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The network core 1

The network core 1






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    The network core 1 The network core 1 Presentation Transcript

    • THE NETWORK CORE Mesh of interconnected Routers  The fundamental question: how is data transferred through network?  circuit switching   University of Education Township Lahore  dedicated circuit per call: telephone net packet-switching  data sent through net in discrete “chunks” 1
    • NETWORK CORE Long distance transmission is typically done over a network of switched nodes  Nodes not concerned with content of data  End devices are stations  Computer, terminal, phone, etc.  A collection of nodes and connections is a communications network  Data routed by being switched from node to node  Node to node links usually multiplexed  University of Education Township Lahore 2
    • NETWORK CORE: CIRCUIT SWITCHING End-to-end resources reserved for “call”  link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required    University of Education Township Lahore  3
    • NETWORK CORE – CIRCUIT SWITCHING Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete  Circuit switched network - a network in which a dedicated circuit is established between sender and receiver and all data passes over this circuit.  The telephone system is a common example.  The connection is dedicated until one party or another terminates the connection. University of Education Township Lahore  4
    • CIRCUIT SWITCHING University of Education Township Lahore 5
    • NETWORK CORE – CIRCUIT SWITCHING Dedicated communication path between two stations  Three phases (Establish, Transfer, Disconnect)  Inefficient (for data traffic)  Channel capacity dedicated for duration of connection  Much of the time a data connection is idle  If no data, capacity wasted  Set up (connection) takes time  Once connected, transfer is transparent  Circuit switching designed for voice  Constant Data rate (Both ends must operate at the same rate)  University of Education Township Lahore 6
    • NETWORK CORE - CIRCUIT SWITCHING Multiplexing in Circuit Switched Networks  Multiplexing is a technique, in which a single transmission medium is being shared among multiple users.  Types of Multiplexing  Frequency Division Multiplexing FDM  Time Division Multiplexing TDM  University of Education Township Lahore 7
    • CIRCUIT SWITCHING: FDM AND TDM University of Education Township Lahore 8
    • Output Stream generated by a synchronous time division multiplexer University of Education Township Lahore 9
    • NETWORK CORE: PACKET SWITCHING  Packet switched network        A network in which data is transmitted in the form of packets Multiple users share network resources No dedicated bandwidth is allocated No resources are reserved, resources used as needed Each packet uses full link bandwidth Good for bursty traffic, simpler, no call setup Packets queued and transmitted as fast as possible Packets are accepted even when network is busy, which causes the delivery to slow down University of Education Township Lahore  12
    • PACKET SWITCHING: STATISTICAL MULTIPLEXING 10 Mb/s Ethernet A  C 1.5 Mb/s queue of packets waiting for output link Sequence of A & B packetsD does not have fixed pattern E  statistical multiplexing University of Education Township Lahore B statistical multiplexing 13
    • NETWORK CORE: PACKET SWITCHING The goal of packet switching is to move packets through routers from source to destination  Packets sent one at a time to the network  Two approaches are used:  Datagram Approach  Virtual Circuits Approach  University of Education Township Lahore 14
    • PACKETS FORWARDING  Two broad classes of packet switched networks are:  Datagram Networks  Virtual Circuit Networks Any network that forwards the packet according to the virtual circuit identifier is called a virtual circuit network  Examples are X.25, Frame Relay, ATM technologies  University of Education Township Lahore Any network that forwards the packet according to the destination address is called a datagram network  The routers in the Internet forwards packets according to host destination addresses; hence the Internet is a datagram network.  15
    • PACKET SWITCHING - DATAGRAM  University of Education Township Lahore Datagram Approach:  Each packet is treated independently  No reference to packets that have gone before  Each node chooses next node on path using destination address  Packets with same destination address may not follow same route  Packets may arrive out of sequence, may be lost  It is up to receiver to re-order packets and recover from lost packets  No Call setup  For an exchange of a few packets, datagram quicker  Analogy: driving, asking directions 16
    • PACKET SWITCHING - DATAGRAM The Internet is a Datagram network  Datagram network is not either connection-oriented or connectionless.  Internet provides both connection-oriented (TCP) and connectionless services (UDP) to applications. University of Education Township Lahore  17
    • DATAGRAM NETWORKS A datagram network is not either a connectionless or a connection oriented network.  It can provide connectionless service to some of its applications and connection-oriented service to other applications.  Example  The Internet, which is a datagram network, provides both connectionless (UDP) and connection oriented (TCP) services to its applications  Networks with Virtual Circuits are, however, always connection-oriented.  University of Education Township Lahore 18
    • PACKET SWITCHING - DATAGRAM University of Education Township Lahore 19
    • PACKET SWITCHING: DATAGRAM APPROACH University of Education Township Lahore 20
    • PACKET SWITCHING – VIRTUAL CIRCUITS  University of Education Township Lahore Virtual Circuit Approach:  Virtual circuit packet switched network create a logical path through the subnet  Call request and call accept packets establish a virtual connection  Virtual route remains fixed through the call.  All packets from one connection follow this path.  Each packet contains a virtual circuit identifier instead of destination address to determines the next hop  Not a dedicated path  No routing decisions required for each packet 21
    • SWITCHING TECHNIQUE – VIRTUAL CIRCUIT Preplanned route established before packets sent  All packets follow same route  Similar to circuit in circuit-switching network   Each packet has virtual circuit identifier    Not dedicated path, as in circuit switching    Nodes on route know where to direct packets No routing decisions Packet still buffered at node and queued for output Routing decision made on before that virtual circuit Network may provide services related to virtual circuit  Sequencing and error control Packets should transit more rapidly  If node fails, all virtual circuits through node lost University of Education Township Lahore  Hence virtual circuit  22
    • PACKET SWITCHING: VC APPROACH University of Education Township Lahore 23
    • VIRTUAL CIRCUITS VS. DATAGRAM VC        No call setup phase  Better if few packets More flexible  Routing can be used to avoid congested parts of the network More reliable  If a node fails, packets may find an alternate route that bypass that node More Processing Delay at a node University of Education Township Lahore  Network can provide sequencing and error control Packets are forwarded more quickly  No routing decisions to make Less reliable  Loss of a node looses all circuits through that node Less Processing Delay at a node Datagram 24
    • CIRCUIT SWITCHING CIRCUITS VS. CS   VC   Route  No dedicated path is established. Only a route is defined. Each switch creates an entry in its routing table for the duration of virtual circuit Shared Links  The link that makes a route can be shard by other connections University of Education Township Lahore  Path  A dedicated path is established between two devices for the duration of session. Reserved Resources  The link (multiplexed / not multiplexed) that makes the path are dedicated, and cannot be used by other connections constant data rates VIRTUAL 25
    • FEATURES OF CIRCUIT AND PACKET SWITCHING University of Education Township Lahore 26
    • NETWORK TAXONOMY University of Education Township Lahore 27
    • NETWORK ACCESS    University of Education Township Lahore Network Access:  The physical link that connects an end system to its Edge Router, which is the first router on a path from the end system to any other distant end system. Classification of Network Access:  Residential Access  Connecting a home end system to an edge router  Dial-up modems, DSL, HFC system  Company Access  Switched Ethernet LANs  Mobile Access  Wireless LAN (802.11b)  Wide Area Wireless Access Networks (GPRS, 3G, WAP) Note: these categories are not hard and fast 28
    • PHYSICAL MEDIA Twisted Pair Cable  UTP Cat 5  Coaxial Cable  Baseband and Broadband Cable  Fiber Optics  Multimode and single mode  Terrestrial Radio Channels  Local Area Radio Channels (Wireless LANs)  Wide Area Radio Channels (WAP, I-mode, 3G)  Satellite Radio Channels  Geostationary Satellites (36000 km)  Low Altitude Satellites  University of Education Township Lahore 29
    • DELAY PACKET SWITCHED NETWORKS  Considering what can happen to a packet as it travels from its source to its destination.   As a packet travels from one node to other node (host or end system), it suffers from several types of delays at each node along the path Most important types of delays are: Processing Delay  Queuing Delay  Transmission Delay  Propagation Delay 
    • TYPES OF DELAY  Processing Delay The time required to process (examine the packet’s header and determine where to direct the packet) is part of the processing delay  Processing delay in high-speed routers is typically on the order of microseconds or less.  After this nodal processing, the router directs the packet to the queue that precedes the link to the next router.  Processing Delay depends on the processing speed of a router. 
    • TYPES OF DELAY  Queuing Delay       At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link. The queuing delay of a packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission across the link If queue is empty, and no other packet is being transmitted, the queuing delay will be zero If traffic is heavy and many other packets are waiting to be transmitted, the queuing delay will be long Thus, queuing delay depends on the intensity and nature of traffic arriving at the queue. Queuing delays can be in the order of microseconds to milliseconds in practice
    • TYPES OF DELAY  Transmission Delay       It is the amount of time required to push an entire packet into the link The time taken by a transmitter to send out all the bits of a packet onto the medium Also called Store and Forward Delay Node receives complete packet before forwarding Transmission Delay is directly proportional to the length of the packet Transmission delays are typically in the order of microseconds to milliseconds in practice
    • TYPES OF DELAY  Transmission Delay Let us denote the length of the packet by L bits.  Denote the transmission rate of the link from Router A to B by R bits/sec  Transmission Delay (L/R) = Packet Length (L) Transmission Rate (R)  Example:   It takes 1 sec to transmit a 10,000 bits packet onto a 10Kbps line. (10,000 / 10 x 1000 = 1) L R R A R B
    • TYPES OF DELAY  Propagation Delay Time it takes a bit to propagate from one node to the next.  The time required by a bit to propagate from the beginning of the link to the next router is called propagation delay  The bit propagates at the propagation speed of the link which depends on the physical medium being used.  It is typically in the range of:    2 x 108 meters/sec to 3 x 108 meters/second In wide area networks, propagation delays are on the order of milliseconds
    • TYPES OF DELAY  Propagation Delay Propagation delay depends on the distance (d) between the two routers/nodes and the propagation speed (s) of the link. Propagation Delay (d/s) = Distance b/w 2 Routers (d) Propagation Speed (s) 
    • TYPES OF DELAY  Total Nodal Delay (the delay at a single router)  If we let dproc, dqueue, dtrans and dprop denote the processing, queuing, transmission and propagation delays respectively, then the total nodal delay is given by: dnodal = dproc + dqueue + dtrans + dprop
    • QUEUING DELAY Queuing delay is most complicated and interested delay as compared to other components of nodal delay (processing, transmission, propagation)  Queuing delay can vary from packet to packet   Example: if ten packets arrive at an empty queue, the first packet will suffer no queuing delay while the last packet will suffer large queuing delay
    • QUEUING DELAY  Queuing delay depends on:       Average Rate at which the packets arrives at a queue (a = packets/sec) Transmission Rate of the link (R = bits/sec) Nature of the incoming traffic (bursty/periodic) Assume that all the packets are of equal length say L bits Then the average rate at which the bits arrive at the queue will be La bits/sec Traffic Intensity = La/R  This ratio helps in estimating the extent of queuing delay
    • TRAFFIC INTENSITY  Traffic Intensity  If La/R is > 1 It means that the average rate at which the bits arrive at the queue exceeds the rate at which the bits can be transmitted from the queue.  In this undesirable situation, the queue will tend to increase without bound and the queuing delay will reach to infinity!   A golden rule in traffic engineering  “Design your systems so that the traffic intensity is no greater than 1s”
    • TRAFFIC INTENSITY  Traffic Intensity  If La/R is > 1 If the traffic intensity is close to one, there will be intervals of time when the arrival rate exceeds the transmission capacity and a queue will form  As the traffic intensity approaches 1, the average queue length gets larger and larger   If La/R is < 1 If the traffic intensity is close to zero, then the packets arrivals are few and far between, and it is unlikely that an arriving packet will find another packet in the queue  Average queuing delay will be close to zero 
    • TRAFFIC INTENSITY Average Queuing Delay 0 1 Traffic Intensity (La/R)
    • PACKET LOSS In reality a queue has a finite capacity  As the traffic intensity approaches 1, a packet can arrive to find a full queue.  With no place to store such a packet, a router will drop that packet; that is the packet will be lost  The fraction of lost packets increases as the traffic intensity increases  Thus, a node performance also includes the probability of packet loss  A lost packet may be retransmitted on an end-toend basis, either the application or transport layer protocol. 
    • END-TO-END DELAY  The total delay from source to destination is referred to as end-to-end delay  Example:  Suppose that the queuing delay is negligible as the network is uncongested, then the end-to-end delay between the source and destination having N-1 routers in between will be: dend-end = N (dproc + dtrans + dprop ) L R R R
    • DELAYS AND ROUTES IN THE INTERNET  Traceroute        A program that sends multiple special packets towards the destination As these packets work their way towards the destination, they pass through a series of routers. When a router receives one of these special packets, it sends a short message back to the source. This message contains the name and address of the router http://www.traceroute.org For Details: Consult Traceroute: RFC 1393 To Do: Explore the Netstat tracert commands
    • LAYERED ARCHITECTURE  Design Philosophy of Layered Architecture       The complex task of communication is broken into simpler sub-tasks or modules Each layer performs a subset of the required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers Helps in troubleshooting and identifying the problem
    • INTERNET PROTOCOL STACK Application Transport Network Data Link Physical
    • TCP/IP PROTOCOL SUITE  Application Layer    Transport layer (End-to-end Communication)    Routing of datagrams from one host to another IP works on this layers Data link Layer (Node-to-node Communication)    Two transport layer protocols (TCP and UDP) Transports messages between client and server applications Network Layer (Host-to-host Communication)    Responsible for supporting network applications Protocols include: HTTP. SMTP, FTP etc. Logical interface between end system and network Examples: Ethernet, PPP, ATM and Frame Relay technologies Physical Layer   Transmission medium Signal rate and encoding