Local Area Networks: Topologies

Packet Identification & MAC Addresses Each packet specifies an intended recipient with an identifier. Demultiplexing uses the identifier known as an address. IEEE (Institute for Electrical and Electronic Engineers) allocates a unique address for each piece of network interface hardware, i.e., a network interface card (NIC) Media Access Control (MAC) address A device may have multiple MAC addresses depending on how many network interface cards it has.

48 bits in hexadecimal number

MAC Addresses Composition Vendor ID + NIC ID IEEE assigns a vendor ID VV:VV:VV:NN:NN:NN IEEE assigns a vendor ID Organizationally Unique ID (OUI) http://standards.ieee.org/regauth/oui/oui.txt The vendor assigns a unique NIC value to each device

Exercise: Find MAC address of your computer Start Command Prompt (Start Runcmd) and type ipconfig /all Do a google search to find the vendor name of the NIC of your computer

Compose a Packet: Step 1 I like you A packet consists of two conceptual parts: Header that contains metadata, such as an address Payload that contains the data being sent header payload 54:DD:91 11:CA:3B E8:25:32 46:C2:8F I like you 6 bytes dest. address 6 bytes source address 46-1500 bytes

Compose a Packet: Step 2 Framing: using extra bits to delineate the start and end of a frame SOH: Start of Header (ASCII code: 0000001) EOT: End of Transmission (ASCII code: 0000100) To enable computer to distinguish two packets, the SOH and EOT must be unique and different from the payload. Frame: Layer-2 packet SOH header payload EOT 1 byte 1 byte

Compose a Packet: Step 3 Add error checking code Cable Ethernet use Cyclic Redundancy Check (CRC), a form of channel coding for high-speed data network SOH header payload EOT CRC Cable modulation More on CRC: Chapter 8.13 – 8.14

How to Transmit a Packet? Depends on the network type and topology Three main categories of packet switching network:

LAN Example

LAN Topologies Each LAN is classified into a category according to its topology or general shape

Star Topology in Detail P-B NIC-B P-S P-A NIC-2 P-C NIC-A NIC-1 NIC-3 NIC-C NIC-4 P-D NIC-D

Star Topology in Detail Steps for P-A to send “I like you” to P-C: P-A will compose the packet P-A forwards the packet to NIC-A NIC-A calculates the CRC, adds SOH, EOT NIC-A modulates the packet  send out After a while, NIC-1 will receive the m-carrier NIC-1 demodulates the m-carrier and put the packet into a queue P-S checks this packet and find destination is “C” NIC-3 modulates the packet  send out After a while, NIC-C will receive the m-carrier NIC-C demodulates the packet and put it into its cache NIC-C does two checkings; If failed, the packet will be discarded Length checking: the payload should be between 46 and 1500 bytes CRC checking If the packet passes all checkings, NIC-C will strip off SOH, EOT, CRC, and forward “|C|A|I like you|” to P-C I like you C A CRC I like you SOH EOT C A

Bus Topology P-B P-D P-A P-C Bus topology usually consists of a single cable to which computers are attached. The ends of a bus network must be terminated. Preventing electrical signals from reflecting back along the bus. P-B P-D NIC-B NIC-D P-A NIC-A P-C NIC-C

Bus Topology in Detail Steps for P-A to send “I like you” to P-C: P-A will compose the packet P-A forwards the packet to NIC-A NIC-A calculates the CRC, adds SOH, EOT NIC-A modulates the packet  send out After a while, NIC-C will receive the m-carrier NIC-C demodulates the packet and put it into its cache NIC-C does address checking; if the packet is not for P-C, discard it NIC-C does length and CRC checkings; If failed, the packet will be discarded If the packet passes all checkings, NIC-C will strip off SOH, EOT, CRC, and send “|C|A|I like you|” to P-C I like you C A CRC I like you SOH EOT C A

Bus Topology When there is a signal on the bus, everybody can sense it. What if multiple computers are sending packets simultaneously? Collisions may happen!

Bus Topology: The Collision Problem Basic solution: CSMA (Carrier Sense Multiple Access) Before any packet is sent out, the sender will first senses its connecting point to see if the bus is idle. If nothing is sensed, it will send out a packet, otherwise it will wait until nothing is sensed. Q1: Do you believe that CSMA can avoid all collisions? No

Bus Topology: The Collision Problem Advanced solution: CSMA/CD (CD = Collisions Detection) During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus. If any other m-carrier is sensed, stop transmitting instantly; wait for a random amount of time; then retransmit Q2: If collision occurs, who will detect the collision? Every computer will sense the collision

Bus Topology: The Collision Problem Advanced solution: CSMA/CD (CD = Collisions Detection) During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus. If any other m-carrier is sensed, stop transmitting instantly; wait for a random amount of time; then retransmit Q3: Why do we wait a random amount of time? So that we are sure that there isn’t as great a chance that the packets collide again

Bus Topology: The Collision Problem Advanced solution: CSMA/CD (CD = Collisions Detection) During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus. If any other m-carrier is sensed, stop transmitting instantly; wait for a random amount of time; then retransmit Q4: When a NIC senses a collided m-carrier, will the NIC card demodulate it? Yes. You have to demodulate the packet in order to know if it is corrupted

Bus Topology: The Collision Problem Advanced solution: CSMA/CD (CD = Collisions Detection) During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus. If any other m-carrier is sensed, stop transmitting instantly; wait for a random amount of time; then retransmit Q5: When a NIC senses a collided m-carrier, will the NIC forward the packet to the computer? No. The packet will be discarded.

Why Different Topologies? Ring: Easy for computers to coordinate access and to detect whether the network is operating correctly An entire ring network is disabled if one of the cables is cut Star: Protecting the network from damage to a single cable No collisions at all Requiring more wires Bus: Fewer wires A network is disabled if someone accidentally cuts the main cable Difficult to add new devices if no tap exists. As such, bus topology is losing popularity but Plenty still around.