Network Topologies An introduction to Network Topologies and the Link Layer

Direct Point to Point Communication Advantages  Exclusive connections between computers  Useful over long distances  Easy to enforce security since only two computers have exclusive access to the one connection  Every computer has a direct connection with each other.

Direct Point-to-Point Communication Disadvantages  Expensive to implement over a Local Area network  The more computers connected, the more direct connections that are required increasing the cost of the network. For LANS shared communications are more feasible.

LAN Local Area Network Use Shared Communication channels where each computer take a “turn” to communicate over a single medium or network. Using shared communication channels are a cost effective way to implement a LAN

LAN Topologies

Star Topology All computers on the network are connected to a central point The central point is usually a Hub or switch The logical layout may resemble a star while the physical layout may not. The Hub or switch is often located in a separate room or closet where networking staff have exclusive access to them.

Example of a Star Topology

Star Topology Continued Advantage: If one computer fails communication on the network remains intact Disadvantage: More wiring is required for a Star Topology and this increases the cost of the network.

Example of a Star network (ATM) Uses an ATM switch in the center of the network Used for Fiber Communication  Fiber communication is in one direction only.  There is a strand for sending and a strand for receiving The switch will receive the data from the sender and transmit it directly to the intended receiver.

Ring Topology Each computer is connected to each other forming a logical ring Communication of the ring is in one direction The Logical structure of the network is a ring, but computers may be located anywhere in a given building.

Example of a Ring Topology

Ring Topology Cont. Advantages: Makes it easier for many computers to coordinate communication with each other on the Network. Disadvantage: If there is a break in the line, i.e. a computer failure, communication on the network stops.

Self Healing Token Ring Networks Fixes the disadvantages of Ring networks (Breaks in the line) The Ring contains two Rings  An outer ring where most of the communication takes place  An Inner ring – only used when there is a break in the communication line When there is a break in the communication line, all traffic is diverted to the Inner Ring.

Example of a Self Healing Token Ring Network SONET

Bus Topology Each computer is attached logically in a straight path. The physical location of the computers are independent from the topology (each computer can be located on separate floors). The ends of a bus network must be terminated or echo noise will result.

Example of a Bus Topology

Bus Topology Cont. Advantage: requires less wiring than star topology networks (often the reason why it was used in earlier LAN’s) Disadvantage: Like the ring, if there is a break in a wire or a computer is down, the network communication is broken.

Ethernet Standards and Protocols

Manchester Encoding As signals are transmitted on the transmission media, they are timed with specific time slices. Each one and zero that is sent over the network are interpreted by the receiver in the middle of each time slice.

Manchester Encoding Cont. Considered to be edge trigered.  The rising edge of a signal will be interpreted as a 1.  The falling edge of a signal will be interpreted as a 0. The sender and receiver are synchronized by the use of a preamble.  64 alternating ones and zero’s are sent to synchronize the sender and receiver.  Once received, the receiver can accurately determine the time slices for the message sent.

CSMA (Carrier Sense on Multiple Access Networks) Uses electrical activity on the network to determine status. Each message that is sent is sent in the form of a frame (packet) Each sender will check the network for a “clear” (No Carrier signals are present) opportunity to send a message. Checking for a Carrier signal is called Carrier Sense (Where CSMA)

CSMA/CD (Collision Detection) CSMA/CD uses the same technology as CSMA, but provides a means to detect multiple computers sending at the same time (collisions). Collisions cause the original message to become changed. Detecting the changes in the original messages detects the collision that occurred.

CSMA/CD Cont. When a collision is detect retransmission is needed. Each computer will retransmit after waiting a random amount of time and then retransmit their original messages. If a collision occurs again, each computer again will pick a greater amount of time to wait and then retransmit their messages.

CSMA/CA (Collision Avoidance) Used primarily in Wireless networks and APPLETALK With wireless networks it is difficult to “Detect” a collision on the network due to distance. The sender sends a control message to the receiver.

CSMA/CA Receiver will send an acknowledgement if it is ready to receive a message. There can still be collisions with the control messages sent. In the case of collisions, both senders can back off a random amount of time before resending the control message.

AppleTalk (Local Talk) A wired implementation of the CMSA/CA Standard. Similar to the wireless version, a sender sends a small signal to reserve the BUS (network) All other computers refrain from using the network until the sender transmits its frame.

AppleTalk (CMSA/CA) Cont. Advantages:  Cheap to implement in hardware.  The control message is relatively small so there is little overhead created for transmission of data. Disadvantages: Even with the low overhead, the transmission speed tops out at kbps

Token Ring Network Standards

The use of Tokens Sometimes called Token Passing Networks An electronic signal is passed around the ring to each computer. If a Sender wishes to send a message, that sender will reserve the “token” and then send its Frame

The use of Tokens Cont. When the frame is sent the token is released Each computer on the ring then has an equal chance to get the token and send a frame. If no computer reserves the “token”, the “token” is just passed from computer to computer on the ring until it is picked up.