Devices for Network and Internetwork Connectivity

Multistation Access Units (MAUs)

• Used exclusively on token ring networks, MAUs can perform the following tasks:

• Connect workstations in a logical ring through a physical star topology
• Move the token and packets around the ring
• Amplify data signals
• Connect in a daisy-chained manner to expand a token ring network
• Provide orderly movement of data)

• A passive hub connects nodes in a star topology, performing no signal enhancement as the packet moves from one node to the next through the hub.
• An active hub connects nodes in a star topology, regenerating, retiming, and amplifying the data signal each time it passes through the hub.

Multiplexers

• Multiplexers are network devices that can receive multiple inputs and transmit them to a shared network medium.
• Multiplexers simply are switches used in old and new technology, such as the following:

• Telephone switching
• Switching telecommunications lines to create multiple channels on a single line.
• Serial communications to enable more than one terminal to communicate over a single line.
• Fast Ethernet, X.25, ISDN, frame relay, ATM, and other networking technologies to create multiple communication channels over a single communications cable.

• A multiplexer is a switch that divides a communication medium into multiple channels so several nodes can communicate at the same time.
• A signal that is multiplexed must be demultiplexed at the other end.
• Work at the OSI physical level, switching from channel to channel using one of three physical methods:

• Time division multiple access (TDMA)
• Frequency division multiple access (FDMA)
• Statistical multiple access

• TDMA enables multiple devices to communicate over the same communications medium by creating time slots in which each device transmits.

• TDMA divides the channels into distinct time slots. Each time slot is designated for a particular network node, as if it were a dedicated line.
• TDMA does not guarantee the most efficient use of the network medium, since transmission occurs on only one channel at a time.
• The timing of node transmission is also important, because a node may transmit at an interval that is out of synchronization with its time slot.

• FDMA creates separate channels on one communication medium by establishing different frequencies for each channel.

• FDMA divides the channels into frequencies instead of time slots.
• Each channel has its own broadcast frequency and bandwidth.
• Statistical multiple access multiplexing allocates the communication resources according to what is needed for the task.
• Is used by X.25, ISDN, and frame relay.
• This method is more efficient than TDMA or FDMA, because the physical medium bandwidth is dynamically allocated based on the application need.
• The multiplexer continuously monitors each channel to determine the communication requirements.

Repeaters

• A repeater amplifies and retimes a packet-carrying signal so it can be sent along all cable segments.
• As used in this context, a segment of cable is one cable run within the IEEE specifications.
• How repeaters are used:

• To extend a cable segment
• To increase the number of nodes beyond the limit of one segment
• To sense a problem and shut down a cable segment
• To amplify and retime a signal (as a component in other network devices)

• When a repeater retransmits a signal along more than one additional cable segment, it is called a multiport repeater.
• An attached unit interface (AUI) connects coax or fiber-optic backbone cable to a network node, such as a repeater.
• A partitioned segment is one that has been shut down because a portion of the segment is malfunctioning.

Bridges

• A bridge is a network device that connects different LAN segments using the same access method.
• Bridges are used to:

• To extend a LAN when the maximum connection limit has been reached
• To extend a LAN beyond the length limit
• To segment LANs to reduce data traffic bottlenecks
• To prevent unauthorized access to LAN (for security)

• The first bridges were developed by DEC in the early 1980s. DEC’s work on bridges was incorporated in the IEEE 902.1 standard.
• A network device that operates in promiscuous mode reads frame destination address information before sending a packet onto other connected segments of the network.
• A local bridge connects networks in close proximity and is used to segment a portion of a network to reduce problems caused by heavy traffic.
• A remote bridge joins networks across the same city, between cities, and between states to create one network.

Token Ring Bridging

• Token ring bridges use source routing to forward packets on the network.
• Originally proposed by IBM, source-route bridging has been incorporated into the 802.5 token ring LAN specification.
• When a node anticipates sending a packet on a bridged network, it issues an explorer packet. Each bride that receives the explorer packet copies it onto all outbound ports. Route information is appended to the data area of the explorer packets as they travel through the inter-network.
• When the sending node’s explorer packets are received by the destination node, the destination node replies to the sending node using the accumulated route information. Then the sending node must select a path to the destination node.
• The path is determined by three factors:

• the route taken by the first packet received back
• the minimum number of hops to the destination
• the path that will enable the largest packet size.

• Hops are the number of times a packet travels point-to-point from one network to the next.

Routers

• Routers can connect networks that have dissimilar data links.
• It forwards packets to networks by using a decision-making process based on:

• Routing table data
• Discovery of the most efficient routes
• Preprogrammed information from the network administrator

• How routers are used:

• To efficiently direct packets from one network to another, reducing excessive traffic
• To join neighboring or distant networks
• To connect dissimilar networks
• To prevent network bottlenecks by isolating portions of a network
• To secure portions of a network from intruders

• A router forwarding a frame to the right network:

• When equipped with the appropriate hardware and software, routers can connect to the following networks, among others:

Ethernet	Frame Relay	Fast Token Ring	Fast Ethernet
Token Ring	Cell Relay	ATM ISDN

• Nodes regularly communicate with a router to confirm their address and presence.
• Routers are designed to send packets along paths with the lowest volume of traffic and with the lowest cost in terms of using network resources.
• As a network grows in complexity, the need to ship packets along the shortest, most efficient path grows proportionally. Bridges often are replaced by routers to ensure that growing network traffic is handled efficiently and network congestion avoided.
• Static and dynamic routing:

• Static routing involves control of routing decisions by the network administrator through preset routing instructions.
• In dynamic routing, the router constantly:
• Checks the network configuration
• Automatically updates routing tables
• Makes its own decisions about how to route frames

Routing Tables and Protocols

• Routers maintain information about node addresses and network status in databases.
• The routing table database contains the addresses of other routers and each end node.
• Routers regularly exchange information about network traffic, the network topology, and the status of network lines.
• Routers exchange information by using one or more routing protocols.
• Local and Remote routers:

• A local router joins networks in the same building or between buildings in close proximity.
• A firewall is software and/or hardware employed to restrict who has access to a network, to specific network segments, or to certain network resources (such as servers).
• A remote router joins networks across large geographical areas, such as between cities, states, and countries.

Brouter

• A brouter, also called a multiprotocol router, is a network device that acts like a bridge or a router, depending on how it is set up to forward a given protocol.
• It is used on networks that operate with several different protocols.
• Brouters are used to:

• For efficient packet handling on a multiprotocol network with some protocols that can be routed and some that cannot
• To isolate and direct network traffic to reduce congestion
• To join networks
• To secure a certain portion of a network by controlling who can access it

• A brouter is used on networks that operate with several different protocols, such as:

• NetBEUI
• IPX/SPX
• TCP/IP

• Also called a multiprotocol router.
• Whether it bridges or routes a protocol can depend on these things:

• The instructions programmed in by the network administrator for handling a protocol
• If the incoming frame contains routing information

• When a brouter is set to forward but not route a protocol, it forwards each frame using addressing information in the MAC sub-layer of the data link layer, just like a bridge.
• That capability is important on a network when one of the protocols is NetBEUI, which cannot be routed.

Hubs

• A hub is a central network device that connects network devices in a star topology.
• Advanced hubs provide very high speed connectivity for FDDI, Fast Ethernet, frame relay, and ATM networks.
• It is also referred to as a concentrator (or switch), which is a device that can have multiple inputs and outputs all active at one time.
• Services offered by hubs:

• Provide a central unit from which to connect multiple nodes into one network
• Permit large numbers of computers to be connected on single or multiple LANs
• Reduce network congestion through centralizing network design
• Provide multiprotocol services, such as Ethernet-to-FDDI connectivity
• Consolidate the network backbone
• Enable high-speed communications
• Provide connections for several different media types
• Enable centralized network management

• Types of Hubs:

• MAUs
• 10BASE-T hubs
• 100BASE-X hubs
• Intelligent and modular hubs

10-Base-T Hubs

• One of the simplest hubs
• Popular way to connect workgroups on small and large LANs
• Uses physical star topology to connect PCs to the central hub
• Additional hubs are added by connecting one hub to the next.

Switching Hubs

• Permit significant increase in the throughput capability of an existing 4 Mbps, 10 Mbps, or 16 Mbps network by taking full advantage of exiting bandwidth capabilities
• Allow an existing network to be separated into multiple smaller segments, each independent of the others
• Can be installed on LANs in a WAN where specific LANs are experiencing increased network traffic.

100-Base-X Hubs

• Multimedia, video, and GUI client/server applications have fostered the need for high-bandwidth, high-speed technologies.

Intelligent and Modular Hubs

• An intelligent hub has network management and performance monitoring capabilities.
• A modular hub, also called a chassis hub, contains a backplane into which different modules can be inserted.
• A backplane is the main circuit board in modular equipment, containing slots as plug-ins for modular cards. It provides connections between the modular boards, a power source, and grounding.

Diagram of a 10-Base-T Star Topology on Two Floors

Network diagram of 10BASE-T network segments connected to an intelligent hub

Gateways

• A gateway enables communications between two different types of networked systems, such as between complex protocols or between different e-mail systems.
• How gateways are used:

• To convert commonly used protocols to a specialized protocol
• To convert message formats from one format to another
• To translate different addressing schemes
• To link a host computer to a LAN
• To provide terminal emulation for connections to a host computer
• To direct e-mail to the right network destination
• To connect networks with different architectures

Systems Network Architecture (SNA) SNA is a layered communications protocol used by IBM for communications between IBM mainframe computers and terminals. It employs seven-layered communications that are similar to the OSI model, but there are differences in the way the services are grouped within the layers.

ATM Switches

• An ATM switch determines the network channel used to transmit an ATM cell received from a node, taking into account the type of information in the cell (voice, video, data) and the transmission speed needed.
• Capabilities of ATM switches:

• Provide high-speed communications on a network backbone
• Provide cell transmissions directly to the desktop
• Enable high-speed communication between network hubs
• Centralize network design for better management
• Connect to very high speed networks, such as SONET
• Enable network design around workgroup members at dissimilar locations (virtual LANs)
• Reduce network bottlenecks through high-speed communications and efficient traffic management through workgroups

Virtual LANs (VLANs)

• A virtual LAN (VLAN) uses switches, routers, and internetworking software to configure a network into sub-networks of logical workgroups, independent of the physical network topology.
• Advantages of VLANs:

• VLANs enable a network to operate at the most efficient level; it is not limited by physical topology.
• Network resources can be managed based on the actual work groupings of users.
• VLANs can be reconfigured to move a user from an old workgroup to new one.

Networking Devices and the OSI Model