2Hubs
Single-hub or single-switch LAN 200 meter max distance between
farthest stations with UTP
100 m
100 m
X
Y
200 m
3Multiple Hubs in 10Base-T
Farthest stations 10Base-T can be 5 segments 100 meters per segment Separated by four hubs
100m
100m
100m
100m
100m
500m, 4 hubs
10Base-T hubs
4Multiple Hubs in 10Base-T
No loops allowed Only one possible path between any two
stations
No LoopsA
B
C1
2
34
5
6AB=1,2,3,4,5AC=1,2,3,4,6BC=5,4,6
No!
5Multiple Hubs in 10Base-T
Practical Limit in 10Base-T is Number of Stations
Degradation of service beyond 100 stations
Unacceptable service beyond 200 stations
Maximum possible span normally embraces more than 200 stations
6Multiple Hubs in 100Base-TX
Limit of Two Adjacent Hubs in 100Base-TX & one hub with Gigabit Hub Must be within a few meters of each
other Maximum span is 200 meters Shorter maximum span than 10Base-T
100m
100m2 Collocated
Hubs
100Base-TXHubs
~200 m
7Ethernet Switched Networks
No Limit in Number of Switches Between the Farthest Stations (No maximum distance span)
Limit between pairs of switches - UTP 100 m, optical : longer distance
No Limit OnNumber of
Switches or Span
EthernetSwitch
8Hierarchies
Ethernet Switches Must be Hierarchical (no loops) Usually, Fastest Switches are at the Top
(Root)Gigabit
EthernetCampusSwitch
100Base-XBuilding Switch
10Base-TWorkgroupSwitch/Hub
Root
9Hierarchies
Single Possible Path (2,1,3,4) Between 2 Stations Single Points of Failure
Switch or Link (trunk line between switches)Divide the network into pieces
EthernetSwitch
A
1 34
5
B
2
X X
10Hierarchies
802.1D Spanning Tree Allows Redundant Links Automatically deactivated to prevent
loops Reactivated if there is a failure
DeactivatedRedundant
Link
EthernetSwitch
11Hierarchies
Link Aggregation Protocol Allows Multiple Links Between Stations If one link fails, others continue Switch failures or cuts of all links still
fatal
MultipleLinks
EthernetSwitch
12Hierarchies
Single Possible Path & Switch Forwarding Tables Frame MAC address = single possible
output port Switch sends frame out that port Switch forwarding table has only one row
for each MAC address
EthernetSwitch Address
A3..B2..
Port35
13Hierarchies
Ethernet switch only has to find the single row that matches the destination MAC address
Only has to examine half the rows on average; less if the table is alphabetized
Comparison at each row is a simple match of the frame and row MAC addresses; much less work that row comparison in routers
Switches work much faster than routers & are lessexpensive
AddressA3..B2..
Port35
14Switch Learning
Situation: Switch with NIC A1-33-B6-47-DD-65 (A1) on Port 1 NIC BF-78-C1-34-17-F4 (BF) on Port 2 NIC C9-34-78-AB-DF-96 (C9) on Port 5
Switch Forwarding Table is Initially Empty
Address Port
A1 BF C9
EthernetSwitch
At Start
15Switch Learning
A1 on Port 1 Sends to C9 on Port 5 Switch does not know port for C9 Broadcasts the frame, acting as a hub Notes from source address that A1 is on Port 1 Adds this information to switch forwarding
table
AddressA1
Port1
A1 BF C9
EthernetSwitch
After Transmission
16Switch Learning
C9 on Port 5 Sends to A1 on Port 1 Table shows that A1 is on Port 1 Switch only sends out Port 1: No broadcast! Source address shows that C9 is on Port 5 Switch adds this information to forwarding
table
AddressA1C9
Port15
A1 BF C9
EthernetSwitch
After Transmission
17Switch Learning
Every Few Minutes, Forwarding Table is Erased To eliminate obsolete information Relearning is very fast
Address Port
A1 BF C9
EthernetSwitch
Erased
18Switch Learning
Switches Can be in Hierarchy Switches only learn that stations are out
certain ports Do not Learn of switch in Between
A1 BF C9
AddressA1BFC9
Port111
Port1
Switch A
Switch B
19Switch Purchasing Decisions
Maximum Number of MAC address-port entries
Small switches may store many MAC addresses
Unknown addresses require broadcasting
Queue Size Incoming frames are placed in queues if
they cannot be processed immediately Small queues result in lost framesSwitch
MatrixQueues
OutputPorts
InputPorts
Frames
20Switch Purchasing Decisions
Switching Matrix Receives input from multiple input ports Switches frame to the correct output
portSwitching Matrix Aggregate Throughput
Maximum number of bits it can switch/second
Nonblocking: aggregate throughput = no. ports X port speed
All ports receive input simultaneously with no delay
QueuesOutputPorts
InputPorts
FramesSwitch Matrix
21Switch Purchasing Decisions
Reliability through Redundancy Redundant power supplies and cooling fans
May even have redundant switch matrix for backup
Manageability Managed remotely from network administrator’s
deskNetwork administrator can check on status of switch
Network administrator can modify how the switch functions
Remote management greatly reduces labor
22Ethernet Virtual LANs
Hubs versus Switches Hubs broadcast bits out all ports Switches usually send a frame out a one
port
Unicasting: message intended for one destination Switches assume unicasting
23Ethernet Virtual LANs
Broadcasting
Broadcasting is occasionally required
Ex: servers send advertisement of their presence
Broadcasting with Ethernet Switches Switch turns single-point delivery off Broadcaster sets destination MAC
address to ones Can create congestion
24Ethernet Virtual LANsMulticasting: messages intended some
stations Ex: from a server only to the clients it
serves If Ethernet switches can implement
multicasting, traffic overload would be avoided
MulticastFrame
25Ethernet Virtual LANsEthernet switches do implement
multicasting A server and its clients are treated as
(VLAN) Can only communicate among
themselves
Frame
MarketingVLAN Server
MarketingVLAN Client
26Ethernet Virtual LANs
VLAN Benefits
VLANs reduce traffic on the switched network
Other benefits
Provide weak security:clients cannot reach all servers (easily defeated but good first line of defense)
Ease of management: user changes organizational membership, VLAN membership is easily changed centrally
27Ethernet Virtual LANs
VLAN Problems
VLANs have not been standardized
A network of switches from different vendors cannot implement VLANs
Standardization is beginning
Using tagging (Chapter 7)
Tag Control Information field has a 12-bit VLAN ID (VID) number, allowing 212 VLANs to be identified
28When are Frames Forwarded?
Cut-Through Ethernet Switches Forward after seeing only part of a
frameMinimum is destination address May need to see tag fields for priority, VLANMay wait until 46 octets of data plus PAD
Fast operation-less latency
PreSFDDASALenDataPADFCS
Forward the Frame
29When are Frames Forwarded?
Store-and-Forward Ethernet Switches Forwarded only after receiving full
frame Allows error checking (CRC field) Brief latency
Hybrid Ethernet Switches Start in cut-through mode but check
errors If many errors, go to store-and-forward
mode PreSFDDASALenDataPADFCS
Forward the Frame
30Bad Switch Organization
One Server for All Clients All traffic goes to and
from server Bottlenecks: no simultaneous
conversations No major benefits compared
to hub
Multiple Servers for Clients Allows simultaneous
conversations Brings switching’s
main benefit
BottleneckEthernetSwitch
EthernetSwitch
31The 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
32Overprovisioning
Overprovisioning: Install More Capacity than Will be Needed Nearly All of the Time Wasteful of capacity Cheapest solution today because of its
simplicity
Overprovisioned Capacity
TrafficPeak
33Priority
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
34Priority
Standardizing Priority 802 Tag Fields are standardizing priority for
Ethernet and other 802 LAN technologies Priority being standardized by the IETF for IPv4 &
IPv6 (Diffserv for differentiated services) 802 and IETF are harmonizing efforts for end-to-
end priority
High-PriorityFrame Goes
Low-Priority FrameWaits Briefly
35Full Quality of Service (QoS)
Quality of Service (QoS) Makes Quantitative Promises for service
Reserves capacity Capacity wasted for bursty
transmissionsNon guaranteed traffic will not benefit &
may be lost voice traffic: strong guarantees data traffic: low or no guaranteesHigh Guarantee
Reserved CapacityLow or No Guarantee
36Traffic Shaping
Traffic Shaping recognizes that congestion is beginning, acts to stop it
Switch Tells Some Sources to Slow or Stop if Congestion is Beginning, based on Policies
Source A
Source BNetwork
Slow or Stop
Continue
37ATM Switches
Asynchronous Transfer Mode
Basic Standards Set by ITU-T Partner with ISO in OSI standards ATM standards developed within OSI
architecture
ATM Forum Sets Detailed Standards Group of mostly ATM vendors Moves quickly Also tests for interoperability
38ATM Switches
Cells of fixed length frame (5 octet header, 48 octet payload)
Small cell reduces latency at each switchOverhead = all bits but payload (5 of 53 or
10%) Not efficient use of transmission capacity
Highly Scalable Comparable to Ethernet
Very sophisticated Offers quality of service guarantees Very expensive to purchase and manage
39ATM Switches
Unfortunately, very expensive Usually cheaper to use high-capacity
Ethernet switches with overprovisioning, so that latency does not grow to the point where QoS is critical
40ATM QoS Categories
ATM Offers Varying Levels of QoS
Parameters Peak cell rate (maximum burst speed) Maximum burst size (bits per burst) Sustainable cell rate (always allowed) Cell Delay Variation Tolerance (CDVT):
how exact cell-to-cell timing is; Critical for voice and video
Cell Loss Ratio: Losses during transmission
41ATM QoS Categories
For Voice and Video ITU-T Class A ATM Forum Service Category: Constant
Bit Rate (CBR) Low latency Low Cell Delay Variation Tolerance
42ATM QoS Categories
For IP and LAN Data ITU-T Class D Several ATM Forum Service Categories
Available bit rate (ABR): send if capacity is available
Unspecified bit rate (UBR):, but can get almost no share of capacity
Guaranteed frame rate (GFR) gets roughly fair share of capacity during congestion
43ATM QoS Categories
For Videoconferencing ATM: Class B ATM Forum Service Category: Variable
Bit Rate-Real Time (VBR-RT)Momentary bandwidth increase for burst of
screen motion Needs Low Cell Delay Variation
Tolerance
For Connection-Oriented Data ATM: Class C ATM Forum Service Category: Variable
Bit Rate-Not Real Time (VBR-NRT) Most data not connection-oriented
44ATM Switches: Virtual Circuits
Often Arranged in a Mesh
Virtual Circuits: Single Possible Path between Any Two Stationssimplify switch operation and lower switch
cost
VirtualCircuit
ATMCell
45ATM Switches
Permanent Virtual Circuits (PVCs) Set up once, for each pair of sites Simplest and least expensive
administratively because rarely changed Most widely used form of virtual circuit
Switched Virtual Circuit (SVC) Set up at time of use Flexible but expensive
46ATM Switches
ATM Frame Header
Does NOT have a destination address field
Instead, has two fields that together contain a hierarchical virtual circuit number
Has Virtual Circuit Number & Port in forwarding table
Virtual Circuit NumberATM Header
47ATM Switches
Hierarchical Virtual Circuit Number Virtual Path Identifier
Higher-level number; Often specifies a site Virtual Channel Identifier
Lower-level number; Often specifies a computer at a site
All traffic between two sites can be given the same VPI numberBut difference VCI values
Switch needs only one VPI table entry for all this trafficDramatically reduces number of table entries in
switches between sites and therefore makes lookups very fast
48Switches Versus Routers
Switches
Fast
Inexpensive
No benefits of alternative routing
Routers
Slow
Expensive
benefits of alternative routing
“Switch where you can; route where you must”
49Early Site Networks
Organization LANs (subnets) based on hubs Routers link hubs Hierarchy of Routers
Router
Hub
50The Switching Revolution
Switches Push Routers to the Edge Switches replace most routers in site networks Because switches are cheaper than routers Routing’s sophistication is still needed at the edge
Layer 3 Switches Traditional switches operate at Layer 2; Switch
based on MAC addresses Layer 3 switches: based on internet layer IP
addresses Layer 3 switches are replacing many Layer 2
switchesExternal
Switch
Router
51The Switching Revolution
Layer 3 Switches versus Routers Layer 3 switches are much faster than routers
Layer 3 switches cost less than routers
Internet layer: Layer 3 switches normally only support IP and sometimes IPX; Routers route many more internet layer protocols, including those of AppleTalk, SNA, and others
At the data link layer, Layer 3 switches normally support only Ethernet on LANs. Routers support many Layer 2 LAN protocols.
52The Switching Revolution
Layer 3 Switches versus Routers Layer 3 switches rarely support Layer 2 WAN
protocols
Routers usually are still needed at the edge of the site network, to communicate with external links
External
Layer 3Switch
53The Switching Revolution
Routers
Forward based on IP addresses and other internet layer addresses
Expensive and slow
Handle multiple internet layer protocols
Handle multiple LAN and WAN subnet protocols
Layer 3 Switches
Forward based on IP addresses, sometimes IPX addresses
Inexpensive and Fast
Do not handle multiple internet layer protocols
Do not handle multiple LAN and WAN subnet protocols
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