Facts About Soft RTS

Transcription

1 Soft - Firm RTS Recall RTS Definitions A Soft RTS is one in which performance is degraded but not destroyed by failure to meet response time constraints (Laplante) Examples Typical Desktop PC Multimedia devices Eg. VoIP service i.e. public IP network Unmanaged non deterministic Note: For MM data, requirement for logical correctness of output can be relaxed somewhat (See G.1010) Recall Unclear distinction between Soft and Firm RTS Firm RTS A Firm RTS is one in which a few missed deadlines will not lead to total failure, but missing more than a few may lead to complete and catastrophic system failure failure (Laplante).. Written with Hard RTS in mind.. Distinction between Soft/Firm typically based on consequences of missed deadlines Eg. Private IP Network governed by SLA SLA may specify jitter/delay/loss/availability Lack of adherence results in :» Irate customers loss of business» Penalties imposed on provider Mobile Phone Technology / Cameras / Games Consoles etc E-Business eg. Reservation Systems 1

2 Soft Firm RTS Embedded Systems Multimedia Systems (MMS) QoS is key issue for MMS QoS in Communication Networks LAN based and WAN based MM Main focus for remainder of course LAN : QoS Support Integrated voice (media rich) /data systems WAN: QoS Support Many issues with public IP Networks Emergence of private IP networks with QoS Support QoS in MM Terminal Move towards RTOS functionality Other S/W and H/W hardware issues : Application Design, Driver performance, buffer design, skew ITU-T G.1010 Useful guideline QoS Much abused term.. broad application Intrinsic versus Perceived QoS Intrinsic (Network) QoS Relates to technical performance of network Packet loss Packet delay Packet jitter : variation in delay (Above 3 influenced by bandwidth) Other critical issues Availability (down time)» POTS designed with % line-available target Connection setup time QoS Perceived QoS: Same Intrinsic QoS may be perceived very differently by different users Influenced by expectations and past experience Expectations rarely drop! Eg. Mobile phone technology» Coverage, voice quality, availability expectations Internet connectivity» Bandwidth.. What does broadband mean?» What will it mean in 5 years?» Contention rates often overlooked» Narrowband (POTS-GSM) versus Wideband codecs» (eg. Skype can use both) G.1010 will need to be continuously updated 2

3 G.1010 Summary Error tolerant Conversational voice and video Voice/video messaging Streaming audio and video Fax Error intolerant Command/control (e.g. Telnet, interactive games) Transactions (e.g. E-commerce, WWW browsing, access) Messaging, Downloads (e.g. FTP, still image) Background (e.g. Usenet) Interactive (delay <<1 s) Responsive (delay ~2 s) Timely (delay ~10 s) Non-critical (delay >>10 s) T G.1010 MM based Soft RTS primarily deal with delay insensitive applications Often are Loss Tolerant Human comprehension can tolerate/compensate for limited loss without significant QoS impact May also have strategies to deal with degree of packet loss Basis for use of UDP versus TCP Note: Many streaming applications use TCP» Delay not an issue as non interactive» Eg Internet Radio Service 3

4 Internet MM Protocols RTP/RTCP (RFC 1889) Delivery of media streams across packet network RTP Deals with some shortfalls of using UDP Sequence numbers o/o/o delivery Detection of lost packets Media-specific timestamps 8000Hz typical for audio (POTS standard) Used for reconstruction of media Note: Silence Detection or VAD No packets sent during silences No assumption about sender/receiver clock synchronization Marker bit to indicate start of talkspurt RTP Header Internet MM Applications RTCP Companion Control Protocol to RTP RR (Recv Reports) and SR (Sender Reports) RR Provides feedback Packets lost / Jitter Enables RTD to be calculated SR Facilitates Lip Synch Mapping between RTP timestamp and system clock time (NTP) Audio/Video streams must be tightly synchronised Audio lead versus audio lag.. Which is best? If NTP implemented Facilitates one way delay measurement Skew elimination 4

5 Internet MM Applications Session Control eg. set-up/disconnect ITU-T H.323 or IETF SIP(Session Initiation Protocol) Alternative approaches SIP gaining ground Interoperability possible Messages carried over TCP TCP (control) plus UDP (media) channels Media Capability Issues Agree on codec for data transport RTP-RTCP RFC

6 Packet Encapsulation H.323 Based VoIP Session VoIP Huge growth in recent years POTS VoIP Deterministic Synchronised Network Non deterministic IP network Dumb Complex Terminals Many challenges with both Streaming vs Interactive Audio Very different requirements VoIP: M2E delay critical G.114/G msec one-way limit Good Echo cancellation facilitates higher delays Strategies reqd to minimise delay / jitter and loss 6

7 VoIP M2E Delay Sender Packetisation delay Encoding (+ look ahead) delays OS/Applic/Driver s/w NIC serialisation delay Network Propagation delay Queuing delays Processing/serialisation delays in routers Receiver NIC delays OS/Applic/Driver s/w Jitter buffer delays Decoding delays VoIP QoS Strategies Sender-based RTCP feedback with adaptive codecs If loss/delay excessive, switch to lower b/w codec? FEC Network-based IntServ (RSVP), DiffServ, MPLS Prioritising delay sensitive traffic flows Receiver-based buffering strategies Human ear very sensitive to short term variations Need to playout voice as close as possible to sampled voice Buffer absorbs variation in network queuing delays BUT.. adds to overall M2E delay tradeoff 7

8 Jitter Buffer Strategies Buffer Playout Delay adds to M2E delay Above strategy Pkt 8 too late for playout normally dropped Increase size of buffer in response to increasing delays? Jitter Buffer Strategies Fixed buffer size: limitations Too large extends overall delay (perhaps beyond G.114) Too small Additional late packet losses due to late arrival Adaptive buffer size Adapt to network conditions Per talkspurt or per packet Mostly operate by elongating/shortening inter talkspurt silence periods less noticeable 8

9 Network QoS LAN Environment Huge growth in integrated voice/data networks WAN Environment Public IP i.e. Internet Private IP networks ATM based solution Never succeeded in LAN environment WAN Deployment ATM System Architecture End Station Switch End Station Voice Data Video A A L A T M P H Y P H Y A T M Cells Adaptation Layer (AAL): Inserts/extracts information into 48 byte payload ATM Layer: Adds/removes 5 byte header to payload Physical Layer: Converts to appropriate electrical or optical format P H Y P H Y A T M A A L Voice Data Video QoS in LAN Environment Integrated voice / data services across Enterprise LANs Original Ethernet Non deterministic Poor performance under high loads Switched LANs Full duplex High Speed Backplane RTOS Engine Deterministic & QoS Support Cost effective Facilitates new generation of applications Collaborative workspaces : voice + video +..etc.. Better speech quality possible via wideband voice codecs Gateway enables connection to POTS Recall Moneypoint : Industrial Switched Ethernet 9

10 CSMA-CD LAN Switching 10

11 QoS in LAN Environment Switched LANs typically QoS enabled Classify traffic according to : TL port number or Protocol (TCP vs UDP) NL IP address, VLAN DLL MAC address Physical port number Specify treatment for each classification Simple priorities Bandwidth min & max levels Eg. UDP traffic (i.e. voice) from designated ports guaranteed 2% min traffic 2% of 100Mbps= 2Mbps» Recall PCM G.711 voice = 64kbps QoS in LAN Environment VLAN Logical rather than physical grouping of machines VLANs used to reflect organisational structure Easy to reconfigure Facilitates greater security Limits broadcast traffic (ARP)» Contained within each VLAN Less scope for active attacks VLAN QoS May be configured according to VLAN grouping VLANs can span multiple switches and physical locations Configuration By switch port / MAC address / IP address VLAN IEEE 802.1Q (1998) Simplifies multi switch environment Each switch needs to know VLAN membership of nodes on other switches VLAN tag Extra 4 byte header field Includes VLAN Identifier Useful to identify VLAN members across multiple switches Becoming more prevalent Note: Fast/Gigabit Ethernet links rarely congested No need (yet) for QoS policies? What Andy giveth.. Bill taketh away! 11

12 Other LAN Based Protocols Token Bus Protocol IEEE Token circulates around logical ring Node can only transmit when in possession of token Different priority levels Developed for production line environment Significant improvement on standard Ethernet Basis for Moneypoint CS275 Bus Upgrades: Switched LAN provides similar capability Largely superceded QoS in WAN Environment Internet default best-effort service TCP provides a reliable bit stream Connection oriented Reliable service Flow control but.. No guarantees on available bandwidth / loss / delay / jitter TCP adds significant overhead and delay (for retransmissions) Non deterministic Core Issue for realtime applications» use of UDP to limit delay Various initiatives to provide IP-based WAN QoS Distinction between Internet and private (managed) IP networks Emergence of SLA (Service Level Agreements) 12

13 QoS Approaches Increase bandwidth LAN solution: Fast / Gigabit Ethernet No real need yet for QoS features? Last mile: Home Environment ADSL / 3G Overprovision networks Costly, limited and temporary solution? Reservation policy Similar to circuit switched POTS approach Implementation / scalability issues Requires admission control policy Traffic categorisation & prioritisation Simpler to implement Category 3 UTP Characteristics 13

14 IntServ IETF IntServ (Integrated Services) RSVP (Resource Reservation Protocol rfc 2205) Resources reserved along path prior to media delivery Routers need to maintain this information for duration of session Similar to virtual circuit Guaranteed and Controlled load service Guaranteed tight adherence to specification Controlled load service equiv to lightly loaded network DiffServ IETF DiffServ (RFC 2475) Prioritise traffic Traffic classification and policing carried out at edge of network Recall IPv4 TOS and IPv6 Traffic Class fields Renamed DS Codepoint field (DSCP) DSCP interpreted by routers in terms of PHB (per hop behaviour) Simpler to implement within network PHB: Expedited Forwarding : Service equiv to a leased line Realtime traffic has reserved bandwidth eg. VoIP Assured Forwarding: 4 priority classes each with 3 different drop precedence values DiffServ EF PHB 14

15 DiffServ AF PHB Classifier Packets sorted into 4 priority classes Sorting may be based on IP address Transport Layer Protocol etc. Marker Packet DSCP marked to indicate priority level Shaper/Dropper Shapes traffic flows into acceptable profile May be governed by a Service Level Agreement Packets proceed into DiffServ aware network DiffServ AF PHB MPLS MPLS : Multi-Protocol Label Switching (RFC 3031) New switching architecture rather than purely QoS focus Works with Frame Relay / ATM or IP Sometimes known as level 2.5 Labels packets according to FEC (Forwarding Equivalent Class) FEC determined at point of ingress to network New label attached at each router which will be correctly interpreted at next downstream router 15

16 MPLS Packet Loss Strategies In absence of Network QoS Use of UDP limits delays but leads to packet loss Need to compensate Use of FEC & PLC Forward Error Correction (FEC) Form of Information Redundancy Media Specific FEC Lower bandwidth version sent» Lower Quality Eg. Dual codec capability Media Indep FEC: Exact replica sent Packet n also sent with Pkt n+1, n+2 etc» If Pkt n lost, must wait for Pkt n+1.. Or n+2 Both strategies deliver correctness at cost of increased delay and bandwidth utilisation Still quicker and more suitable than TCP Packet Loss Strategies Packet Loss Concealment PLC Various Strategies: Silence : simplest Repetition : repeat last packet Interpolation Packet n, n+ 2 arrived ok» Reconstruct n+1 using both» Processing power reqd can be significant eg. WSOLA delay also 16

17 MM Terminal QoS Issues Underlying OS Need for Realtime support Application Software Driver Software Driver fragment Application Packet Mismatch will lead to further delay jitter Jitter Buffer Design Skew Lack of synchronisation between system/media clocks Dedicated IP terminals perform better than PC based Terminal Clock Skew Terminals may have multiple clocks System Clock to measure actual/trends in delay Used to optimise playout buffer Media Clock Used for sampling/playback Simplest Case Media Clock Skew Buffer Underfill Discontinuity Buffer Overflow High Delay & Packet loss 17

18 Media Clock Skew Other Clock Skew Scenarios Gateway scenario Gateway must deal with multiple concurrent streams Mixer Skew will result in cumulative timing error & lack of alignment Wireless Speaker Environment End terminal has to deal with Varying delay characteristics Skew differences between speakers Haas effect 18

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