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Transcript
ICSA 745 Transmission Systems Foundations for Emerging Technologies Nirmala Shenoy Dept. of Information Technology Rochester Institute of Technology Scope • Virtual circuits • Connection oriented and connectionless services • Variable and constant bit rate services • Flow control and congestion management Scope • • • • User payload integrity management Address and identification schemes Multiplexing methodologies Switching, routing and relaying Virtual Circuits • Physical circuit – two users communicate directly with each other through a circuit • Virtual circuit – User unaware of sharing – Intermediate systems relay information as they arrive Types of Virtual Circuits • Permanent Virtual Circuit - PVC Allocated by contract • Switched Virtual Circuits - SVC allocated on demand • Semi-Permanent Virtual Circuits – SPVC – Not guaranteed continuous service Permanent Virtual Circuits • Requires a contract agreement • Provisioned on a continuous basis • Entries stored at intermediate nodes for appropriate handling • Payloads should carry assigned identifier • Quality of service - throughput, delay, security, error-rates, etc. specified Switched Virtual Circuit • Not pre-provisioned • Call setup essential • Connection request -virtual circuit number allocation, connection tear down • Virtual circuit number is used during the session to route information – • Resources released when session is over Semi-permanent Virtual Circuits • Pre-provisioned similar to PVC • Does not guarantee that users will obtain level of requested service Connection-Oriented System • Systems using virtual circuits • Connection setup, connection tear down • Forwarding nodes maintain connection information for packet handling • Processing overheads • Normally in-sequence delivery Connectionless Systems • Does not employ virtual circuits • No information maintained by intermediate nodes • Ad hoc processing • Known as datagram networks Comparison • Connection-Oriented • Connectionless – more control over traffic and servicing user’s QOS – Faster payload processing – Low end-to-end delays Foundation updated 12/2001 – No Qos guarantees – Robust – forwarding decisions to be made 11 Coexistence • Telephone networks – throughout connection oriented • TCP/IP based transfers – end-to-end connection oriented, within the network connection less • UDP/IP – connectionless throughout • Protocol Layer based Traffic Characteristics • Different applications have different patterns of traffic – Continuous – Bursty – Highly Bursty Variable Bit Rate • Service preferred by bursty traffic – Periods of lull and periods of heavy traffic • Transmit and receive traffic asynchronously • Loose timing and synchronisation between sender and receiver • Traffic may be queued during bursts • Non-real-time, Real time • Example: email, client-server apps, video(?) Constant Bit Rate • Can be used by services requiring continuous bandwidth – Predictable and bound delay, low jitter • Normally preferred by real-time services • Example: voice, video, video-conferencing (?) Virtual Multiple Networks • Integrated VBR and CBR service support • All traffic is digitized • Manage the different traffic and provide fair and suitable service for all traffic • User perspective – one network Flow Control • Network QoS Requirement – – traffic should not saturate a network – Should not exceed the network’s capacity • Methods of flow control – Explicit – Implicit – None Explicit Flow Control • Network explicitly informs end systems via “explicit messages” on the amount of traffic the end systems can send • Users can increase traffic on congestion clearance Implicit Flow Control • Informs user about congestion – Packet dropping, messages • User should reduce traffic, else data will be dropped • Control messages – user is violating service contract – network is congested No Flow Control • Network discards any traffic that is creating congestion – Simple for network to implement – User service requirements not met • Connectionless networks have traffic regulation mechanisms – carrier-sense collision – issuance of tokens User Payload Integrity Mgmt • All payload correctly received – Network responsibility? – End system responsibility? • Do all applications need this feature? – EFT, voice, video, data – service based? Addresses & Labels • Connectionless systems – Explicit Address – each PDU uses explicit addresses for routing – IP address - network/subnetwork/device • Connection-oriented systems - Labels – uses a short label or Virtual Circuit Identifier – label used to index into state tables to determine the explicit location or forwarding information Multiplexing • Frequency Division Multiplexing (FDM) – each user has fixed portion of freq. spectrum • Time Division Multiplexing (TDM) – each user is given a slot – slots are rotated among the attached users • Statistical TDM (STDM) – dynamically allocate time slots • Code Division Multiplexing – Usage of codes Network Interface • User-to-Network Interface - UNI or SNI – protocol that governs the interface between the user and the network • Network-to-Network Interface - NNI or ICI – protocol that governs the interface between networks Switching, Routing and Relaying • Relaying – forward data as it comes • Switching – comes in at one input is switched on to another • Routing – decision made on which path to take – Source routing – Non-source routing Network Interfaces • UNI or SNI – User network interface, subscriber network interface • NNI or INSI – within network – Network- network interface, intra-network switching interface • NNI or ICI – between networks – inter-carrier interface Convergence Operations • Performed at UNI – Add synchronization and accommodate different transfer rates • Convergence Sublayer - CS • Application specific • Example: error detection, synchronization Segmentation and Reassembly • Segmentation is performed on the user traffic before it enters the network • Reassembly is performed before data reaches end user