1. Chapter 5: Devices for Connecting Networks

2. 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

3. 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

4. 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

5. 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

6. Repeater
Multiple repeater ports enable several types of cable connections
Example: inbound to fiber-optic, outbound to twisted pair

7. 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

8. 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

9. 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

10. 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

11. 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)

12. Figure 5-2 Simple hub connecting networked computers

13. 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

14. Bridge Three frame scenarios Protocol independent
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

15. Figure 5-3 Cascade bridging

16. Bridge Translational bridge Three primary bridge functions
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

17. 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

18. 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

19. 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

20. 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

21. 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

22. Figure 5-5 Router forwarding capabilities

23. 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

24. 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

25. 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

26. 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

27. Figure 5-11 OSPF protocol border areas

28. 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

29. 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

30. 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

31. Gateway Software or hardware interface Functions of a gateway
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

32. 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 messages from one format to another

33. 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

34. 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)

35. 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

36. 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

37. 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

38. 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

39. 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)

40. 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

41. Figure 5-12 DSL monitoring and management software

42. 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

43. Figure 5-17 Using an access server

44. 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

45. Table 5-3 WAN connectivity devices

46. 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

47. 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

48. Figure 5-18 A router-based network

49. 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

50. 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

51. 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