Quality of service Flashcards
Networks mutlimedia applications
Different features with respect to traditional applications
Features of streaming:
- continuos flow of data
- Profile of generated flow must be the same profile to playback → continuos playout
Features of interactivity:
- With other humans/terminals
- short reponse time
General features:
- large trasmission bandwidth
- group communications: many to many communications
Multimedia requirements on the network
Streaming:
- limited loss (some app. tolerate it)
- constant delays
Interactivity:
- Low delay
Large Transmission bandwidth:
- High Resource Availability
- Transmission capacity
- Memory in network nodes
- Processing power
- Switching
Main problem of multimedia network
DELAYS.
Countermeasures:
- Traffic classification
- Sophisticated scheduling algorithms
- WFQ, RR, WRR, CBQ
- Control on traffic entering the network at various levels
- (QoS) routing
In other words:
- Limit the amount of packets arriving at network nodes
- Handle apprioriately packets need specific QoS
Levels of Control on Traffic
Packet level
- shaping/policing
Call/flow level
- Signaling with resource reservation
- RSVP (IP)
- UNI (ATM)
A priori
- Network engineering
- Network dimensioning according to estimates of traffic
- Limit number of users
- Traffic engineering:
- controlled distribution of traffic accross the network
QoS Support: Classification
Identify packets to which quality is to be guaranteed.
In other words: in which queue to store packets.
Based on information inside TCP/UDP and IP headers:
- Destination IP address
- Source IP
- Transport portocol
- Dest./source port
Classification is based on complex algorithms, to reduce forwarding time, routers integrate ASICs and CAM (content addressable memory).
QoS Support: packet scheduling
- Simple queuing: FIFO
- Statistical multiplexing
- Multiple queues and scheduling
Scheduling algos:
- priority queuing
- round robin
- class based queuing
- weighted fair queuing
- Deadline queuing
- It is not possible to satisfy everyone
Switching is a limited resource
- guaranteed immediate switching requires speedup
- the transfer speed of the switching fabric is higher than the input link speed
- Particularly critical when operating with high capacity links
QoS Support: Control on traffic
- Policing and shaping
- makes sure the traffic entering the network has the expected profile
- Token bucket
- capacity: b tokens
- token generation frequency: r token per second if bucket is not full
- on a interval of length t: admitted packets are <= r*t + b
- Non conformant packets are
- delayed (made conformant)
- discarded
- set at a lower priority (possibly best effort)
- CAC (Call admission control)
- Signalling
- Description of generated traffic
- description of required service
- RSVP, UNI
- Resource reservation
- Signalling
QoS routing
- Routing decisions based on resource availability, not only topological information
- instability with connectionless data transfer
*
Network engineering and traffic engineering for QoS
Preventive actions:
- Network is dimensioned for almost worst case
- Traffic matrix is determined (traffic distribution)
Actions throughout:
- Networks state is continuosly monitored
- Network dimensioning and traffic matrix can be changed if needed
Distinctive properties:
- Low resource utilization (low efficiency)
- simplicity and scalability
QoS frameworks
IntServ
- Ambitious solution
DiffServ (Lowering ambitions)
IntServ
IntServ
- Ambitious solution
- Per-flow resource reservation
- RSVP
- Guaranteed quality of service
- per-flow queuing inside routers
- Limits
- High complexity
- Low scalability
- State of the art
- standard completed
- implemented by routers vendors
- RSVP message handling
- queuing algos (?)
- unusable on large scale public networks
DiffServ
DiffServ (Lowering ambitions)
- No quality of service guarantees
- no resource reservation
- Different service to different types (class) of traffic: class of service
- DS (DiffServe) field
- per-class queuing
- Uses network engineering aud traffic engineering
- Complexity is moved to the edge router that perform the policing:
- marking in-profile (high-priority) and out-profile packets
- Features:
- low efficiency: large portion of traffic is best effort
- simplicity and scalability
- Increasingly used
- IP telephony
