Chapter 5: Devices for Connecting Networks

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LAN Transmission Devices• Uses of LAN transmission equipment

• Connecting devices on a single network• Creating and connecting multiple networks or subnetworks• Setting up some enterprise networks

• Connecting devices that will be discussed • Repeaters, MAUs, hubs, bridges, routers, switches, and gateways

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Repeater• Connects two or more cable

segments• Retransmits incoming signal to all

other segments• Cable segment is one cable run

within IEEE specifications• Example: Ethernet segment in star-bus

topology

• Can perform four Physical layer functions• Filter out signal disturbance caused by

EMI and RFI• Amplify and reshape incoming signal • Retime the signal (in Ethernet

applications)• Reproduce the signal on all cable runs

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Repeater• Uses of repeaters

• Extend cable segments• Extend a wireless signal• Increase number of nodes beyond segment• Sense a network problem and shut down a segment• Connect to components in other network devices• Connect segments using different media• Extend backbone cable segments in LANs, CANs, and MANs• Extend long, fiber-optic cable segments• Increase communication distance of T-carrier lines

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Repeater• Collision domain: segments where collisions occur

• Caused by two or more nodes transmitting at once

• Partitioning: detecting and closing down bad segment• Examples: missing terminator or broken cable• Nodes cannot communicate in partitioned segment• Segment must be reset at repeater after problem fixed

• Depending on network topology, media, and type of repeater, a single packet can travel through as many as four repeaters

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Repeater• Multiple repeater ports enable several types of cable

connections• Example: inbound to fiber-optic, outbound to twisted pair

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Multistation Access Unit• Multistation access unit (MAU or MSAU)

• Central hub on a token ring network• May have intelligence built-in to detect problems

• Smart multistation access unit (SMAU)

• Tasks performed by MAU • Connect nodes in a logical ring through a physical star topology• Move the token and frames around the ring• Amplify data signals• Expand token ring network by daisy-chain connections • Provide for orderly movement of data• Shut down ports to malfunctioning nodes

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Multistation Access Unit

• Functions at OSI Physical and Data Link layers• MAU technology evolved into newer devices:

• Control Access Unit (CAU): allows several connected, stackable units to count as one MAU

• CAUs also come with options for gathering information used in network performance management

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Hub• Central network device connecting nodes in a star

topology • Functions of a hub

• Centrally connect multiple nodes into one network• Permit connections on single or multiple LANs• Provide multi-protocol services• Consolidate the network backbone• Provide connections for several different media types • Enable centralized network management and design

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Hub• Unmanaged hub (simplest)

• Used for very small networks (up to 12 nodes)• Do not have management software to provide network

management information or functions• Passive hub – performs no signal amplification as the

signal moves through the hub• Active hub – retimes and amplifies the carrier signal

• Functions like a multiport repeater• Both passive and active hubs operate at the Physical

layer of the OSI model

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Hub• Intelligent (managed) hub

• Gathers information about network performance • Enables remote shut down of port or entire hub

• Some hubs have ports that can operate at multiple speeds• Automatically senses the speed of the connected device

• Hubs can partition network segments (like repeaters)

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Figure 5-2 Simple hub connecting networked computers

Bridge

• Network device connecting LAN segments• Functions of a bridge

• Extend a LAN when the maximum connection limit is reached• Example: the 30-node limit on an Ethernet bus

• Extend a LAN beyond the length limit• Example: beyond 185 meters for a thinnet segment

• Segment LANs to reduce data traffic bottlenecks• Prevent unauthorized access to a LAN

• Operates in promiscuous mode • Examine frame's physical destination address• Occurs at MAC sublayer of OSI Data Link layer

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Bridge• Three frame scenarios

• Destination of frame is on same segment as source• Bridge drops frame, since no forwarding needed

• Destination of frame is on another segment known to bridge• Bridge transmits frame to the known segment only

• Destination of frame is not known to bridge• Bridge transmits frame to all segments but the source

• Protocol independent• Look only at MAC address• May forward different protocols on same network

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Figure 5-3 Cascade bridging

Bridge• Translational bridge

• Converts frame to new access method and media type • Example: from token ring to Ethernet

• Discards addressing information not used in Ethernet

• Three primary bridge functions• Learning: learn network topology and device addresses

• Information is stored in a bridging table• Filtering: do not flood certain frames, discard others

• Enables the bridge to be used for security purposes• Forwarding: transmit frames to destination

• Based on data built-in to the bridging table

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Bridge• Multiport bridges tie several LANs into one network• Advantages of bridge over repeaters and hubs

• Ability to segment network traffic• May serve as a firewall to keep intruders out

• Two types of bridges• Local: directly connects two LANs in close proximity

• Also used to segment traffic to reduce bottlenecks• Remote: join distant networks

• Used to join networks in different cities or states

• Wireless bridges (access points)• Link to nodes equipped with wireless NIC (WNIC)• Data transmission rate is adjusted with each WNIC

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Spanning Tree Algorithm

• Defined by the IEEE 802.1d standard• Bridges frames in networks with more than two bridges• Sets up a system of checks performed by bridges

• Spanning tree algorithm has two goals:• Ensure a frame does not enter an endless loop

• Causes congestion that may intensify to broadcast storm• Forward frames along the most efficient route

• Efficiency based on distance and utilization of resources

• Improves network efficiency:• Creates a one-way path around network• Establishes maximum number of hops (hop count)• Enable bridges to send frames along best route

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Router• Learns, filters, and forwards like a

bridge• Differs from a bridge in significant ways

• Connect LANs at the Network layer of the OSI model

• Contains built-in intelligence to direct packets to different networks

• General functions of a router • Reduce traffic by efficiently directing packets

from one network to another• Join neighboring or distant networks• Connect dissimilar networks• Prevent bottlenecks by isolating portions of a

network• Secure portions of a network by acting as a

firewall

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Router• Hop – a regeneration, amplification, and movement of a

packet from one network onto another by a router• Hop count can be included in packets retransmitted by

routers• May be used to determine the fastest route to a particular

destination

• Routers receive regular communication from nodes confirming their address and presence

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Router

• Uses a metric to determine optimal routes• A metric can be calculated using any of the following:

• Number of incoming packets waiting at a particular router port • Number of hops between sending and receiving segments• Number of packets that can be handled in a specific amount of time• Size of the packet (large packet may be subdivided)• Bandwidth (speed) between two communicating nodes• Whether a particular network segment is available

• May isolate segments to prevent congestion

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Figure 5-5 Router forwarding capabilities

Static and Dynamic Routing

• Static routing requires routing tables • Routing tables specify paths between routers• Tables are set up & updated by a network administrator

• Dynamic routing - routing tables are updated automatically• Functions automatically performed in dynamic routing

• Determine which other routers can be reached• Determine shortest paths to other networks with metrics• Determine when path to a router is down or unusable• Use metrics to reconfigure alternative routes • Rediscover a router and network path after restoration

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Routing Tables and Protocols

• Routers maintain two important databases• Routing table: contains addresses of other

routers• Network status: contains information about

traffic, topology, and status of links

• Databases updated by regular exchange of data

• Routers forward packets on the basis of metrics

• Routers use one or more protocols• Multiprotocol router: an address database is

kept for each protocol supported

• Two common communication protocols: RIP and OSPF

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Routing Tables and Protocols• Routing Information Protocol (RIP)

• Determines shortest number of hops to other routers• Information added to each router's table• Disadvantages

• Updates containing entire routing table create traffic• Only uses hop count as a metric

• Open Shortest Path First (OSPF) protocol• Sends only a portion of table related to immediate links

• Called “link-state routing message”• Link state information consists of router interface IP address, subnet

mask, type of network connection (wired or wireless), other immediate routers, and router’s relationship to other routers on the network

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Routing Tables and Protocols• Advantages of OSPF over RIP:

• Routing information is packaged in a more compact format• Only updated routing table information is shared among routers• There is no hop count limit as with RIP• It does not slow down on networks with different speeds• It enables better load balancing of network traffic• It enables better authentication security for routing information

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Figure 5-11 OSPF protocol border areas

Switch• Switches serve two purposes:

• To provide bridging capacity• To increase bandwidth

• Bridge-like characteristics of switch• Operates at Data Link MAC sublayer• Uses table information to filter and forward traffic• Can use the spanning-tree algorithm

• LAN uses two switching techniques (unlike bridges)• Cut-through: forward portions of frame before entire frame is received• Store-and-forward: frame is buffered until entire frame is received

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Switch• Reduces collisions and improves bandwidth on Ethernet

• Example: hub with eight 100 Mbps segments • Has capacity of 8 x 100 (800) Mbps

• Store-and-forward switching is more popular than cut-through• Some store-and-forward switches use CPUs

• Switches can be unmanaged or managed• Unmanaged switches have fixed configurations that

cannot be changed

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Switch• Management options in managed switches:

• Activating or deactivating specific ports• Assigning priorities to ports• Aggregating multiple links into one for higher bandwidth• Using SNMP for monitoring• Employing the spanning tree algorithm protocol• Employing MAC filtering

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Gateway• Software or hardware interface

• Enables two networked systems or software to connect• Functions of a gateway

• Convert common protocols to specialized type • Convert message formats from one format to another• Translate different addressing schemes• Link a host computer to a LAN• Provide terminal emulation for connections to host • Direct electronic mail to the right network destination• Connect networks with different architectures

• Can function at any OSI layer

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Gateway• The most traditional type of gateway is a network device

that translates one type of protocol to another• Example: Translates IBM’s Systems Network Architecture (SNA) to

TCP/IP

• Another common use of the term “gateway” is for software that converts e-mail messages from one format to another

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WAN Transmission Devices• WAN transmission devices work over two network types

• PSTN (public switched telephone networks) • Leased telephone lines such as T-carrier or ISDN

• Characteristics of WAN transmission equipment• May have analog component or be completely digital• Converts signal for long distance communications• Creates multiple channels in medium (grow bandwidth)

• Frequently used WAN transmission devices• Telephone modems, ISDN adapters, cable TV modems, DSL

modems/routers, access servers, remote routers

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Analog Telephone Modems

• Modem (modulator/demodulator)• Converts outgoing digital signals to analog signals • Converts incoming analog signals to digital signals

• Two ways to attach a modem to a computer• Internal: installed in a computer’s expansion slot• External: attached to serial port connector via cable

• Common types of connectors• DB-25 connector, DB-9 connector, PS/2, and USB

• Modem data transfer rate measured in bits per second (bps)

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Analog Telephone Modems

• Data terminal equipment (DTE)• Device that prepares data for transmission• Data transfer speed of PC is DTE communications rate

• Data communications equipment (DCE)• Device (modem) that converts data from DTE• Speed of modem is DCE communications rate

• Modems use two communication formats• Synchronous: continuous data bursts controlled by a clock signal • Asynchronous: discrete signals delimited by start and stop bits

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ISDN Adapters

• Connect PCs to ISDN lines with a terminal adapter • Terminal adapter (TA): modem-like device

• Converts digital signal for transmission over digital telephone line• Typically includes analog phone jacks

• ISDN hardware connects to copper telephone lines• Separate channels for computer data and analog telephone signals

• Analog and digital lines may be used simultaneously

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Cable TV Modems• Uses two channels (frequencies) to communicate

• Upstream: transmit outgoing data, sound, TV signals• Downstream: receive and blend incoming signals

• Factors affecting transmission speed• Modem speeds may differ upstream and downstream

• Example: 30 Mbps upstream, 15 Mbps downstream• Maximum bandwidth reduced by other subscribers

• Cable service may impose policy limits

• Data Over Cable Service Interface Spec (DOCSIS)• Also called Certified Cable Modem Project• Provides standards and certifications

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Cable TV Modems• DOCSIS standards in use for Internet access

• DOCSIS 1.0: 5 Mbps upstream and downstream • DOCSIS 1.1: Doubles speed of DOCSIS 1.0, includes data

encryption• DOCSIS 2.0 (Adv PHY): triples speed of DOCSIS 1.1 (up to 30

Mbps), protects from interference• DOCSIS 3.0: enables cable channels to be bound together to

achieve higher speeds

• May be internal or external device• Advantage of cable communications

• System dynamically allocates unused bandwidth

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DSL Modems and Routers• Digital Subscriber Line (DSL)

• Works over copper wire likes ISDN• Requires intelligent adapter in connecting computer or router

• Intelligent adapter: sends digital signal over copper wire

• Simplex communication over copper wire• One pair of wires is used for incoming transmissions and another pair

is used for outgoing transmissions• Maximum upstream and downstream transmission rates

are 200 Mbps• Maximum distance from user to telco without a repeater is

5.5 kilometers (3.4 miles)

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DSL Modems and Routers• Advantages of DSL over cable

• Dedicated DSL line is more secure • Dedicated DSL line provides full bandwidth for the link (unlike cable

modem, which is shared by other users)• DSL networks utilize combined DSL adapter/router

• Device can be used to direct network traffic and to create a firewall so that only authorized users can access network services

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Figure 5-12 DSL monitoring and management software

Access Servers• Combines WAN communications into one device

• Example: combine capabilities of modem, DSL, T-1, T-3, ISDN, and frame relay

• Small access servers may have:• 8 or 16 asynchronous ports • One or two synchronous ports

• Large access servers are modular• Contain slots for multiple communication cards• Example: separate cards for T-1 and DSL communications

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Figure 5-17 Using an access server

Remote Routers• Enables networks to be connected to WANs over long

distances• Connect ATM, ISDN, frame relay, high-speed serial, and X.25

networks• Example: connect networks from NY to LA into WAN

• Similarities with local routers• Can support multiple protocols• Can be set up as a firewall

• Most routers connect to WAN through serial interface• CSU/DSU for T-carrier communications

• Channel service unit (CSU): interface to T-carrier line• Data service unit (DSU): digital interface to CSU

• Modular adapter for other high-speed connections

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Table 5-3 WAN connectivity devices

Putting It All Together: Designing A Router-Based Network

• Guidelines to consider when designing a network:• Use the most efficient devices for your application• Understand which devices have repeater functions and stay within

the limits for maximum number of repeaters• Use routers to segment network (IP) traffic on mid-sized and large

networks to reduce congestion• Use routers on networks for a firewall between you and the outside

world• If you share an Internet connection on a small network, bring the

WAN connection into a router• Consider using an access server on larger networks• Purchase the best Internet connected you can afford

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Putting It All Together: Designing A Router-Based Network• Scenario: design a network for one-story office building• Implementing the network design

• Bring the DSL connection into a router• Put all appraisers on one workgroup switch• Put all social workers on different workgroup switch• Connect both switches to the router • Use router to segment traffic through each switch • Use router as a firewall between user groups and the outside world

• Enable both user groups to access DSL line through the router

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Figure 5-18 A router-based network

Summary

• Early networks use repeaters to expand network communications when the IEEE limits are reached or to extend the range of wireless communications

• Some network devices incorporate repeater functions as they implement more complex network options such as filtering and forwarding packets and frames

• Routers and switches incorporate some bridging functions for networking and are used in centralized star-based networks to connect segments and to link networks to one another

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Summary

• Routers and switches can be equipped with intelligence to help in collecting network data and for centralized network management

• Routers are popular because they control traffic patterns and they play a dual role providing both LAN and WAN connectivity

• Switches are popular because they are faster than hubs• Analog modems used over PTSN lines have been used for

many years in the past

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Summary• Cable modems have a widespread presence because

they can be used over existing cable TV lines and offer high-speed access

• Access servers provide a single unit in which to combine all types of telecommunications connectivity (modems, T-1, ISDN, and DSL)

• Remote routers are used to join LANs at remote sites into WANs

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