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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
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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
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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
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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
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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
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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
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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
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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
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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, T1, ISDN, and DSL)
• Remote routers are used to join LANs at remote sites into
WANs