Networking Topologies & Models

Physical and Logical Topologies

• Every network has a physical and logical topology

Physical topologies

• The physical topology represents the physical layout of the network

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• Bus
◇ Host devices are connected to one centralized cable or hardware device by twisted pair wiring
◇ Network data travel is bi-directional, which means that data can collide
• Full Mesh
◇ All devices are connected to each other for redundancy. This network is the most reliable
◇ It is also the most expensive and difficult to set up
◇ You can create a partial mesh topology as an alternative
• Star
◇ A central device connects to network and host devices using twisted pair wiring
• Ring
◇ All devices are connected using a single cable and data is routed through each device
◇ The data travels in a single direction until it reaches its intended target
◇ This topology averts network collisions, but is susceptible to failure if a single node fails
• Dual Ring
◇ Two rings enable data to be sent in opposite directions, creating redundancy
◇ If one ring fails, the other continues to transmit data

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Logical topologies

• The logical topology is a virtual representation of how the physical topology operates, and is not necessarily identical to the physical topology
• For example, a network's physical topology might be laid out in the form of a star topology, but its logical operation functions like a ring topology

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Network Wiring and Cables

Types of wires

• These wires are typically twisted together in pairs to reduce interference
• Types: Shielded and Unshielded

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Network cable categories

Cable rating	Frequency	Data Performance	Channel length limit	Max data speed	Length limit at max	Notes
Cat 5e	100 MHz	100 MBS	100 m	1 GBPS	100 m	Can handle 1G Ethernet
Cat 6	250 MHz	1 GBPS	100 m	10 GBPS	37 m	Can handle 10G Ethernet for a length up to 55 meters depending on cabling quality and installation practices
Cat 6a	500 MHz	10 GBPS	100 m	10 GBPS	100 m

• A network engineer cannot run cables to an unlimited length. Resistance in the wires causes the signal to degrade past the maximum length, which causes data loss
• Hardware devices called repeaters are sometimes needed to connect cables to repeat and strengthen the signal that is being sent

Straight and crossover network cables

	Hub	Switch	Router	Workstation
Hub	Crossover	Crossover	Straight	Straight
Switch	Crossover	Crossover	Straight	Straight
Router	Straight	Straight	Crossover	Crossover
Workstation	Straight	Straight	Crossover	Crossover

This table reveals certain patterns
• Use a crossover cable to connect identical device types, except in the case of hub-switch and router-workstation connections. Router-workstation connections are uncommon
• Use a straight cable to connect different device types

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The OSI Reference Model

• To understand how data is processed through the network, you need to understand the communication model
• Open System Interconnection (OSI ) is created by Organization for Standardization (ISO) in 1984. The OSI model is a tool used to describe how network data packets are transported through the network
• In the OSI model there are seven layers

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Layer	Name	Function	Example protocols	Format	Network hardware
7	Application	Interface between software applications and the network	Telnet, HTTP, FTP, SSH	Data
6	Presentation	Handles encryption, message formatting, and compression	ASCII, JPEG, PNG	Data
5	Session	Manages applications and establishes, maintains, and terminates user connections	Operating systems, scheduling	Data
4	Transport	Provides reliable or best-effort data delivery with optional error and flow control	TCP, UDP	Segments
3	Network	Provides logical end-to-end network addressing and routing	IP	Packets	Router
2	Data Link	Uses MAC addresses to access network devices. Provides error detection but no correction.	802.3, 802.2, HDLC, FDDI, PPP, Frame relay	Frames	Switch, Bridge
1	Physical	Bit stream. Specifies voltage, wire speed, and cable pin outs	EAI/TIA, V.35	Bits	Hub, Repeater, NIC

• When data is being prepared, headers are added at each layer. This is called encapsulation
• Removing the headers is called de-encapsulation

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The TCP/IP Suite

• Another prevalent network communication model is the TCP/IP suite, also known as the TCP/IP Stack or IP Stack
• There are 4 layers In the TCP/IP Suite

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• The TCP/IP Stack is sometimes presented as a 5-layer model because layer 1 ("Physical") and layer 2 ("Data Link") are split rather than combined into one layer ("Network Access")

Layer	Name	Function	Example protocols	Format	Network hardware
4	Application	Provides network services to applications via services and protocols. Sockets and port numbers are used to differentiate the path and sessions on which applications operate. Message formatting and compression is provided.	Telnet, HTTP, FTP, SSH, JPG, Operating Systems Scheduling	Data
3	Transport	Provides reliable or best-effort data delivery with optional error and flow control	TCP, UDP	Segments
2	Internet	Provides logical end-to-end network addressing and routing.	IP	Packets	Router
1	Network Access	Encapsulates IP packets into frames for transmission. Maps IP addresses to physical hardware addresses (MAC addresses) and uses protocols for physical data transmission.	802.3, 802.2, HDLC, FDDI, PPP, Frame Relay	Frames, Bits	Switch, Bridge, Hub, Repeater, NIC

Index