1. Datornätverk A – lektion 11
Kapitel 16: Connecting LAN:s, Backbone Networks and Virtual Lans.
(Kapitel 18: Frame Relay and ATM översiktligt)

2. Connecting LANs, Backbone Networks,
Chapter 16
Connecting LANs, Backbone Networks,
and Virtual LANs

3. Limitations of Ethernet Technologies
Distance (the length of the cable)
200 m in Thin Ethernet (10Base2)
100 m in twisted pair Ethernet (10BaseT or 100BaseT or Fast Ethernet)
Number of collisions when too many stations are connected to the same segment
The situation is similar in other LAN technologies

4. Figure Repeater

5. A repeater connects segments of a LAN.
Note:
A repeater connects segments of a LAN.

6. Note:
A repeater forwards every frame bit- by-bit; it has no packet queues, no filtering capability and no collision detection.

7. A repeater is a regenerator
Figure Function of a repeater
A repeater is a regenerator

8. Hubs A hub is a multiport repeater used in 10BaseT and Fast Ethernet
Hubs give a possibility to have a physical star topology but logical bus topology.

9. Hub’s Limitations
Hubs and repeaters resolve the problem with the distance, but does not resolve the problem with collisions.
A hub network can have lower throughput than several separate networks.
The maximum througput of the three separate networks = 3x10Mbps
The throughput of the connected network = 10Mbps

10. Bridges – A Simple Example
The frame from H1 to H4 is forwarded by the bridge
The frame from H1 to H3 is dropped by the bridge H1 H2 H3 H4 H5 H6
LAN1 P2 B1 P1
LAN2
Traffic within the same group
Traffic between the two groups

11. A bridge has a table used in filtering decisions.
Note:
A bridge has a table used in filtering decisions.

12. Figure Bridge

13. Figure 16.6 Learning bridge

14. Figure Loop problem

15. Cycles in Bridged Network
1. host writes frame F
to destination which is
unknown for B1 and B2
2. B1 and B2
forward the
frame, F1 and F2
are generated
3. B2 receives F1,
B1 receives F2 F B1 B2 B1 B2 B1 B2 F2 F1 F1 F2
4. B1 and B2
forward the
frames F1 and F2
5. The situation in 3. is repeated and the frames are sent back
6. The frames can circulate in the network for ever F2 F1 F1 F2 B1 B2 B1 B2 B1 B2 F1 F2

16. Figure 16.10 Forwarding ports and blocking ports
Dotted lines = blocking (non-active redundant) ports. May be used if one of the other bridges or links fails.
Continuous black lines = forwarding (active) ports. These constitute a spanning tree (ett spännande träd) without loops.

17. Spanning Tree Algorithm – Definitions
Root Path Cost: For each bridge, the cost of the min-cost path to the root. Costs are assigned to each port or hop count is used, based on for example bandwith, delay or number of hops (1 per port).
Each bridge is assigned a unique identifier: Bridge ID
If not assigned, the lowest MAC addresses of all ports is used as the bridge ID.
Low ID number means high priority.
Each port within a bridge has a unique identifier (port ID). Typically the MAC address of the port is used.

18. The Spanning Tree Algorithm
Elect the root bridge. (The bridge with lowest ID.)
Choose a root port for every bridge. (For lowest cost to the root bridge.)
Chose one designated bridge for each LAN, for minimum cost between the LAN and the root bridge. Mark the corresponding port as a designated port.
If two bridges have the same cost, select the one with lowest ID.
If the min-cost bridge has two or more ports on the LAN, select the port with the lowest identifier
Mark the root ports and designated ports as forwarding (active) ports, the others as blocking (non-active) ports.

19. Root ports: Minimum one star. Designated ports: Two stars.
Figure Applying spanning tree
Root ports: Minimum one star. Designated ports: Two stars.
The other ports are blocking ports.

20. Spanning Tree - Example
The corresponding graph
The network 1 B1 B2 1 2 3 4 B1
Network 1
Network 2
Network 4
Network 3 B2
Networks are graph nodes, ports are graph edges
A spanning tree is a connected graph which has no loops (cycles)
The dotted links are the blocked ports on the bridge, in order to prevent loops and duplicated frames

21. Another example Cost for each port is 1 (hop-count) B8 B3 B5 B7 B2 B1

22. The Root Bridge and the Spanning Tree** B8 ** *
Spanning Tree: B3 ** * B5 B1 ** * ** B7 B2 * * B2 B4 B5 B7 ** ** ** B1 ** **
Root B8 * * B6 B4 **
A spanning tree is a connected graph which has no loops (cycles) **

23. Multiple LANs with Bridges with Costs Assigned4 4
LAN 1 6 B1 B5 B6 2
Cost=4 1 5 B1
Cost=6
LAN 2
Cost=2 L2 L3 B6
Cost=4 2 3
Cost=5
Cost=6
Cost=2 B3 6 B5 6 B3 B2
LAN 3
Cost=1 B4
Cost=3 B2 4
Cost=4 5 L4
Cost=6 B4
The cost of sending from L1 to L4 via B1 and B2 is 6
Only costs for going from a bridge to a LAN are added
Cost=5
LAN 4

24. Example: Root Bridge and Root Ports4 2 6 5 3 1
Lowest cost from each bridge to the root bridge are calculated.
The root bridge and root ports are marked in red
Root
Cost=3
Cost=6
Cost=2
Cost=8
Cost=6

25. Example: Designated Ports and the Spanning TreeB1 B2 B6 B5 B4 B3 4 2 6 5 3 1 * *
Lowest cost from each LAN to the root bridge are calculated (= the cost from an adjacent bridge.)
The designated ports are marked “*”.
Root L1 L2
Cost=3
Cost=6 * *
Cost=2 L3
Cost=8
Cost=6 L4 *

26. Example: Designated Ports and the Spanning Tree
The rest of
the ports are blocked.
This results in
a spanning
tree. 4 B1 B5 B6 2 3 L2 2 L3 B3 6 B4 B2 4 L4

27. Figure 16.13 Connecting remote LANs

28. LAN Switches
LAN switching provides dedicated, collision-free communication between network devices, with support for multiple simultaneous conversations.
LAN switches are designed to switch data frames at high speeds.
LAN switches can interconnect a 10-Mbps and a 100-Mbps Ethernet LAN. H1 H2 H3 H1 H3 H2

29. A LAN Switch
The computer has a segment to itself – the segment is busy only when a frame is being transfered to or from the computer
As a result, as many as one-half of the computers connected to a switch can send data at the same time

30. Figure Star backbone

31. Virtual LANs
Membership
Configuration
IEEE Standard
Advantages

32. Figure 16.15 A switch using VLAN software

33. VLANs create broadcast domains.
Note:
VLANs create broadcast domains.

34. Figure 16.16 Two switches in a backbone using VLAN software

35. Switching: Frame Relay and
Chapter 18
Virtual Circuit
Switching: Frame Relay and
ATM

36. Two Approaches to Packet Switching
Datagram networks (For example IP)
Analogous to the postal service
The inteligence is in the end devices (computers), the network should not be trusted
Each packet carries the destination address
Destination addresses are global internationally
Virtual circuit networks (For example X.25, Frame Relay and ATM)
Analogous to the telephone service
The network should take all the responsibility, the end devices should be as simple as posible
The path that the packets follow is determined at the beginning of the transmission, but store and forward switching is used.

37. Characteristics of WANs
Circuit
Datagram
Virtual Circuit
Dedicated path
No dedicated path
No dedicated path
Continuous data
Packets
Packets
transmission
No data storage
Store and forward
Store and forward
Connection
Route established
Route established
established for
for every packet
for every packet
entire conversation
Call
setup delay;
Packet transmission
Call
setup delay;
low transmission
delay
Packet transmission
delay
delay
Busy signal
Possible notification
Notification of
of no/bad deliveries
connection denial
Blocking at network
Delay at network
Blocking/delay at
overload
overload
network overload
Fixed bandwidth
Dynamic bandwidth
Dynamic bandwidth
No overhead/data
Overhead/packet
Overhead/packet

38. Figure 18.1 Virtual circuit wide area network

39. Figure VCI phases

40. Virtual Circuit Network
Three Phases
Setup phase
Network protocol establishes a logical path called virtual circuit (VC). The path remains the same during transmission (all packets use it)
Data transfer phase
Each packet carries “tag” or “label” (virtual circuit id, VCI), which determines next hop (the link to which the packet should be forwarded).
At each node, the forwarding is done by inspecting the input line, the VCI and consulting the forwarding table at the switches.
Teardown phase
All switches remove the entries about the VCI from their tables

41. Figure VCI

42. Figure 18.4 Switch and table

43. X.25 Networks
Developed in 1970s in European countries under the auspices of ITU
Public packet-switched networks
Uses virtual circuit connections
Switched virtual circuits – analog to dial-up in circuit switching
Permanent virtual circuits – analog to leased lines in circuit switching.
Operates on the three lowest layers (physical, data-link and network layer)
Performs error-contol and flow-control on the node-to-node basis
Work at speed up to 64Kbps
Nowadays it is obsolete

44. Frame Relay
X.25 data rates were not stisfactory for users looking for higher data rates and lower costs
Checking frames for error at every node is inefficient
Only one fourth of traffic is message traffic, the rest is overhead (necessary for transmission media that are more error prone)
Frame relay – public data network that have improved performance
Developed having in mind new transmission media that have much lower probability of error
Does not provide error checking and acknowledgement at both, the data-link layer and the network layer

45. X.25 versus Frame Relay
Data
Data
Data
Data
Frame ack
Frame ack
Frame ack
Frame ack
Ack
switch
Ack
switch
Ack
Ack
switch
X.25 traffic (ACKs at both data-link and transport layer)
Data
Data
Data
Data
Frame relay traffic (ACKs are required at the transport layer only)

46. Frame Relay in the Internet
The virtual circuits in frame-relay are called DLCI (Data Link Connection Identifier)

47. Figure 18.8 Frame Relay network

48. Frame Relay operates only at the physical and data link layers.
Note:
Frame Relay operates only at the physical and data link layers.

49. Note:
Frame Relay does not provide flow or error control; they must be provided by the upper-layer protocols.

50. ATM – Basic Idea Uses small fixed-size packets called cells
The cells are 53 bytes long (48 bytes payload + 5 bytes header)
The length of the cell compromise between American and European telephone companies (average of 32 and 64)
Uses packet switching
Connection oriented (uses virtual circuits)
Speeds of 155 Mbps or 622 Mbps are achieved over SONET
Was heavily promoted by telephone companies as BISDN (Broadband Integrated Services Digital Network) technology.

51. Figure 18.13 Multiplexing using different frame sizes

52. Figure 18.14 Multiplexing using cells

53. Note:
A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-sized block of information.

54. ATM Basic Concepts Nagotiated Service Contract
Logical connections called Virtual circuits
The sender nagotiates a ”requested path” with the network for a connection to the destination
End-to-end Quality of Service
When setting up a connection the sender specifies the atributes of the call (type, sped, ...) which determine end-to-end quality of service
Virtual Circuit Network
Well defined connection procedures
Dedicated capacity per connection
Flexible access speeds
Cell based (short packets with fixed size)
All kinds of data look same to the network

55. ATM Switching
When a site has an information to send to another, it requests a connection by sending a message
The message passes through vasious switches, setting up a virtual path
Subsequent data cells contain a virtual path ID which the switch uses to to route the cell through outgoing links
Using the input port and VP ID, the switch locates the table entry, changes the cell VP ID with one paired with the asssociated output port and sends the cell through that port
End System B
End System A
Connect to B OK

56. Virtual Circuit and Paths
Virtual Circuits (VC)
ATM Physical Link
(STM-1, OC-12, E1)
Virtual Channel Connection (VCC)
Virtual Path (VP)
Virtual Path (VP)
Virtual Circuit (VC)
= Logical Path between
ATM End Points
VCC - contains multiple VPs
VP - contains multiple VC

57. Figure 18.18 Example of VPs and VCs

58. Note:
Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI.

59. Figure 18.19 Connection identifiers

60. Figure 18.20 Virtual connection identifiers in UNIs and NNIs

61. Figure An ATM cell

62. Figure 18.22 Routing with a switch

63. ATM Service Models CBR (Constant Bit Rate) UBR (Unspecified Bit Rate)
Carries real time (constant bit rate) traffic
Guaranties rate, delay and loss of cells
UBR (Unspecified Bit Rate)
No other guarantee besides in-order delivery of cells
ABR (Available Bit Rate)
No guarantee on transmision rate, but if possible the user can use a higher rate than in UBR.
Congestion feedback from the network
VBR
The variable bit-rate is requested by the sender
Targeted toward real-time services like CBR