Datorn tverk a lektion 11
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Datornätverk A – lektion 11. Kapitel 16: Connecting LAN:s, Backbone Networks and Virtual Lans. (Kapitel 18: Frame Relay and ATM översiktligt). Chapter 16. Connecting LANs, Backbone Networks, and Virtual LANs. Limitations of Ethernet Technologies. Distance (the length of the cable)

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Datorn tverk a lektion 11

Datornätverk A – lektion 11

Kapitel 16: Connecting LAN:s, Backbone Networks and Virtual Lans.

(Kapitel 18: Frame Relay and ATM översiktligt)


Datorn tverk a lektion 11

Chapter 16

Connecting LANs,Backbone Networks,

and Virtual LANs


Limitations of ethernet technologies

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


Datorn tverk a lektion 11

Figure 16.2Repeater


Datorn tverk a lektion 11

Note:

A repeater connects segments of a LAN.


Datorn tverk a lektion 11

Note:

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


Datorn tverk a lektion 11

Figure 16.3Function of a repeater

A repeater is a regenerator


Datorn tverk a lektion 11

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.


Hub s limitations

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


Bridges a simple example

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

LAN1

H6

H5

H4

P2

B1

P1

LAN2

Traffic within the same group

Traffic between the two groups


Datorn tverk a lektion 11

Note:

A bridge has a table used in filtering decisions.


Datorn tverk a lektion 11

Figure 16.5Bridge


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Figure 16.6Learning bridge


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Figure 16.7Loop problem


Cycles in bridged network

Cycles in Bridged Network

2. B1 and B2

forward the

frame, F1 and F2

are generated

1. host writes frame F

to destination which is

unknown for B1 and B2

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


Datorn tverk a lektion 11

Figure 16.10Forwarding 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.


Spanning tree algorithm definitions

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.


The spanning tree algorithm

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.


Datorn tverk a lektion 11

Figure 16.9Applying spanning tree

Root ports: Minimum one star.Designated ports: Two stars.

The other ports are blocking ports.


Spanning tree example

1

B1

2

1

4

3

B2

Spanning Tree - Example

The corresponding graph

The network

B1

  • 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

Network 1

Network 2

Network 4

Network 3

B2


Another example

Another example

B8

Cost for each

port is 1

(hop-count)

B3

B5

B7

B2

B1

B6

B4


The root bridge and the spanning tree

The Root Bridge and the Spanning Tree

**

B8

*

**

Spanning Tree:

B3

**

*

B5

B1

**

**

*

B7

B2

*

*

B2

B4

B5

B7

**

**

**

B1

**

**

Root

B8

*

*

B6

**

A spanning tree is a connected graph which has no loops (cycles)

B4

**


Multiple lans with bridges with costs assigned

Multiple LANs with Bridges with Costs Assigned

L1

4

4

6

LAN 1

B1

B5

B6

2

1

Cost=4

5

B1

Cost=6

LAN 2

Cost=2

L2

L3

B6

3

2

Cost=4

Cost=5

B3

6

Cost=6

Cost=2

B5

6

B3

B2

B4

LAN 3

Cost=1

B2

Cost=3

4

5

Cost=4

L4

Cost=6

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

B4

Cost=5

LAN 4


Example root bridge and root ports

L1

4

4

6

B1

B5

B6

2

1

5

3

L2

2

L3

B3

6

6

B4

B2

4

5

L4

Example: Root Bridge and Root Ports

Root

  • Lowest cost from each bridge to the root bridge are calculated.

  • The root bridge and root ports are marked in red

Cost=3

Cost=6

Cost=2

Cost=8

Cost=6


Example designated ports and the spanning tree

L1

4

4

6

B1

B5

B6

2

1

5

3

L2

2

L3

B3

6

6

B4

B2

4

5

L4

Example: Designated Ports and the Spanning Tree

*

*

Root

L1

L2

  • Lowest cost from each LAN to the root bridge are calculated (= the cost from an adjacent bridge.)

  • The designated ports are marked “*”.

Cost=3

Cost=6

*

*

Cost=2

L3

Cost=8

Cost=6

L4

*


Example designated ports and the spanning tree1

Example: Designated Ports and the Spanning Tree

L1

The rest of

the ports areblocked.

This results in

a spanning

tree.

4

B1

B5

B6

2

3

L2

2

L3

B3

6

B4

B2

4

L4


Datorn tverk a lektion 11

Figure 16.13Connecting remote LANs


Lan switches

LAN Switches

H2

H3

H1

  • LAN switching provides dedicated, collision-free communication between network devices, withsupport 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

H3

H2


A lan switch

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


Datorn tverk a lektion 11

Figure 16.12Star backbone


Datorn tverk a lektion 11

16.3 Virtual LANs

Membership

Configuration

IEEE Standard

Advantages


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Figure 16.15A switch using VLAN software


Datorn tverk a lektion 11

Note:

VLANs create broadcast domains.


Datorn tverk a lektion 11

Figure 16.16Two switches in a backbone using VLAN software


Datorn tverk a lektion 11

Chapter 18

Virtual Circuit

Switching:Frame Relayand

ATM


Two approaches to packet switching

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.


Characteristics of wans

Characteristics of WANs

Virtual Circuit

Circuit

Datagram

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


Datorn tverk a lektion 11

Figure 18.1Virtual circuit wide area network


Datorn tverk a lektion 11

Figure 18.3VCI phases


Virtual circuit network

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


Datorn tverk a lektion 11

Figure 18.2VCI


Datorn tverk a lektion 11

Figure 18.4Switch and table


X 25 networks

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


Frame relay

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


X 25 versus frame relay

Data

Data

Data

Data

Data

Data

Data

Data

Frame ack

Frame ack

Frame ack

Frame ack

Ack

Ack

Ack

Ack

X.25 versus Frame Relay

switch

switch

switch

X.25 traffic (ACKs at both data-link and transport layer)

Frame relay traffic (ACKs are required at the transport layer only)


Frame relay in the internet

Frame Relay in the Internet

  • The virtual circuits in frame-relay are called DLCI (Data Link Connection Identifier)


Datorn tverk a lektion 11

Figure 18.8Frame Relay network


Datorn tverk a lektion 11

Note:

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


Datorn tverk a lektion 11

Note:

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


Atm basic idea

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.


Datorn tverk a lektion 11

Figure 18.13Multiplexing using different frame sizes


Datorn tverk a lektion 11

Figure 18.14Multiplexing using cells


Datorn tverk a lektion 11

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.


Atm basic concepts

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


Atm switching

Connect to B

Connect to B

OK

End System A

OK

OK

Connect to B

Connect to B

OK

End System B

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


Virtual circuit and paths

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


Datorn tverk a lektion 11

Figure 18.18Example of VPs and VCs


Datorn tverk a lektion 11

Note:

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


Datorn tverk a lektion 11

Figure 18.19Connection identifiers


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Figure 18.20Virtual connection identifiers in UNIs and NNIs


Datorn tverk a lektion 11

Figure 18.21An ATM cell


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Figure 18.22Routing with a switch


Atm service models

ATM Service Models

  • CBR (Constant 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


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