Network Topolog

Network Topologies

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Objectives

• Describe the basic and hybrid LAN physical topologies, their uses, advantages, and disadvantages

• Describe a variety of enterprise-wide and WAN physical topologies, their uses, advantages, and disadvantages

• Compare the different types of switching used in data transmission

• Understand the transmission methods, or logical topologies, underlying Ethernet, Token Ring, LocalTalk, and FDDI networks

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Simple Physical Topologies

• Physical topology– Physical layout of a network

• A Bus topology consists of a single cable—called a bus— connecting all nodes on a network without intervening connectivity devices

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Advantages of Bus Topology

• Works well for small networks

• Relatively inexpensive to implement

• Easy to add to it

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Disadvantages ofBus Topology

• Management costs can be high

• Potential for congestion with network traffic

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• Ring topology– Each node is connected to the two nearest nodes so the entire

network forms a circle– One method for passing data on ring networks is token passing

• Active topology– Each workstation transmits data

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Advantages of Ring Topology

• Easier to manage; easier to locate a defective node or cable problem

• Well-suited for transmitting signals over long distances on a LAN

• Handles high-volume network traffic

• Enables reliable communication

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Disadvantages ofRing Topology

• Expensive

• Requires more cable and network equipment at the start

• Not used as widely as bus topology– Fewer equipment options– Fewer options for expansion to high-speed

communication

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• Star topology– Every node on the network is connected through

a central device

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Advantages of Star Topology

• Good option for modern networks

• Low startup costs

• Easy to manage

• Offers opportunities for expansion

• Most popular topology in use; wide variety of equipment available

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Disadvantages ofStar Topology

• Hub is a single point of failure

• Requires more cable than the bus

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Hybrid Physical Topologies

• Hybrid topology– Complex combination of the simple physical topologies

• Star-wired ring– Star-wired topologies use physical layout of a star in

conjunction with token ring-passing data transmission method

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• Star-wired bus– In a star-wired bus topology, groups of workstations are

star-connected to hubs and then networked via a single bus

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• Daisy-Chained– A Daisy chain is linked series of devices

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• Hierarchical hybrid topology– Uses layers to separate devices by priority or function

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Enterprise-Wide Topologies

• Enterprise– An entire organization

• Backbone networks– Serial backbone– Distributed backbone– Collapsed backbone– Parallel backbone

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• Serial backbone– Two or more hubs connected to each other by a single cable

• Distributed backbone– Hubs connected to a series of central hubs or routers in a hierarchy

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• Collapsed backbone– Uses a router or switch as the single central

connection point for multiple subnetworks

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• Parallel Backbone– Collapsed backbone arrangement that consists of more than one

connection from central router or switch to each network segment

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• Mesh networks– Routers are interconnected with other routers, with at least two

pathways connecting each router

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Wide Area Network (WAN) Topologies

• Peer-to-peer topology– WAN with single interconnection points for each location– Dedicated circuits

• Continuous physical or logical connections between two access points that are leased from a communication provider

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• Ring WAN topology– Each site is connected to two other sites so that entire

WAN forms a ring pattern

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• Star WAN topology– Single site acts as the central connection point for several

other points

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• Mesh WAN topology– Many directly interconnected locations forming a complex mesh

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Logical Topologies

• Refers to the way in which data are transmitted between nodes

• Describes the way:– Data are packaged in frames

– Electrical pulses are sent over network’s physical media

• Logical topology may also be called network transport system

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Switching

• Component of network’s logical topology that determines how connections are created between nodes– Circuit switching

• Connection is established between two network nodes before they begin transmitting data

– Message switching• Establishes connection between two devices, transfers

information to second device, and then breaks connection

– Packet switching• Breaks data into packets before they are transmitted

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Ethernet

• Carrier Sense Multiple Access with Collision Detection (CSMA/CD)– The access method used in Ethernet

– Collision• In Ethernet networks, the interference of one network node’s

data transmission with another network node’s data transmission

– Jamming• Part of CSMA/CD in which, upon detection of collision,

station issues special 32-bit sequence to indicate to all nodes on Ethernet segment that its previously transmitted frame has suffered a collision and should be considered faulty

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Ethernet

CSMA/CD process

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Ethernet

• On an Ethernet network, an individual network segment is known as a collision domain– Portion of network in which collisions will occur if

two nodes transmit data at same time

• Data propagation delay– Length of time data take to travel from one point

on the segment to another point

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Ethernet

• Demand priority– Method for data transmission used by 100BaseVG Ethernet networks– Demand priority requires an intelligent hub

CSMA/CD versus demand priority

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Ethernet

• Traditional Ethernet LANs, called shared Ethernet, supply fixed amount of bandwidth that must be shared by all devices on a segment

• Switch– Device that can separate network segments into

smaller segments, with each segment being independent of the others and supporting its own traffic

• Switched Ethernet– Newer Ethernet model that enables multiple nodes to

simultaneously transmit and receive data over logical network segments

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Ethernet

A switched Ethernet network

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Ethernet

• Gigabit Ethernet– 1 Gigabit Ethernet

• Ethernet standard for networks that achieve 1-Gbps maximum throughput

– 10 Gigabit Ethernet• Standard currently being defined by IEEE 802.3ae

committee• Will allow 10-Gbps throughput• Will include full-duplexing and multimode fiber

requirements

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Ethernet

• Padding– Bytes added to data portion of an Ethernet frame to

make sure this field is at least 46 bytes in size

• Ethernet frame types:– IEEE 802.3 (“Ethernet 802.2” or “LLC”)– Novell proprietary 802.3 frame (or “Ethernet 802.3”)– Ethernet II frame– IEEE 802.3 SNAP frame

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IEEE 802.3 (“Ethernet 802.2” or “LLC”)

• Default frame type for versions 4.x and higher of Novell NetWare network operating system– Sometimes called LLC frame– In Novell’s lexicon, this frame is called Ethernet 802.2 frame

An IEEE 802.3 frame

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IEEE 802.3 (“Ethernet 802.2” or “LLC”)

• Service Access Point (SAP)– Identifies node or internal process that uses LLC

protocol

• Frame Check Sequence (FCS)– This field ensures that data are received just as

they were sent

• Cyclical Redundancy Check (CRC)– Algorithm used by FCS field in Ethernet frames

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Novell Proprietary 802.3 (or “Ethernet 802.3”)

• Original NetWare frame type• Also called:

– 802.3 Raw– Ethernet 802.3 frame

A Novell proprietary 802.3 frame

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Ethernet II

• Original Ethernet frame type developed by DEC, Intel and Xerox, before IEEE began to standardize Ethernet

An Ethernet II frame

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IEEE 802.3 SNAP

• Adaptation of IEEE 802.3 and Ethernet II• SNAP stands for Sub-Network Access Protocol

An IEEE 802.3 SNAP frame

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Understanding Frame Types

• Learning about networks is analogous to learning a foreign language, with the frame type being the language’s syntax– Just as you may know the Japanese word for go but

how to use it in a sentence, you may know all about the IPX/SPX protocol but not how devices handle it

• Autosense– Feature of modern NICs that enables a NIC to

automatically sense what types of frames are running on a network and set itself to that specification

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Design Considerations for Ethernet Networks

• Cabling

• Connectivity devices

• Number of stations

• Speed

• Scalability

• Topology

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LocalTalk

• Logical topology designed by Apple Computer, Inc.

• Uses a transmission method called Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)

• A teleconnector is a transceiver used on a LocalTalk network

• Macintosh version of TCP/IP is called MacTCP

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Token Ring

• Token Ring networks use the token passing routine and a star-ring hybrid physical topology

• The 100-Mbps Token Ring standard is known as High-Speed Token Ring (HSTR)

• On a Token Ring network, one workstation, called the active monitor, acts as the controller for token passing

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Token Ring

• Multistation Access Unit (MAU)– Regenerates signals

Interconnected Token Ring MAUs

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Token Ring

• Control Access Unit (CAU)– Connectivity device used on a Token Ring

network

• Lobe Attachment Module (LAM)– Device that attaches to a CAU to expand the

capacity of that device

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Token Ring

• Token Ring networks with STP cabling may use a type 1 IBM connector

• A DB-9 connector is another type of connector found on STP Token Ring networks

Type 1 IBM and DB-9 Token Ring connectors

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Token Ring

• Media filter– Device that enables two types of cables or connectors to be linked

• Token Ring media filter– Enables DB-9 cable and type 1 IBM cable to be connected

A Token Ring media filter

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Token Ring

• Token Ring switching– Like Ethernet networks, Token Ring networks can take

advantage of switching to better utilize limited bandwidth

• Token Ring frames– IEEE 802.5 Token Ring frame– IBM Token Ring frame

An IBM Token Ring frame

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Design Considerations for Token Ring Networks

• Cabling

• Connectivity devices

• Number of stations

• Speed

• Scalability

• Topology

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Fiber Distributed Data Interface (FDDI)

• Logical topology whose standard was originally specified by ANSI in mid-1980s and later refined by ISO

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Asynchronous Transfer Mode (ATM)

• Logical topology that relies on a fixed packet size to achieve data transfer rates up to 9953 Mbps

• The fixed packet in ATM is called a cell• A unique aspect of ATM technology is that it

relies on virtual circuits

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Asynchronous Transfer Mode (ATM)

• ATM uses circuit switching, which allows ATM to guarantee a specific quality of service (QOS)

• ATM technology can be integrated with Ethernet or Token Ring networks through the use of LAN Emulation (LANE)

Technology

Network Topolog