1 Kommunikatsiooniteenuste arendus IRT0080 Loeng 7 Avo Ots telekommunikatsiooni õppetool, TTÜ raadio- ja sidetehnika inst.

2 Packet switching versus circuit switching Great for bursty data –resource sharing –simpler, no call setup Bad for applications with hard resource requirements –Excessive congestion: packet delay and loss –Need protocols for reliable data transfer, congestion control –Applications must be written to handle congestion How to provide circuit-like behavior? –bandwidth guarantees needed for audio/video apps –Common practice: over-provision packet switching

3 How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn when packet arrives to full queue, packet is dropped (aka lost) –lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all A B packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers

4 Four sources of packet delay 1. nodal processing: –check bit errors –determine output link A B propagation transmission nodal processing queueing 2. queueing –time waiting at output link for transmission –depends on congestion level of router

5 Delay in packet-switched networks 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x10 8 m/sec) propagation delay = d/s A B propagation transmission nodal processing queueing s and R are very different quantities!

6 Nodal delay d proc = processing delay –typically a few microsecs or less d queue = queuing delay –depends on congestion d trans = transmission delay –= L/R, significant for low-speed links d prop = propagation delay –a few microsecs to hundreds of msecs

7 Queueing delay R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay infinite!

8 Transport service requirements of common apps Application file transfer Web documents real-time audio/video stored audio/video interactive games instant messaging Data loss no loss loss-tolerant no loss Bandwidth elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Time Sensitive no yes, 100’s msec yes, few secs yes, 100’s msec yes and no

9 QoS architectures for the Internet Best Effort Integrated Services Differentiate d Services 1.Best effort: no guarantees 2.Integrated Services (IntServ): Per-flow guarantees 3.Differentiated Services (DiffServ): Per aggregate guarantees

10 Edge Router/Host Functions Classification: marks packets according to classification rules to be specified. Metering: checks whether the traffic falls within the negotiated profile. Marking: marks traffic that falls within profile. Conditioning: delays and then forwards, discards, or remarks other traffic.

11 Motivation Traffic sensitive flow can be redirected to long path Loss sensitive flow can be redirected to lossy network path Jitter sensitive flow may be sent along two paths of high jitter/ or on a single path w/ queuing delay and have high jitter

12 Traffic Engineering Objectives Traffic Engineering (TE) concerned with performance optimization The key performance objectives –traffic oriented e.g. minimization of packet loss –resource oriented - optimization of resource utilization e.g. efficient management of bandwidth

13 Objectives (cont’d) Minimizing congestion is a major traffic and resource oriented performance objective Congestion manifest under two scenarios –Network resources insufficient or inadequate Solved by capacity expansion or classical congestion control techniques –Inefficient mapping of traffic streams onto available resources Reduced by adopting load balancing policies

14 Example All links have capacity of What if these are the delay sensitive flows?

15 Voice over IP Issues modify capacity management and routing methods in IP to support IP telephony –delay less than 300ms –loss rate <1% First Model: RSVP Second Model: voice service uses Virtual Private Network

16 RSVP Routing Shortest Path First Shortest Available Path First Widest Available Path First

17 Virtual Private Networks interconnects telephony switches Direct Path Only Success to the Top State-Dependent Routing Approximate State-Dependent Routing

18 Lingid ng.pdf %28telecommunications%29