Topologies

Topology is simply the "shape" of the connections. The type of topology you choose will be constrained by the media, and by other physical considerations.

Physical topologies (layer 1)

Physical topologies are the lowest level. They describe how the wires (fibers, airwaves) are used to connected together stations. You would call each layer 1 physical connection a segment of a layer 2 network.

Point to Point

Dial up, Serial lines, full duplex Ethernet are examples. Each station talks only to one other station. Sometimes station A and station B can each only talk when the other is not talking, this is half duplex. Sometimes station A and station B and talk to each other even when the other is talking, this is full duplex (and really two point to point links, one in each direction).

Bus topologies

"10 base 5" or "10 base 2" are Ethernet examples, as are most Radio Frequency media. You have a medium shared by all stations. Only one station may transmit to the media at any time.

Ring topology

"Token ring" or "FDDI" are examples. Each station has connection to two other stations on the ring. Data is passed from station to station, and only the addressed destination pays attention to it, all others just pass it along. Can also be thought of as a ring of point to point connections. Usually a "token" is used to control media access.

Star topology

"100 base TX" or "10 base FL" Ethernet are examples. A "hub" or "repeater" device sits in the middle of all stations. It takes input from each station and retransmits it to all others. This device in the "center" can be passive (IE just electrically repeating things) or active (receiving the entire frame and the retransmitting).

Mesh

Point to point links between all (Full Mesh) or many (Partial Mesh) stations. Not really practical for Ethernet. Sometimes used at layer 3 (IP) to improve redundancy.

Hybrid

Building a network by mixing the above topologies. Linking together multiple layer 1 (physical) topologies into a single layer 2 (network) network. Or linking together multiple layer 2 networks into a single layer 3 network.

Sometimes this Hybrid linking of physical layers is used to provide redundancy or multiple paths between end stations.

Network topologies (layer 2)

A set of stations that use the same layer 2 frame format and addressing can be thought of as being part of the same Layer 2 "network". These networks may have multiple physical topologies, but share in common the layer 2 address space and frame format. An example is Ethernet which might have Bus and Star physical topology networks linked together to build a single set of stations addressable on the network. Layer 2 networks are always bordered by layer 3 devices (routers) which link them into Inter-networks.

Inter network topologies (layer 3)

When you talk about topologies used when connecting layer 2 networks together and running a common layer 3 protocol, you are creating an "internet". The layer 1 and layer 2 topologies of the component networks can be quite dissimilar and can have stars linked to buses linked to rings. (Hybrid). Devices called routers will separate and link the various layer 2 networks.

Sometimes this layer 3 linking of layer 2 networks is used to provide redundancy or multiple paths between networks.

Media

Shared vs Non-Shared

Shared means all stations "share" the wire electrically (or optically), and only one can talk at any one time. Since other stations share the media there must be some sort of Media Access Control (MAC) to determine who can transmit when.

Non-Shared means *only two* stations share the medium. Typically you have two channels, one in each direction. Since the media is not shared then the transmitting station has no need to determine if the media is available. The media always is available.

Logical Bus vs Point to Point

Media can be shared between a number of stations or can be shared between two stations only. If the medium is shared, it is considered a logical bus topology (no matter the physical characteristics of the connections). Non-shared media is essentially point to point.

In some networks, the media is directly connected only to end stations. (10 Base 2, 10 Base 5 and Radio Frequency are examples) In other networks there are devices that typically sit in the center of a physical star that aid in the access to the medium. (10 Base T is an example).

Coaxial Copper cable

RG8 (thick) and RG58 (thin) cables are used as media for a number of network technologies. In Ethernet the RG8 (thick wire) cable is connected via a vampire tap MAU (media access unit). In Ethernet with RG58 (thin wire) a BNC T connector is used to connect to the MAU.

Unshielded Twisted Pair

There are two types of twisted pair cables shielded and unshielded (you hardly ever see shielded) There are different categories of unshielded twisted pair (UTP), higher is capable of higher speed data transmission. 4 pairs (8 wires) in each typical UTP cable

4 pairs used by Gigabit Ethernet. 4 pairs used by 100BaseT4 100 mbps Ethernet (not common).

Only 2 pairs used by 10BaseT 10Mbps Ethernet and only two pairs used in 100BaseTX 100Mbps Ethernet

Cables usually built to T568B and T568A standards. The end on the twisted pair cable is known as a RJ45 connector. It is similar but larger than the RJ11 connector used for most phones. RJ45 connector is also known as a 8P8C connector.

Eric's cable building crib sheet

Good link on EIA/TIA T568A and T568B http://www.duxcw.com/faq/network/diff568ab.htm

Fiber optics


Multimode 62.5/125 or 50/125 (smaller = better range) 62.5 micrometers is core diameter, 125 micrometers is cladding diameter. Singlemode 9/125 Fiber optic cables use different types of connectors, all designed to line up the fibers precisely.
examples: SC, ST, LC
Here is some good details on the different types of fiber optic connectors.

Though most fiber optic cables carry only one frequency of light modern Fiber Optic systems can carry multiple frequencies of light, each being a separate signal. This is known as WDM (Wavelength division multiplexing). Each signal is called a Lambda (from the greek symbol for wavelength) and can carry the same amount of information as a signal normal signal. There are two broad categories of WDM, DWDM (Dense) and CWDM (Course). DWDM has more lambda's per cable, CWDM has less.

Classic (older) Ethernet media and topologies

Here are the 10 Mbps (classical) Ethernet physical layers. MAU is the Media Attachment Unit. The MAU connects to the transceiver (in fact is often built into the transceiver) and the transceiver connects to the Ethernet card via the AUI (Attachment Unit Interface) cable.

Many modern Ethernet cards have the transceiver built in and as such only make the MAU interface available.

Modern Ethernet media

Ethernet topologies

Ethernet can be a physical star or bus. It is a physical star if using unshielded twisted pair or fiber optic media. Ethernet is a physical bus if using older 10Base2 or 10Base5 media.

In the case of a physical star topology, Ethernet requires a device to sit in the center of the star and link up all systems. This is a Repeater or Hub (synonyms) in older half duplex systems. It is a bridge or switch in modern full duplex systems.

An Ethernet network can be made up of multiple segments separated by Ethernet repeaters (sometimes called Ethernet hubs. Same thing). A repeater/hub simply receives and electrical signal and replicates and amplifies it.

As you combine older 10 Mbit Ethernet segments you have to be certain that you do not exceed guidelines for electrical signal strength and signal timing. A good rule of thumb for 10 MB Ethernet is the 5-4-3 rule. 

	5-4-3 - 5 segments, 4 repeaters, 3 mixing (more than one station)
You can find distance and segment guidelines for 10M Ethernet at http://www.ethermanage.com/ethernet/ch13-ora/ch13.html

Modern ethernet (IE 100BaseTX or newer/faster) can only exist as a star topology. It must have a hub or switch in the center. More on this later.

Ethernet networks can be divided by devices called switches instead of repeaters. Switches separate Ethernet networks into different collision domains. With a switch instead of a hub you have a star topology of point to point networks.

When you have a switch, each port on the switch can be a separate collision domain. But all ports on a normal (not vlan/802.1q capable) switch are in the same *broadcast* domain.

Some terms

Broadband - multiple signals over a single medium

Baseband - Only one signal per medium

Ethernet is a primarily a baseband technology.

End stations vs Intermediate Stations

The difference between Bits Per Second and Baud.

A signal that has 9600 state changes per second, each state change encoding 4 bits of data, is 9600 baud and 38400bps.

Collision Domain

Ethernet networks can be thought of as all station within a collision domain. IE, if two stations can electrically interfere with each other (produce a collision) then they are on the same physical Ethernet network.

Cat 5 Twisted pair pin configuration 

Pin Order RJ45
(looking down with cable to you and pins on connector faceing up)
12345678

Our color scheme conforms to T568B

Pairs(pins)  	1(4/5) Blue		Crossover 		1(4/5) Brown
MDI		2(1/2) Orange		MDIX			3(1/2) Green
		3(3/6) Green					2(3/6) Orange
		4(7/8) Brown					4(7/8) Blue

Pair 2 and 3 are used for ethernet.
Pair 2 is transmit (tx+,tx-) Pair 3 is receive (rx+,rx-) (in MDI)
Pair 3 is receive (rx+,rx-) Pair 2 is transmit (tx+,tx-) (in MDIX)

Workstations are normally MDI (tx on pair 2, rx on pair 3)
Hubs/Repeates are normally MDIX (tx on pair 3, rx on pair 2)

	

	RJ45 pin colors		
	Straight through - both ends of cable 

1	orange/white	
2	orange	
3	green/white	
4	blue
5	blue/white
6	green		
7	brown/white	
8	brown	


	RJ45 pin colors		
	Cross over  - one end is done as stright through, the other as below

1	green/white	
2	green		
3	orange/white	
4	brown/white
5	brown
6	orange	
7	blue
8	blue/white