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)○ 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

Figure 16.2 Repeater

A repeater connects segments of a LAN.

NoteNote::

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

detection.

NoteNote::

Figure 16.3 Function of a repeater

A repeater is a regenerator

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• 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

B1B1

Traffic within the same group Traffic between the two groups

P1P2

LAN1

LAN2

H1

H4

H2 H3

H6H5

The frame from H1 to H4 is forwarded by the bridge

The frame from H1 to H3 is dropped by the bridge

A bridge has a table used in filtering decisions.

NoteNote::

Figure 16.5 Bridge

Figure 16.6 Learning bridge

Figure 16.7 Loop problem

Cycles in Bridged Network

1. host writes frame F to destination which is unknown for B1 and B2

B2B1

F1

2. B1 and B2 forward the frame, F1 and F2 are generated

F2

B2B1

F

B2B1

F1

4. B1 and B2 forward the frames F1 and F2

F2

B2B1

F1

3. B2 receives F1, B1 receives F2

F2

B2B1

F2

5. The situation in 3. is repeated and the frames are sent back

F1

B2B1

F1

6. The frames can circulate in the network for ever

F2

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.

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

1. Elect the root bridge. (The bridge with lowest ID.) 2. Choose a root port for every bridge. (For lowest cost to

the root bridge.)3. 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

4. Mark the root ports and designated ports as forwarding (active) ports, the others as blocking (non-active) ports.

Figure 16.9 Applying spanning tree

Root ports: Minimum one star.Designated ports: Two stars.The other ports are blocking ports.

Spanning Tree - Example

B1B1

B2B2

1

• 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 3Network 4

B1

B2

1 2 3 4

The networkThe corresponding graph

Another example

B3

B5

B7B2

B1

B6 B4

B8

Cost for each

port is 1

(hop-count)

The Root Bridge and the Spanning Tree

B3

B5

B7B2

B1

B6 B4

Root

B8

B2 B4 B5 B7

B8

B1

Spanning Tree:

*

*

*

*

*

*

**

**

**

**

**

**** **

**

***

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

**

Multiple LANs with Bridges with Costs Assigned

B1

B6

B5B2 B3

B4

LAN 1

LAN 2

LAN 3

LAN 4

Cost=6

Cost=5Cost=4

Cost=1

Cost=6

Cost=5

Cost=4

Cost=2

Cost=6

Cost=4

L1L1

L2L2 L3L3

L4L4

B1B1

B2B2

B6B6B5B5

B4B4

B3B3

4

2

46

5

3Cost=2

Cost=3

6

4 5

6

1

The cost of sending from L1 to L4 via B1 and B2 is 6Only costs for going from a bridge to a LAN are added

2

Example: Root Bridge and Root Ports

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

• The root bridge and root ports are marked in red

L1L1

L2L2 L3L3

L4L4

B1B1

B2B2

B6B6B5B5

B4B4

B3B3

4

2

46

5

3

6

4 5

6

1

2

Cost=6

Cost=8Cost=2

Cost=6 Cost=3

Root

Example: Designated Ports and the Spanning Tree

L1L1

L2L2 L3L3

L4L4

B1B1

B2B2

B6B6B5B5

B4B4

B3B3

4

2

46

5

3

6

4 5

6

1

2

Cost=2L3

Cost=6 Cost=3

RootL1L2

Cost=6L4

Cost=8

*

*

*

*

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

• The designated ports are marked “*”.

Example: Designated Ports and the Spanning Tree

L1L1

L2L2 L3L3

L4L4

B1B1

B2B2

B6B6B5B5

B4B4

B3B3

4

2

3

6

4

2

The rest ofthe ports areblocked.

This results ina spanningtree.

Figure 16.13 Connecting remote LANs

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

H2

H3

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

Figure 16.12 Star backbone

16.3 Virtual LANs16.3 Virtual LANs

Membership

Configuration

IEEE Standard

Advantages

Figure 16.15 A switch using VLAN software

VLANs create broadcast domains.

NoteNote::

Figure 16.16 Two switches in a backbone using VLAN software

Chapter 18

Virtual CircuitSwitching:Frame Relay

andATM

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 WANsCircuit Datagram Virtual Circuit

Dedicated path No dedicated path No dedicated pathContinuous datatransmission

Packets Packets

No data storage Store and forward Store and forwardConnectionestablished forentire conversation

Route establishedfor every packet

Route establishedfor every packet

Call setup delay;low transmissiondelay

Packet transmissiondelay

Call setup delay;Packet transmissiondelay

Busy signal Possible notificationof no/bad deliveries

Notification ofconnection denial

Blocking at networkoverload

Delay at networkoverload

Blocking/delay atnetwork overload

Fixed bandwidth Dynamic bandwidth Dynamic bandwidth

No overhead/data Overhead/packet Overhead/packet

Figure 18.1 Virtual circuit wide area network

Figure 18.3 VCI phases

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

Figure 18.2 VCI

Figure 18.4 Switch and table

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• 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

Frame ackData

Frame ack

Data

Frame ackData

Frame ack

Ack Ack Ack Ack

Data Data Data Data

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

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

switch switch switch

Frame Relay in the Internet

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

Figure 18.8 Frame Relay network

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

NoteNote::

Frame Relay does not provide flow or error control; they must be provided by

the upper-layer protocols.

NoteNote::

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.

Figure 18.13 Multiplexing using different frame sizes

Figure 18.14 Multiplexing using cells

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.

NoteNote::

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

• 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

End System B

End System A

Connect to BConnect to B

Connect to B

Connect to B

OK

OK

OK

OK

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 Path (VP)

Virtual Path (VP)

ATM Physical Link(STM-1, OC-12, E1)

Virtual Channel Connection (VCC)

Virtual Circuits (VC)

Virtual Circuit (VC)= Logical Path between

ATM End PointsVCC - contains multiple VPs

VP - contains multiple VC

Figure 18.18 Example of VPs and VCs

Note that a virtual connection is defined by a pair of numbers:

the VPI and the VCI.

NoteNote::

Figure 18.19 Connection identifiers

Figure 18.20 Virtual connection identifiers in UNIs and NNIs

Figure 18.21 An ATM cell

Figure 18.22 Routing with a switch

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