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Network Equipment

Technologies 

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Repeaters

Transmits in both directions Joins two segments of cable No buffering No logical isolation of segments If two stations on different segments

send at the same time, packets will collide

Only one path of segments and repeaters between any two stations

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Baseband Configuration

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Hubs, Bridges, and Switches Used for extending LANs in terms of

geographical coverage, number of nodes, administration capabilities, etc.

Differ in regards to:– collision domain isolation– layer at which they operate

Different than routers– plug and play– don’t provide optimal routing of IP packets

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Hubs Physical Layer devices: essentially

repeaters operating at bit levels: repeat received bits on one interface to all other interfaces

Hubs can be arranged in a hierarchy (or multi-tier design), with a backbone hub at its top

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Hubs (more)

Each connected LAN is referred to as a LAN segment

Hubs do not isolate collision domains: a node may collide with any node residing at any segment in the LAN

Hub Advantages:– Simple, inexpensive device– Multi-tier provides graceful degradation:

portions of the LAN continue to operate if one of the hubs malfunction

– Extends maximum distance between node pairs (100m per Hub)

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Hubs (more) Hub Limitations:

– Single collision domain results in no increase in max throughput; the multi-tier throughput same as the the single segment throughput

– Individual LAN restrictions pose limits on the number of nodes in the same collision domain (thus, per Hub); and on the total allowed geographical coverage

– Cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)

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Hubs and Switches

Shared medium hub– Central hub– Hub retransmits incoming signal to all outgoing

lines– Only one station can transmit at a time– With a 10Mbps LAN, total capacity is 10Mbps

Switched LAN hub– Hub acts as switch– Incoming frame switches to appropriate

outgoing line– Unused lines can also be used to switch other

traffic– With two pairs of lines in use, overall capacity is

now 20Mbps

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Switched Hubs No change to software or hardware of

devices Each device has dedicated capacity Scales well Store and forward switch

– Accept input, buffer it briefly, then output

Cut through switch– Take advantage of the destination address being

at the start of the frame– Begin repeating incoming frame onto output line

as soon as address recognized– May propagate some bad frames

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Bridges

Link Layer devices: they operate on Ethernet frames, examining the frame header and selectively forwarding a frame base on its destination

Bridge isolates collision domains since it buffers frames

When a frame is to be forwarded on a segment, the bridge uses CSMA/CD to access the segment and transmit

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Bridges (more)

Bridge advantages:– Isolates collision domains resulting in higher

total max throughput, and does not limit the number of nodes nor geographical coverage

– Can connect different type Ethernet since it is a store and forward device

– Transparent: no need for any change to hosts LAN adapters

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Backbone Bridge

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Interconnection Without Backbone

Not recommended for two reasons:- Single point of failure at Computer Science

hub- All traffic between EE and SE must path over

CS segment

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Bridges Ability to expand beyond single LAN Provide interconnection to other

LANs/WANs Use Bridge or router Bridge is simpler

– Connects similar LANs– Identical protocols for physical and link

layers– Minimal processing

Router more general purpose– Interconnect various LANs and WANs– see later

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Why Bridge?

Reliability Performance Security Geography

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Functions of a Bridge

Read all frames transmitted on one LAN and accept those address to any station on the other LAN

Using MAC protocol for second LAN, retransmit each frame

Do the same the other way round

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Bridge Operation

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Bridge Design Aspects No modification to content or format of frame No encapsulation Exact bitwise copy of frame Minimal buffering to meet peak demand Contains routing and address intelligence

– Must be able to tell which frames to pass– May be more than one bridge to cross

May connect more than two LANs Bridging is transparent to stations

– Appears to all stations on multiple LANs as if they are on one single LAN

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Bridge Protocol Architecture

IEEE 802.1D MAC level

– Station address is at this level Bridge does not need LLC layer

– It is relaying MAC frames Can pass frame over external comms system

– e.g. WAN link– Capture frame– Encapsulate it– Forward it across link– Remove encapsulation and forward over LAN link

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Fixed Routing Complex large LANs need alternative routes

– Load balancing– Fault tolerance

Bridge must decide whether to forward frame Bridge must decide which LAN to forward

frame on Routing selected for each source-destination

pair of LANs– Done in configuration– Usually least hop route– Only changed when topology changes

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Multiple LANs

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Loop of Bridges

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Bridges Spanning Tree For increased reliability, it is desirable to have

redundant, alternate paths from a source to a destination

With multiple simultaneous paths however, cycles result on which bridges may multiply and forward a frame forever

Solution is organizing the set of bridges in a spanning tree by disabling a subset of the interfaces in the bridges:

Disabled

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A Connection To A Network Can Be Extended

Fiber optic cable can provide

connections across long distances.

– Requiring optical modems

– Using light instead of electricity to carry

data

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A Computer Can Have Multiple Connections

A computer can connect to two or more networks.– Using multiple network cards

– Allowing different network technologies

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Using A Computer To Interconnect Networks

Special-purpose computers

are used to interconnect

networks.

– Using standard hardware (CPU,

memory, and network

interfaces)

– Running special-purpose

software

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Interconnecting Computer Packets/

A computer that interconnect networks has one task:– Forwarding packets from one network to

another

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Figure 13.2

A computer, D used to interconnect two networks. The networks can be the same type or different types.

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WWF Bridges vs. Routers Smackdown Both are store-and-forward devices, but

Routers are Network Layer devices (examine network layer headers) and Bridges are Link Layer devices

Routers maintain routing tables and implement routing algorithms, bridges maintain filtering tables and implement filtering, learning and spanning tree algorithms

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Routers

Interconnecting computers are called routers.– Determining where to send packets

Network with Router

Router

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Routers Are the Building Blocks of the Internet

The Internet consists of thousands of

computer networks, interconnected by

routers.– Interconnecting LAN to LAN

– Interconnecting LAN to WAN

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Routers Accommodate Multiple Types of Networks

Routers can interconnect different

network technologies.

– Using diverse networking technologies

– Actively changing computer networking

technologies

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Routers Can Interconnect WANs and LANs

It is least expensive to connect to the

Internet by connecting a router to an

existing network with Internet

connection.

35An example of a wide area backbone. At each backbone site, a router connects a local are network to the backbone. At some sites, additional routers connect additional networks. Networks at site 7 do not connect directly to the backbone.

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Interconnecting Networks Was Revolutionary

Using a dedicated computer to interconnect two networks was a fundamental idea.

Routers enable groups to: – Choose appropriate network technologies

– Interconnect to other networks

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Routers vs. Bridges Bridges + and -

+ Bridge operation is simpler requiring less processing bandwidth

- Topologies are restricted with bridges: a spanning tree must be built to avoid cycles

- Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)

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Routers vs. Bridges

Routers + and -

+ Arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing prots)

+ Provide firewall protection against broadcast storms

- Require IP address configuration (not plug and play)

- Require higher processing bandwidth

Bridges do well in small (few hundred hosts) while routers are required in large networks (thousands of hosts)

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Ethernet Switches

A switch is a device that incorporates bridge functions as well as point-to-point ‘dedicated connections’

A host attached to a switch via a dedicated point-to-point connection; will always sense the medium as idle; no collisions ever!

Ethernet Switches provide a combinations of shared/dedicated, 10/100/1000 Mbps connections

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Ethernet

Some E-net switches support cut-through switching: frame forwarded immediately to destination without awaiting for assembly of the entire frame in the switch buffer; slight reduction in latency

Ethernet switches vary in size, with the largest ones incorporating a high bandwidth interconnection network

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Ethernet Switches (more)Dedicated

Shared

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• The link between a client or a network device and the network cable is the network adapter card.

• The network adapter card is inserted into an expansion slot in a client or network server.

• The network adapter card has an address called the I/O port address on the device's bus.

• The address is determined by the configuration of the network adapter card. • Communications software tells the CPU to move the data from an address in

memory to the address of the expansion slot that contains the network adapter card. • The network adapter card converts the information into a data packet, and then

transmits the data packet across the network cable. • Data transmission across the network is performed a bit at a time. This is called

serial transmission. • The network adapter card must take the parallel data received from the bus and

convert it to serial data.• The network adapter card conducts a handshaking routine with the destination

network adapter card. • Handshaking is the method by which network adapter cards agree on the details of

how they will transmit and receive data packets.

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• Performance features are incorporated in network adapter cards to ease bottlenecks. • Efficient use of memory improves throughput performance. • Network adapters must accept data packets quickly and process at the same pace. • Incoming data packets are stored in memory called a buffer or cache. • Data packets are processed from memory rather than from the stream of data

flowing from the network.• Memory must be allocated by the network adapter card before data packets can be

stored. • There are several memory techniques used to allocate memory. • Direct memory access (DMA)—Network Adapter Card directly controls client’s

memory without using the client’s CPU.• Shared memory access for storage—Network Adapter Card uses client’s memory by

requesting data transfer by CPU. Overcomes the limitation of a slow processor on the

client.• Provides memory on the network adapter card called RAM buffering. No client’s

memory is used.• Fine-tune the network adapter card for performance by allocating more or less of the

client's memory for data communications.

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