Chapter 4Real-World Networks

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What is Meant By LAN Architecture?

• LAN architecture is the overall design of a LAN. It includes:– LAN hardware– LAN software– LAN topology– Media access control (MAC) protocol

• The LAN’s network operating system is sometimes also considered to be part of LAN architecture

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LAN Topologies• There are two types of LAN topologies: physical and

logical• Physical LAN topology refers to the physical layout

of the network– The way in which the communication is configured and

how nodes attach to the network– Because the focus is on physical connections among

hardware component, physical topologies correspond to the physical layer of the OSI reference model

• Logical topology is concerned with how messages are passed from node to node within the network

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LLC and MAC Sublayers

• LANs employ two primary data link protocols: contention and token passing

• In IEEE 802 standards, the data link layer is divided into two sublayers LLC and MAC (see Figure 4-1)– LLC (logical link control) is responsible for flow control,

message sequencing, message acknowledgement, and error checking

– MAC (media access control) enables network nodes to access the communication medium via contention or token passing

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Figure 4-1

4-1

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How Ethernet Works-CSMA/CD• CSMA/CD (Carrier Sense Multiple Access and Collision Detection) is

most widely used contention-based MAC used in LANs– It is the MAC protocol used in Ethernet LANs

• In a true contention MAC (like CSMA/CD), each node has equal access to the medium

• As noted in Table 4-1, each node monitors the medium for data traffic and if none is detected, it begins transmitting data

• A collision occurs when two or more nodes begin to transmit at the same time

• To avoid collision recurrence, each node waits a random time interval (hardwired in its NIC) before attempting to retransmit

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Table 4-1

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10Mbps Ethernet

• The four commonly used 10Mbps Ethernet cabling systems are:– 10Base5, or thicknet, which uses thick coaxial cable– 10Base2, or thinnet, which uses thin coaxial cable– 10BaseT, which uses unshielded twisted pair cable– 10BaseFL, which uses single or multimode optical

fiber

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10Base5 (Thicknet) Ethernet

• 10Base5 (thickenet) Ethernet has the following specifications:– Maximum segment length 500 meters (1650 feet)– Maximum taps 100– Maximum segments 5– Maximum segments with nodes 3– Maximum distance between taps 2.5 meters (8.25 feet)– Maximum repeaters 4– Maximum overall length with repeaters 2.5 kilometers (1.5 miles)– Maximum AUI drop cable length 50 meters (165 feet)

We use BNC connectors, DIX connectors, Transceiver, Transceiver cable, BNC barrel connectors, one BNC terminator, one Grounded terminator.

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10Base2 (Thinnet Coax) Ethernet

• 10Base2 (Thinnet) Ethetnet has the following specifications:

– Maximum segment length 185 meters (610.5 feet)– Maximum segments 5– Maximum segments with nodes 3– Maximum repeaters 4– Maximum devices per segments 30– Maximum overall length with repeaters 925 meters (3052.5 feet)

The term 10Base2 is little misleading since the maximum length is not actually 200 meters (660 feet) but only 185 meters (610.5 feet). The specification of thinnet is 50 Ohms RG-85A/U or RG-85 Coaxial cable. You use in thinnet BNC connectors, T connectors, and two terminators at both ends

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10BaseT (Twisted-Pair) Ethernet• 10BaseT(twisted-pair) Ethernet has the following

specifications:– Maximum segments 1024– Maximum segments with nodes 1024– Maximum segment length 100 meters (330 feet)– Maximum nodes per segments 2– Maximum nodes per network 1024– Maximum hubs in a chain 4

10BaseT is wired as a Star which means that each device has its own set of wires connected directly to a hub. The connection to the hub and the LAN cards is made with an RJ-45 connector. UTP cable is classified in categories, Categories 1and 2 are voice grade cable, Categories 3, 4, and 5 are data-grade.

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10BaseFL• 10BaseFL Ethernet has the following specifications:

– Maximum segments 1024– Maximum segments with nodes 1024– Maximum segment length 2000 meters– Maximum nodes per segments 2– Maximum nodes per network 1024– Maximum hubs in a chain 4

10BaseFL (10Mbps data rate, baseband signaling over a fiber-optic cable) uses light rather than electricity to transfer Ethernet frame. 10BaseFL is a Star-wired network because it requires a hub (also called a concentrator) to receive the light signal from each network station and send the same signal to all stations.

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100VG-AnyLAN• 100VG-AnyLAN (aka 100BaseVG) is capable of transporting both IEEE

802.3 and IEEE 802.5 frames

• It is faster• It provides a mechanism for interconnecting 100 Mbps token ring and

100BaseT Ethernet LANs via specialized hubs and routers (see Figure 4-2)– 100VG-AnyLAN-compliant adapters are also needed

• It uses demand priority access (DPA) rather than CSMA/CD as the MAC protocol in order to enable real-time voice and video frames to be given priority over other data frames

• Hubs can filter individually addressed frames for enhanced privacy.

100VG-anyLAN can use over Categories 3, 4, and 5 Twisted-pair and Fiber-optic cables. It uses a Star topology and defines how child hubs can be connected to a parent hub to extend the network. Several Hubs are cascaded in Figure 4-3.

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Figure 4-2

4-2

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100VG-AnyLAN 100VG-AnyLAN (Figure 4-3)

4-3

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100BaseT Ethernet• 100BaseT, also called Fast Ethernet, is simply regular Ethernet

run at a faster data rate over category 5 twisted-pair cable. 100BaseT uses the CDMA/CD protocol in a star wired bus as 10BaseT.

100BaseT has been specified for three media types:– 100BaseT4 (four pairs categories 3, 4, and 5 UTP or STP)– 100BaseTX (two pairs category 5 UTP or STP)– 100BaseFX (two-strand fiber-optic cable)

In addition to the faster data rate and the higher quality cable required, 100BaseX has the same advantages and drawbacks as 10BaseT.

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Segmentation

• Segmentation is the process of splitting a large Ethernet network into two or more segments linked by bridges or router.

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Token Passing• Token passing is the other major MAC protocol found in LANs• It is used in token ring and FDDI LANs and other networks with

logical ring topologies• The token is a pre-defined bit pattern that is passed among network

attached computers until one of them wants to use the medium to transmit data

• Token passing is summarized in Table 4-2• In token ring networks that resemble physical star topologies, token

passing takes place within MAUs (see Figure 4-7)• Token passing can be used in bus topologies as well as in physical ring

topologies.• Table 4-3 compares token passing and CSMA/CD

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Table 4-2

4-2

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IEEE 802.5 and 802.6 LANs• The most widely used microcomputer ring network is the

token passing ring. It conforms to the IEEE 802.5 standard• Token ring networks physically look like a star topology,

but technically they are physical rings– Token ring nodes attach to multistation access units (MAUs) – see

Figure 4-4– MAUs can be described as “a ring in a box”, because nodes attach

to the physical ring by connecting to the MAU (see Figure 4-5)– MAUs can be interconnected to form larger rings (see Figure 4-6)

• IEEE 802.6 addresses dual-ring metropolitan area network (MAN) architectures.

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Figure 4-44-4

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Figure 4-5

4-5

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Figure 4-6

4-6

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Figure 4-7

4-7

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Table 4-34-3

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Cabling for Token Ring• Type 1: STP cable used to connect terminals and distribution panel and to run through walls to

wiring closets in the same building. It is two twisted pairs of solid core 22 gauge copper wire surrounded by a shield.

• Type 2: It is made just as type1 cable with the exception that it incorporates four twisted pairs of telephone wires.

• Type 3: UTP cable with four pairs, each twisted two time for every 3.6 meters (12feet) of length. It is cheaper than Type 1 and 2. It is made of either 22 or 24-gauge wire.

• Type 5: Optical cable used only on the main ring path. It consists of 62.5-micron diameter or 100-

micron diameter fiber-optic cable.

• Type 6: STP cable that does not carry signals as for as Type 1 and 2. It is generally used only as patch cable or as extensions in wiring closets. It is made with 26-gauge copper wire twisted together in shielded jacket.

• Type 8: Used for runs under carpets. Likes Type 6 cable, it is made of two 26-gauge stranded-core wires twisted together.

• Type 9: Basically the fire-retardant version of Type 6 cable. It is made with two 26-gauge stranded-core copper wires twisted together in a shielded jacket.

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Token RingToken Ring has the following specification:

Cable type UTP, STP, or Fiber-opticMaximum MSAUs 33Maximum Nodes 260Maximum distance between 45.5 meters (150 feet) for UTP; up tonodes and MSAU 100 meters (330 feet) for STP or Fiber-

Optic cableMaximum patch cable distance 45.5 meters with UTP cable; 200 meters connecting MSAUs (660 feet) with STP, 1 kilometer (0.6 mile)

with Fiber-optic cable.Minimum patch cable 2.5 meters (8 feet)distance connecting MASUsMaximum cumulative patch cable 121.2 meters (400 feet) with UTP cable; distance connecting all MSAUs Fiber-optic cabling can span several

kilometers

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How token Ring WorksActive Monitor and Standby Monitors• Active Monitor in a Token Ring network (usually the device

that has been operating on the network for the longest time) that periodically checks the status of the network and monitors for the network errors. Whereas Standby Monitor in Token Ring network monitors the status and may become the active monitor in the case of failure of the active monitor.

Beaconing : • The process on Token Ring network by which a device, in the

event of a cable fault, determines the state of the network and location of the fault. (If a station does not hear from its upstream neighbor in the seven seconds, it assume something bad has happened and acts on its own. This action is called beaconing.) It occurs when the Nearest Active Upstream Neighbor Notification (NAUN) fails.

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Token RingAdvantages:• Unlike Ethernet, Token Ring continuous to operate reliably

under heavy loads.• Built-in diagnostic and recovery mechanism, such as

beaconing and auto-configuration, makes the protocol more reliable.

• Token Ring makes connecting a LAN to an IBM mainframe easier since IBM created it and supports it.

• Fault-tolerance features are provided through ring reconfiguration, called ring-wrap. A single cable can create a ring when attached to two MSAUs.

Disadvantages• Token Ring cards and equipment are more expensive than

Ethernet or ARCnet system.• Token Ring can be very difficult to troubleshoot and requires

considerable experience.

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Medium Access Control Sublayer

IEEE 802 Frame Formats

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Medium Access Control Sublayer

IEEE 802 Frame Formats

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Some Key Ethernet Implementations

• Particularly important Ethernet implementations include:– Fast Ethernet (e.g. 100BaseT and 100BaseFX)

• The IEEE 802.3u specification covers Fast Ethernet– Gigabit Ethernet (e.g. 1000BaseT,

1000BaseSX, and 1000BaseLX)• The IEEE 802.3z specification addresses Gigabit

Ethernet

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FDDI LANs

• Fiber Distributed Data Interface (FDDI) was first recognized in ANSI’s X3T9.5 specification

• Physically, it has a dual ring topology• It has a sequential/ring logical topology and uses a

variation of token passing as the MAC protocol• Key FDDI technologies are identified in Figure 4-8

– These include single attached stations (SAS), dual attached stations (DAS), FDDI concentrators, and FDDI/Ethernet bridges

• FDDI is often used as a backbone network architecture.

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Figure 4-84-8

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ATM LANs• ATM (asynchronous transfer mode) is a switched network

architecture that employs 53-octet cells to transmit data• Two data link layers are defined:

– ATM adaptation layer (AAL)– ATM

• ATM physical topologies are stars• ATM NICs with 25 speeds of 25, 100, or 155 mbps are

available for workstations• Ethernet and token ring LANs can interface with an

organization’s ATM backbone network via ATM gateway/access switches (see Figure 4-9)

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ATM• ATM is a circuit-based network, in that a virtual circuit is

setup between two devices to communicate over the network. There are two types of circuits in an ATM network.

– Permanent Virtual Circuit (PVC): A circuit that is setup once in the switches to allow communication between two devices.

– Switch Virtual Circuit (SVC): A circuit that is temporarily setup just for the duration of a communication between two devices.

Several Classes of service have been defined for ATM- Circuit emulation with constant bit rate.- Audio and/or video with a variable bit rate.- Connection-oriented service for data transmission.- Connectionless service for data transmission

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Figure 4-94-9

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Wireless LAN Architectures• IEEE 802.11x standards are the most important wireless LAN

(WLAN) specifications that exist today (see Table 4-4)• WLANs are typically implemented as physical stars

– Nodes connect to wireless hubs called access points• CSMA/CA is the MAC protocol for IEEE 802.11-compliant LANs• IEEE 802.11 addresses FHSS (frequency hopping spread spectrum),

DSSS (direct sequence spread spectrum), and diffuse infrared transmission– User “roaming” capabilities are also addressed

• WiFi (Wireless Fidelity) certification has been developed to promote interoperability among WLAN products

• The WISPR (Wireless ISP Roaming) standard is designed to enable users to roam from one publicly accessible WLAN to another

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Table 4-4

4-4

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Choosing Among LAN Architectures

• A number of factors should be considered when selecting among LAN architectures

• Some of the major factors are described in Table 4-5.• Especially important factors to consider include:

– Immediate and recurring LAN costs– Total cost of ownership (TCO)– Number of concurrent users that can be supported– Transmission speed and data throughput– Vendor support– Manageability– Scalability/expandability– Security– Adherence to widely accepted standards

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Table 4-5

4-5

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CSMA/CA

• CSMA/CA (Carrier Sense Multiple Access and Collision Avoidance) is a variation of CSMA/CD used in wireless LANs because it is difficult to detect collisions in such networks

• When CSMA/CA is used, each node must wait a random time interval (hardwired in the wireless NIC) after detecting a clear medium before transmitting