problems into small pieces ….. • Link Layer or MAC layer (link layer and MAC layer) • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include congestion control, media access, error detection/correction • Media access. The “air” is a shared medium. If two nodes transmit at the same time, there will be a collision. Thus, a scheme must be developed to determine which node transmits when. • Error detection/correction. If interference does occur, then errors might occur. If an error is detected, then • the error could be corrected with forward error correction, or • the receiving link could request a retransmission The protocol stack

  13. Layers 2-4 client server • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols Top down approach of breaking problems into small pieces • Transport layer • Reliability: The server must make sure that the client gets the data Congestion control (or lack there of) • Congestion Control: The server should send data as fast as possible, but not too fast • TCP provides these features (services), while UDP does not • Network layer (could be called the routing layer, but it isn’t) • The packets must find their way through the network. • Each packet has the IP address of the destination • By examining the IP address, routers decide where to send the packet next • Link Layer or MAC layer • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include • congestion control, media access, error detection/correction • Physical layer • Logical bits are encoded as physical quantities, e.g., as voltage levels, as shifts in phase, … • This course does not cover the physical layer The protocol stack

  14. Protocols TCP connection response Get http://www.awl.com/kurose-ross <file> time • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Hi TCP connection request Hi Got the time? 2:00 The protocol stack

  15. Core components • End-hosts • Applications • ? • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • ? • Protocols

  16. Core components • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols

  17. Application Layer – where the applications live Email: Rules/protocols for how an end-host gets mail from the mail server Web: Rules/protocols for how the end-hosts gets a web page from the web servers Question: How is a networking application different from a non-networking application (e.g., MS Word). That is, why, when talking about networking application, do we focus on protocols, but do not focus on protocols when discussing non-networked applications such as MS-Word? Answer: The networking applications must communicate, and rules are required to define the communication. Roles that end-hosts play: Client, server, and peer The client asks the server for a service. E.g., The client asks the server to send a mail for it. The client asks the server for a web page The client asks the server to translate a web address to an IP address. Peer: A host can act as both a client and a server. But usually in one transaction, the host takes only one role • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols The protocol stack

  18. Layers 2-4 • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols Which are the end-host? client server Routers The protocol stack

  19. Layers 2-4 Goal: move messages from server to the client Approach: break the problem into little pieces. Each piece is a layer in the “protocol stack” Why is this a good approach? • Small problems are easier to understand/solve. 2.Different solutions can be mixed and matched • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols client server The protocol stack

  20. Layers 2-4 client server • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols Top down approach of breaking problems into small pieces • Transport layer • Reliability: The server must make sure that the client gets the data Congestion control (or lack there of) (http://www.youtube.com/watch?v=RjrEQaG5jPM) • Congestion Control: The server should send data as fast as possible, but not too fast • TCP provides these features (services), while UDP does not • Network layer (could be called the routing layer, but it isn’t) • The packets must find their way through the network. • Each packet has the IP address of the destination • By examining the IP address, routers decide where to send the packet next • Link Layer or MAC layer (link layer and MAC layer) • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include • congestion control, media access, error detection/correction The protocol stack

  21. Layers 2-4 • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols Top down approach of breaking problems into small pieces ….. • Link Layer or MAC layer (link layer and MAC layer) • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include congestion control, media access, error detection/correction • Media access. The “air” is a shared medium. If two nodes transmit at the same time, there will be a collision. Thus, a scheme must be developed to determine which node transmits when. • Error detection/correction. If interference does occur, then errors might occur. If an error is detected, then • the error could be corrected with forward error correction, or • the receiving link could request a retransmission The protocol stack

  22. Layers 2-4 client server • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols Top down approach of breaking problems into small pieces • Transport layer • Reliability: The server must make sure that the client gets the data Congestion control (or lack there of) • Congestion Control: The server should send data as fast as possible, but not too fast • TCP provides these features (services), while UDP does not • Network layer (could be called the routing layer, but it isn’t) • The packets must find their way through the network. • Each packet has the IP address of the destination • By examining the IP address, routers decide where to send the packet next • Link Layer or MAC layer • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include • congestion control, media access, error detection/correction • Physical layer • Logical bits are encoded as physical quantities, e.g., as voltage levels, as shifts in phase, … • This course does not cover the physical layer The protocol stack

  23. Protocols TCP connection response Get http://www.awl.com/kurose-ross <file> time • End-hosts • Applications • Web • Email • File transfer • File sharing • Packets • TCP • UDP • Routers and gateways and groups of routers (ISPs) • Links • Fiber • Coaxial • Twisted pair • Wireless • Protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Hi TCP connection request Hi Got the time? 2:00 The protocol stack

  24. Internet protocol stack application: supporting network applications FTP, SMTP, HTTP transport: process-process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire” application transport network link physical The protocol stack

  25. ISO/OSI reference model presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions session: synchronization, checkpointing, recovery of data exchange Internet stack “missing” these layers! these services, if needed, must be implemented in application needed? application presentation session transport network link physical The protocol stack

  26. Layers 1-5 (7) • Why is L3 Communications called L3? • What does the L7 filter web page discuss? Why is it called L7

  27. switch router pkt pkt pkt pkt pkt pkt pkt pkt pkt pkt pkt pkt pkt pkt pkt pkt It was a dark and… pkt It was a dark and… pkt pkt pkt pkt pkt pkt pkt pkt It was a dark and… It was a dark and… pkt pkt pkt Internet components

  28. Today – networking basics • Core components of the Internet – the protocol stack • Multiplexing, circuit switching, and packet switching • Loss and delays • The structure of the Internet • This lecture covers much of chapter 1 in the textbook.

  29. Circuit switching versus Packet switching • Packet switching brought the networking revolution • Circuit switching • Virtual circuit networking • A half-way point between packet switched and circuit switched networking

  30. Circuit switching • Circuit switching • Old style phone system • Each connection gets its own wire or bandwidth • Note: calls must be set-up. • E.g., • Me: operator, get my the president. • Operator: one moment please. • Then she plugs a cable into a socket so now I have a physical wired between me and the president. • Instead of each connection getting a whole wire, connections can share a wire via multiplexing • The first automatic circuit switching was developed by Almon Strowger – an undertaker. There were two undertakers in a small town and the switch board operator was the wife of the other undertaker. So Strowger invented an automatic circuit switch to rid both husband and wife of employment. Division of resources: FDM, TDM

  31. Frequency division multiplexing phone end office 300 100300 200300 300 3400 103400 203400 3400 On each hop, the connection gets its own bandwidth toll office end office phone Frequency division multiplexing is used in • TV & radio • Cell phones (not so much today) Division of resources: FDM, TDM

  32. Time division multiplexing 1 byte for each channel every 1/8000 seconds Or 24×7×8000+overhead = 1.544Mbps (DS1 or T1) phone bytes 1 byte for each channel every 1/8000 seconds Or 28×24×7×8000+overhead = 44.736Mbps (DS-3) 4 3 2 1 4 3 2 1 2 1 3 4 3 2 1 1 byte every 1/8000 seconds Or 7×8000=56Kbps (1Kbps=1000bps) (7 bits of data & 1 bit of control) There are standard bit-rates that support multiplexing different numbers of calls Multiplex 28 DS1 = 28*24*64kbps + overhead = 44.736Mbps DS-3 Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1 STS is electrical and oc is optical OC3 = 155.52Mbps (150.336 payload) OC12 = 633.08 Mbps (601.344 payload) OC48 = 2.488Gbps (2.405Gbps) OC192 = 9.953Gbps (9.6Gbps payload) Division of resources: FDM, TDM

  33. Time division multiplexing 1 byte for each channel every 1/8000 seconds Or 24×7×8000+overhead = 1.544Mbps (DS1 or T1) phone bytes 1 byte for each channel every 1/8000 seconds Or 28×24×7×8000+overhead = 44.736Mbps (DS-3) 4 3 2 1 4 3 2 1 2 1 3 4 3 2 1 1 byte every 1/8000 seconds Or 7×8000=56Kbps (1kbps=1000bps) (7 bits of data 1 bit of control) There are standard bit-rates that support multiplexing different numbers of calls Multiplex 28 DS1 = 28*24*64kbps + overhead = 44.736Mbps DS-3 • Note all the control overhead: if the bit is 1, then payload is control. • Lots of control is needed to setup a circuit. How is it possible to get channels at each hop? • Also, if there is not data, then nothing is sent. This wastes data. • But the circuit is yours, guaranteed! Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1 STS is electrical and oc is optical OC3 = 155.52Mbps (150.336 payload) OC12 = 633.08 Mbps (601.344 payload) OC48 = 2.488Gbps (2.405Gbps) OC192 = 9.953Gbps (9.6Gbps payload) Division of resources: FDM, TDM

  34. Packet switching - Statistical multiplexing Packet format specification specifies where the address is data 1 client Server: address = 1 • Data is in packets, not streams. • Must be digital • Each packet has an address • A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward Division of resources: Packet Switching

  35. Packet switching - Statistical multiplexing data data data data 1 1 1 1 • Data is in packets, not streams. • Must be digital • Each packet has an address • A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward If destination is 1, then next hop is C If destination is 1, then next hop is B B If destination is 1, then next hop is A C D client Server: address = 1 F E Division of resources: Packet Switching

  36. Packet switching - Statistical multiplexing • Data is in packets, not streams. • Must be digital • Each packet has an address • A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward • No reservations are needed. First come first serve. • Major benefit: • If you need more bandwidth, then you can get it, it you don’t need it, then maybe someone else can use it. • Major drawback: • What happens if two packets arrive at a switch and both need to go to the same output interface. Picture. One packet is either dropped, or is placed in a buffer. Either way, something bad has happened, the packet is gone or is delayed. This would never happen on a circuit switched network.  queuing delay and packet loss  Division of resources: Packet Switching

  37. Packet switching - Statistical multiplexing other data data 1 1 1 • Data is in packets, not streams. • Must be digital • Each packet has an address • A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward client Server: address = 1 other client Division of resources: Packet Switching

  38. Packet switching - Statistical multiplexing other other other other data data data data 1 1 1 1 1 1 1 1 • Data is in packets, not streams. • Must be digital • Each packet has an address • A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward client Server: address = 1 other client Division of resources: Packet Switching

  39. Packet switching - Statistical multiplexing • Data is in packets, not streams. • Must be digital • Each packet has an address • A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward • No reservations are needed. First come first serve. • Major benefit: • If you need more bandwidth, then you can get it, it you don’t need it, then maybe someone else can use it. • Major drawback: • What happens if two packets arrive at a switch and both need to go to the same output interface. One packet is either dropped, or is placed in a buffer. Either way, something bad has happened, the packet is gone or is delayed. This would never happen on a circuit switched network.  queuing delay and packet loss  Division of resources: Packet Switching

  40. Packet vs. Circuit Switching If usage is random (e.g., web surfing) statistical multiplexing is better. Suppose that • We have a 5Mbps link • Each user needs 50kbps • And each user is active 20% of the time. (Note that this condition does not matter for circuit switching. Why?) How many users can be accommodated under circuit switching and how many can be accommodated under packet switching? Circuit switching case The total number of users that can be accommodated with circuit switching is 5×106/50×103 = 100 users Division of resources: Packet Switching

  41. Packet Switching Case Is the number of ways that you can select k out of n Now suppose there are 200 users, what is the probability that there are 150 or more active users? In this case, there would be a problem, since the network cannot support more than 100 active users. Simpler questions: What is the probability of 150 particular users being active and 50 other being inactive? How many different ways can I select these 150 active users? Is the number of ways that you can select 150 people out of 200 The probability of any 150 users being active and the rest in active is This is the Binomial distribution Division of resources: Packet Switching

  42. Packet Switching Case What is the probability of more than 100 users being active? The probability of 101 users being active plus, 102 users being active, plus …., plus 200 users being active, which is This is the binomial complementary cumulative distribution We conclude that if there are 200 users, then in “pretty much always” things will work fine Still pretty good Suppose that there are 300 users: Might be acceptable performance (if there is some other mechanism to recover!) Suppose that there are 400 users: Therefore: circuit switching could support 100 users, while packet switching can support 400 users. A factor of 4 more!!! Division of resources: Packet Switching

  43. Packet Switching vs. Circuit Switching A couple of things: What does this probability really mean? This means that • when you walk into the switching center, the probability of finding overload is 10-8. • Or, if you random access the link, the probability of finding it in overload. • Once you find it in overload, or not, the probability that is will be in overload in the next second is more complicated and requires queuing theory. This analysis might reveal worst performance. In this example, we assumed 20% user utilization (they were active 20% of the time) If it the user utilization is smaller, then the difference between packet switching and circuit switching is even greater. But if it is larger, then there is less of a difference. What is your user utilization? • For web surfing • For cell phone usage • VoIP call • For music streaming • P2P Division of resources: Packet Switching

  44. Packet Switching vs. Circuit Switching • If loss and delay are permissible and usage is random, then packet switching is better than circuit switching. • If usage is very regular (e.g. TV!), circuit switching is best. • If losses and delay are not permissible, then circuit switching is best (e.g., remote controlled surgery). • With packet switching, congestion control is required. Also, there is more overhead for each packet. • For circuit switching, once the circuit is setup, it can be very efficient. But circuits must be set-up. • So, for short file transfer, packet switching is good but for long file transfers, circuit switching might be better. Division of resources: Packet Switching

  45. Packet Switching vs Statistical Multiplexing • There is a subtle difference between packet switching and statistical multiplexing. • Statistical multiplexing means to use the resource as needed. • This leads to the performance improvements mentioned but also the complications (delay and loss). • Statistical multiplexing requires packet switching to put data into chunks • Circuit switching can work with data packets/chunks, but there is no need for an address • The phone network uses circuit switching, but the circuits are statistically multiplexed between users. • In packet switching, links are statistically multiplexed. Division of resources: Packet Switching

  46. Packet Switching: Statistical Multiplexing Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand  statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. D E 100 Mb/s Ethernet C A statistical multiplexing 1.5 Mb/s B queue of packets waiting for output link Division of resources: Packet Switching

  47. Time division multiplexing 1 byte for each channel every 1/8000 seconds Or 24×7×8000+overhead = 1.544Mbps (DS1 or T1) phone bytes 1 byte for each channel every 1/8000 seconds Or 28×24×7×8000+overhead = 44.736Mbps (DS-3) 4 3 2 1 4 3 2 1 2 1 3 4 3 2 1 1 byte every 1/8000 seconds Or 7×8000=56Kbps (1kbps=1000bps) (7 bits of data 1 bit of control) There are standard bit-rates that support multiplexing different numbers of calls Multiplex 28 DS1 = 28*24*64kbps + overhead = 44.736Mbps DS-3 • Note all the control overhead: if the bit is 1, then payload is control. • Lots of control is needed to setup a circuit. How is it possible to get channels at each hop? • Also, if there is not data, then nothing is sent. This wastes data. • But the circuit is yours, guaranteed! Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1 STS is electrical and oc is optical OC3 = 155.52Mbps (150.336 payload) OC12 = 633.08 Mbps (601.344 payload) OC48 = 2.488Gbps (2.405Gbps) OC192 = 9.953Gbps (9.6Gbps payload) Division of resources: Packet Switching

  48. Packet-switching: store-and-forward takes L/R seconds to transmit (push out) packet of L bits on to link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link delay = 3L/R (assuming zero propagation delay) Example: L = 7.5 Mbits R = 1.5 Mbps transmission delay = 15 sec L R R R more on delay shortly … Division of resources: Packet Switching

  49. Today – networking basics • Core components of the Internet – the protocol stack • Multiplexing, circuit switching, and packet switching • Loss and delays • The structure of the Internet • This lecture covers much of chapter 1 in the textbook.

  50. Losses and delay in packet switched networks A B packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers • Losses • Transmission losses • In fiber links, bit-error is 10-12 or better (i.e., less). • What is the probability of packet error when there are 1500 bytes in a packet? • 1 - (1 - 10-12)1500×8 = 1.2×10-8 • In wireless links, the bit-error rate can be very high • Congestion losses. • If too many packets arrive at the same time, then the buffers will fill up and packets are lost. • Increasing the link speeds or reducing the number of users can reduce the probability of loss. • Increasing the size of the buffer reduces losses, but also increases delay. • Delay • Queuing delay • Transmission delay • Propagation delay • Processing delay Loss and Delay

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