Chapter 5: Devices for Connecting Networks

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

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

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

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

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

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

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

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

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

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)

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

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

Figure 5-3 Cascade bridging

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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)

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

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

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

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

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

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

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

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