Media and Topologies

NETWORK TOPOLOGIES

A logical topology depicts the route the signal takes on the network.
A physical topology depicts how network devices are connected physically, the cabling.

The 4 diagrams below represent the four topologies:

	Bus - Devices in a bus topology are connected to a central cable. In this type of network, both cable ends must be terminated. A defective cable segment, and changes and additions can affect the entire network.
	Star - Devices in a star topology are connected through a central hub. In this type of network, new nodes can be easily added making it easy to expand. Multiple connected star networks can form a large star or hierarchical topology. The central hub , which physically can be a hub, switch, or router, forms a single-point-of-failure. Another disadvantage is the increased amount of required cabling.
	Ring - In a ring topology, every node is logically connected to two other nodes , forming a ring. Traffic flows through the entire ring until it reaches its destination.
	Mesh - In a full mesh, every device in the network is connected to every other device. In reality, a partial mesh is commonly used in backbone environments to provide fault-tolerant connections between critical servers and network devices.

NETWORK TECHNOLOGIES

802.2 (LLC)

The IEEE 802.2 standard specifies the Logical Link Control (LLC) layer, which is the upper sub-layer of the Data Link layer (Layer 2) in the OSI model. LLC masks the underlying physical network technologies by hiding their differences, hence providing a single interface to the Network layer. The interface acts as an intermediate between the different network protocols (IPX, TCP/IP, etc.) and the different network types (Ethernet, Token Ring, etc.).

802.3 (Ethernet)

Ethernet is a LAN standard developed by DIX (Digital, Intel and Xerox) in the 1970s. In 1980, version 1 of the IEEE 802.3 standard was released. Two years later version 2 of the IEEE 802.3 standard was introduced, which in turn is the basis for today's Ethernet networks. It specifies an implementation of the physical layer and the MAC sub-layer of the data link layer. The older 10Base2 and 10Base5, and the modern Fast Ethernet, Gigabit Ethernet, and 10Gigabit Ethernet extensions and variations are all based on the original IEEE 802.3 standard.

The access method – how the wire is accessed and signals are places on it – for Ethernet networks is Carrier Sense Multiple Access/Collision Detection (CSMA/CD). In a CSMA/CD network, a stations listen to check if the network is busy transmitting data before starting its own data transmission. If the network is free, the station transmits data. When two stations listen and both determine the network is not busy and start sending the data simultaneously, a collision occurs. When the collision is detected, both stations will retransmit the data after a random wait time created by a backoff algorithm.

An Ethernet network is a broadcast system; this means that when a station transmits data, every other station receives the data. The frames contain a destination address in the frame header and only the station with that address will pick up the frame and pass it on to upper-layer protocols to be processed.

802.3 ETHERNET STANDARDS
10Base2 and 10Base5

CompTIA removed 10Base2 and 10Base5 from the exam objectives with the Network+ 2005 update, but you may still find these technologies being part of networks in some organizations. 10Base2 is commonly referred to as Thinnet, and 10Base5 is known as Thicknet, both offering data transfer rates up to 10Mb/s. These names refer to the diameter of the coaxial cable employed by these network technologies. This rigid type of cabling is shielded and provides fairly good protection against electromagnetic interference (EMI) and eavesdropping. Both outer cable ends are terminated using a 50-Ohm terminator.

10Base2 uses a bus topology as depicted in the following diagram:



The maximum length of a 10Base2 segment is 185 meters, which can be extended by using repeaters. The maximum number of nodes that can be attached per segment is 30. Stations are attached using BNC T-connectors as shown in the following picture:


BNC (British Naval Connector) T-connector.

10Base5 also employs a bus topology, as depicted in the following diagram, but uses a different method to attach network nodes to the central cable in the bus.



Stations are attached using a MAU (Medium Attachment Unit), a transceiver that is attached to the central cable using vampire taps that pierce the cable. A cable with AUI connectors is used to connect the transceiver to the network interface on for example a computer, hub or repeater.

AUI connectors	MAU transceiver

The maximum length of a 10Base5 segment is 500 meters, which also can be extended by using repeaters. The maximum length of the cable between a MAU and the AUI connector on pc is 50 meter. The maximum number of nodes that can be attached per segment is 100.

10BaseT (802.3i)

The 10BaseT Ethernet specification specifies Ethernet over Cat 3, 4 and 5 UTP cabling and provides a maximum data transfer rate of 10 Mb/s. This specification is commonly referred to as Ethernet, just plain Ethernet. Devices on the network are connected through a central hub or switch in a star/hierarchical topology.



The maximum cable length of 10BaseT segment is 100 meters. The maximum number of attachments per cable segment is 2, i.e. a hub and a client. 10BaseT employs Cat 3, 4 and 5 Unshielded Twisted Pair (UTP) cabling with RJ-45 connectors as depicted below. Older network devices with AUI interfaces can use a transceiver to employ UTP cabling.



RJ-45 connectors

A wire crimper, depicted in the image below, is used to attach the RJ-45 connector to the cable.



Another tool commonly used to attach UTP cabling to a jacket, in a patch closet for example, is the punch down tool, shown in the following image:

100BaseTX (Fast Ethernet, 802.3u)

100BaseTX, Fast Ethernet, is similar to 10BaseT but requires Category 5 UTP or Category 1 STP (Shielded Twisted Pair) cabling. It uses only four of the eight wires in the cable, just as 10BaseT does. The maximum cable segment distance is still 100 meters, but the maximum data transfer rate is 100 Mb/s.

10BaseFL (802.3j)

10BaseFL is the successor of the FOIRL (Fiber Optic Inter-Repeater Link) specification, and defines Ethernet over fiber optic cabling. FOIRL allowed a point-to-point link between two repeaters up to 1000 meters apart. When fiber optic cabling started to ‘reach’ desktops and other end-devices, new standards where developed, starting with the 10BaseF set including 10BaseFL, 10BaseFB, and 10BaseFP. 10BaseFL is the most common of the three, and is the only one of importance for the CompTIA Network+ exam. 10BaseFL is similar to 10BaseT but designed to operate over two strands of multimode fiber cabling and provides a maximum data transfer rate of 10 Mb/s. One strand is used for sending, the other is used for collision detection and receiving. It is designed to be able to work with existing FOIRL hardware, allowing a smooth migration to 10BaseFL. The maximum cable segment length is 2000 meters for a 10BaseFL multimode fiber link. 10BaseFL uses primarily ST or SC connectors as depicted below. Media converters can be used to provide fiber optic connections to systems that have regular Ethernet network interface cards.

SC connectors	ST connectors

MMF (Multimode Fiber) optic cable

A Multimode Fiber (MMF) fiber optic cable contains a single strand of relatively thick fiber core with a glass or plastic cladding surrounding it. Light rays bounce against the cladding when they travel through the fiber core. Light rays can take different paths, as depicted in the image below, allowing multiple signals to pass the fiber cable simultaneously. The bouncing off the cladding causes signal loss, known as attenuation, because the energy level of a light ray decreases as it transfers heat to cladding. Multimode fiber is primarily used in local area networks.

SMF (Single Mode Fiber) optic cable

A Single Mode Fiber (SMF) optic cable contains a single strand of fiber and allows for only one transmission mode. The relatively small fiber core forces the light to travel in a single direction straight through the core without bouncing off the cladding. This results in less attenuation and support for higher bandwidths, faster transmissions speeds, and much greater distances than multimode fiber. However, it is also more expensive. Single-mode fiber is particularly suitable for long-distance network, telephony and television broadcast systems.

100BaseFX (802.3u)

100BaseFX is the fiber optic equivalent of 100BaseTX. As 10BaseFL, it specifies operation over two strands of multimode fiber cabling. The maximum length of a 100BaseFX link is 400 meters in half-duplex mode and 2000 meters in full-duplex mode. There are non-standard 100BaseFX–based solutions available that allow distances up to 75 km for single-mode fiber optic cabling. 100BaseFX specifies ST, SC, and MIC connectors, but MT-RJ connectors are also used in 100BaseFX-based product:

SC to ST cable	MIC connector	MT-RJ connector

The Mechanical Transfer Registered Jack (MTRJ) is part of a family of Small Form Factor (SFF) adapters that are compact in size compared to the more popular SC and ST adapter types. This increases fiber density per rack unit in data closets.

Gigabit Ethernet

The two 802.3 standards that specify Gigabit Ethernet systems are described below. A major difference with previous Ethernet specifications, is that it uses a different encoding type named 8B/10B with simple NRZ (Non Return to Zero), which results in 10 bits being send per byte (instead of 8). By running pulses of 1250 MHz, the maximum data transfer rate is 1 Gb/s even with the 20% overhead.

1000BaseT (802.3ab)

1000BaseT specifies Gigabit Ethernet over Cat 5e UTP cabling and provides data transfer rates of 1000 Mb/s. It utilizes all four pairs of cable wires for transmission. The maximum cable segment length is 100 meters. 1000BaseTX s pecifies Gigabit Ethernet over Cat 6 UTP cabling, but is not part of the IEEE 802.3ab standard.

1000BaseX (802.3z)

The IEEE 802.3z Gigabit Ethernet standard includes two Physical Layer specifications for fiber optic media, 1000BaseSX and 1000BaseLX, and one for shielded copper media, 1000BaseCX.

1000BaseLX uses multimode fiber with a maximum length of 550 meters or single-mode fiber with a maximum length of 5 km.

1000BaseSX uses multimode fiber with a cable length up to 500 meters IEEE standard specifies SC connectors.

Both 1000BaseLX and 1000BaseSX use SC connectors or the newer LC (Local Connector) connectors. The LC connectors are half the size as their predecessors and reduce the loss of light entering or leaving the cable. LC connectors are available in single-mode and multimode versions.

LC Connector

1000BaseCX specifies Gigabit Ethernet over a special 150-Ohm shielded coaxial cable, also known as twinax, with DB-9 connectors. It is specifically designed for short cable runs such as server-to-server connections and specifies a maximum cable length of 25 meters.

10Gigabit Ethernet (802.3ae)

The IEEE 802.3ae standard specifies 10Gigabit Ethernet, also referred to as 10GbE, over multimode and single-mode fiber optics. In addition to additional bandwidth, 10GbE increases the maximum fiber optic cable lengths up to 40 kilometers. Just as Gigabit Ethernet is based on the original Ethernet standard, 10 Gigabit Ethernet continues still uses the same frame format and size. However, since it is a full-duplex and employs only fiber optic cabling, it does not need CSMA/CD access method protocol. The most common 10GbE specifications that are relevant for the CompTIA Network+ exam are 10GBaseSR, 10GBaseLR, and 10GBaseER. These specifications use a much more efficient encoding type named 64B/66B, which results in data transfer rates of 10.3 Gb/s. All three of the following use SC or LC connectors.

10GBaseSR operates over multimode fiber up to 300 meters.
10GBaseLR operates over single-mode fiber up to 10 km.
10GBaseER operates over single-mode fiber up to 40 km.

RING NETWORK TECHNOLOGIES

802.5 (Token Ring)

Token Ring was originally developed by IBM in the 1970s. Later the IEEE 802.5 specification was developed based on IBM's Token Ring. Despite of what the exam objective implies, Token Ring and the IEEE 802.5 specification are not exactly the same, but the differences are minor. For example, the IEEE 802.5 specification does not specify a physical topology and media, while Token Ring does. The term Token Ring usually refers to either specification. In a Token Ring network, a token is passed around the network from station to station. When a station does not need to transmit data it passes the token to the next station in the logical ring. A station that receives the token and needs to transmit data, seizes the token and sends a data frame. The receiving station marks the data frame as read and passes it forward along the ring to the source station. During this entire process, no other station can transmit data, which rules out collisions on the wire. The source station releases the token (passing it to the next station in line) when it received the data frame and verified it was read. While the logical topology is a ring, the physical topology is star/hierarchical as illustrated in the diagram below. Stations connect to Multi-Station Access Units (similar to a hub) using UTP cabling, which in turn are connected in a physical ring. If one station in the ring fails, it generally doesn't mean the ring is broken. Instead, the MSAU will bypass the individual port and exclude it from the ring.



Token Ring specifications:
- Data transfer rate is 4 or 16 Mb/s
- Maximum attachments per segment is 250
- Uses Twisted Pair cabling (Cat 3 for 4 MB/s, Cat 5 for 16 Mb/s)
- Access method is token passing
- Logical topology ring, physical topology is star
- Connector type is RJ-45

The original IBM Token Ring specification uses IBM Class 1 STP cabling with IBM proprietary connectors. This connector is called the IBM-type Data Connector (IDC) or Universal Data Connector (UDC), and is neither male nor female.

FDDI

Another token-passing network technology is Fiber Distributed Data Interface (FDDI), created by ANSI (American National Standards Institute) in the mid 1980s. FDDI networks are typically used as backbones for wide-area networks to provide data transfer rates up to 100 Mb/s using fiber optic media over large distances. FDDI provides some fault tolerance by using a dual counter-rotating ring configuration – an active primary ring, and a secondary ring used for backup. Some stations are connected to both rings directly (Dual-Attached Stations) and others are connected to a single ring using concentrators (Single-Attached Stations). FDDI uses fiber optic cabling with SC, ST or MIC connectors. There is also an implementation of FDDI that runs on traditional Copper wiring (UTP) that is known as CDDI but is beyond the scope of the Network+ exam.

IEEE1394 (FireWire)

The IEEE 1394 standard specifies a high-speed serial connection. It is originally designed primarily for transferring digital video between a PC and a video camera, but is also used to connect printers, external hard disks, and other peripherals. IEEE 1394 is also known under the trademark FireWire from Apple, and i-Link from Sony. It is often not considered for a corporate network design while it can be a very suitable and affordable solution for short cable runs between servers for example. Operating systems such as Windows XP include built-in support for IP over IEEE1394, which allows the interface to act as a regular network interface providing data transfer rates up to 400 Mb/s. That’s almost just as fast as the effective data throughput of a Gigabit Ethernet link. The maximum cable distance of an IEEE 1394 link is of 4.5 meters. The cable consists of six copper wires, of which two carry power and four are grouped into two twisted pairs. The updated IEEE 1394b standard released in 2002 specifies data transfer rates up to 3.2 Gb/s, over 100 meter Cat 5 UTP or fiber cabling.

6-pin FireWire and 4-pin i-Link (without power wires) connector

Tools

Media tester/certifier	There are several types of cable testers, of which some only monitor the electrical signal and others are capable of recognizing errors such as collisions, traffic congestion, error frames, and protocol errors even. A certifier typically measures frequencies to determine the maximum MHz for a cable.
Tone generator	This device is used to find outer ends of a cable. Place the tone generator on one end of the cable you want to find the other end of, and use a tracer (or probe) on the other end, or usually, what you think is the other end.
Optical tester	This device can be used to find a break or kink in fiber optic cabling.
Time Domain Reflectometer	This device sends pulses through a cable to detect a break or other inconsistencies.
Loopback adapter	As a physical device, a loopback adapter is a kind of terminator you can connect directly to a NIC, allowing you to configure it with an IP address and simulate as if a network were attached, hence test the NIC’s functionality.
Digital Volt meter	A very common electrical measurement tool that can be used to track down breaks in the cable and shortage with other cabling or metal.
Protocol Analyzers (Sniffers)	Typically a tool implemented in software, which analyzes data packets to determine network problems related to software, clients/servers, network addressing and much more.