Larger Site Networks

Part2

2

Ethernet Virtual LANs

• Hubs versus Switches– Hubs broadcast bits out all ports– Switches usually send a frame out a one port

• More fundamentally– In unicasting, a message is only intended to go to one

machine, as when a client sends a message to a server

– Switches assume unicasting; it is the basis for sending a frame out a single port

3

Ethernet Virtual LANs

• Broadcasting

– Sometimes, station needs to send a frame to all other stations; this is broadcasting

– For example, servers send a frame to advertise their presence with a broadcast message every minute or so

4

Ethernet Virtual LANs

• Broadcasting with Ethernet Switches– Broadcaster sets the destination MAC

address to all ones (48 ones)– When switch sees this address, it broadcasts

frame out all stations– All stations read frames with this address

BroadcastFrame

EthernetSwitch

5

Ethernet Virtual LANs

• Broadcasting is a Problem in Large Switched Networks– Server broadcasts go to all stations, creating

a great deal of network traffic– Create congestion

BroadcastFrame

6

Ethernet Virtual LANs

• In multicasting, messages are only intended to go to some stations– For instance, from a server only to the client

PCs it serves– If Ethernet switches can

implement multicasting,traffic overload wouldbe avoided

MulticastFrame

7

Ethernet Virtual LANs

• Ethernet switches do implement multicasting– A server and the clients it serves are treated as a

single virtual LAN (VLAN)– Can only communicate among themselves, as if they

were on their own LAN

Frame

MarketingVLAN Server

MarketingVLAN Client

8

Ethernet Virtual LANs

• VLAN Benefits

– VLANs reduce traffic on the switched network

– Other benefits

• VLANs give ease of management because if a user changes organizational membership, VLAN membership is easily changed centrally

9

Ethernet Virtual LANs

• VLAN Problems

– Communication across VLANs, when necessary, can be difficult

– Often, routers must be used to connect members of different VLANs

VLAN 1 (Subnet 1)

VLAN 2 (Subnet 2)

Router

10

Ethernet Virtual LANs

• VLAN Problems

– For cross-VLAN communication, routers actually connect multiple switches

EthernetSwitch

11

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

BottleneckEthernetSwitch

12

Bad Switch Organization

• Multiple Servers for Clients– Allows simultaneous conversations– Brings switching’s main benefit

EthernetSwitch

Congestion, Latency, and Remedies

•Peak Loads

•Congestion and Latency

•Overprovisioning Capacity

•Priority

•Quality of Service

•Traffic Shaping

14

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

CapacityTrafficPeak

15

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 CapacityTrafficPeak

16

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-PriorityFrame Goes

Low-Priority FrameWaits Briefly

17

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

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

High-PriorityFrame GoesLow-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 GuaranteeLow 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 GuaranteeReserved CapacityLow or No Guarantee

20

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 GuaranteeReserved CapacityLow or No Guarantee

21

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

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

Source A

Source BNetwork

Slow or Stop

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