When are Frames Forwarded

1
When are Frames Forwarded?

• Store-and-Forward Ethernet Switches
• Forwarded only after receiving full frame
• Allows error checking (CRC field)

Forward the Frame
Pre
SFD
DA
SA
Len
Data
PAD
FCS
2
When are Frames Forwarded?

• Cut-Through Ethernet Switches
• Forward after seeing only part of a frame
• Minimum is destination address to determine
  output port
• May need to see tag fields for priority, VLAN
• May wait until 46 octets of data plus PAD
• Faster operation than store-and-forward

Forward the Frame
Pre
SFD
DA
SA
Len
Data
PAD
FCS
3
When are Frames Forwarded?

• Hybrid Ethernet Switches
• Operate mostly in cut-through mode
• Do check for errors sometimes
• If too many errors, go to store-and-forward mode

4
Bad Switch Organization

• One Server for All Clients
• All traffic goes to and from server
• Bottlenecks no simultaneous conversations
• No major benefits compared to hub

Bottleneck
Ethernet Switch
5
Bad Switch Organization

• Multiple Servers for Clients
• Allows simultaneous conversations
• Brings switchings main benefit

Ethernet Switch
6
Congestion, Latency, and Remedies

• Peak Loads
• Congestion and Latency
• Overprovisioning Capacity
• Priority
• Quality of Service
• Traffic Shaping

7
The Peak Load Problem

• Capacity Sufficient Most of the Time
• Otherwise, get bigger switches and trunk lines!
• Brief Traffic Peaks can Exceed Capacity
• Frames will be delayed in queues or even lost if
  queue gets full

Traffic Peak
Capacity
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Overprovisioning

• Overprovisioning Install More Capacity than Will
  be Needed Nearly All of the Time
• Wasteful of capacity
• Still, usually the cheapest solution today
  because of its simplicity

Overprovisioned Capacity
Traffic Peak
9
Priority

• Assign Priorities to Frames
• High priority for time-sensitive applications
  (voice)
• Low priority for time-insensitive applications
  (e-mail)
• In traffic peaks, high-priority frames still get
  through
• Low-priority applications do not care about a
  brief delay for their frames

High-Priority Frame Goes
Low-Priority Frame Waits Briefly
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Priority

• Standardizing Priority
• 802 Tag Fields are standardizing priority for
  Ethernet and other 802 LAN technologies
• Priority is also being standardized by the IETF
  for IPv4 and IPv6 (Diffserv for differentiated
  services)
• 802 and IETF are harmonizing efforts for
  end-to-end priority

High-Priority Frame Goes
Low-Priority Frame Waits Briefly
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Priority

• Once Widely Available, Priority Should Replace
  Overprovisioned Capacity as the Least Expensive
  Solution to Peak Load Problems

High-Priority Frame Goes
Low-Priority Frame Waits Briefly
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Full Quality of Service (QoS)

• Priority Makes no Quantitative Promises of
  Maximum Latency, etc.
• Quality of Service (QoS) Makes Quantitative
  Promises for such things
• Different Frames may have Different QoS
  Guarantees Some may have no guarantee

High Guarantee
Low or No Guarantee
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Full QoS is Expensive

• For high guarantees, capacity must be reserved at
  each switch and trunk line along the way
• Capacity is wasted if not used
• Like circuit switching but at data link layer
• (Circuit switching is at physical layer)

High Guarantee Reserved Capacity
Low or No Guarantee
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Full QoS is Not a Cure-All

• Traffic with no guarantees will not benefit
• It may not get through at all
• Often, voice traffic is given strong guarantees
  while data traffic is given low or no guarantees

High Guarantee Reserved Capacity
Low or No Guarantee
15
Traffic Shaping

• Overprovisioning, Priority, and QoS are Ways to
  Cope with Brief Congestion
• Traffic Shaping Prevents recognizes that
  congestion is beginning, acts to stop it
• Switch Tells Some Sources to Slow or Stop if
  Congestion is Beginning

Slow or Stop
Source A
Network
Continue
Source B
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Traffic Shaping

• Called Shaping Because Governs the Shape of the
  Traffic
• Mix of traffic of various kinds is governed by
  policies about traffic

Slow or Stop
Source A
Network
Continue
Source B