Local Area Networks: Topologies

2 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.

3 48 bits in hexadecimal number

4 MAC Addresses Composition – Vendor ID + NIC ID VV:VV:VV:NN:NN:NN – IEEE assigns a vendor ID Organizationally Unique ID (OUI) – The vendor assigns a unique NIC value to each device

Exercise: Find MAC address of your computer 1.Start Command Prompt (Start  Run  cmd) and type ipconfig /all 2.Do a google search to find the vendor name of the NIC of your computer

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

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

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

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

LAN Example

11 LAN Topologies Each LAN is classified into a category according to its topology or general shape 1980s: IBM Token Ring1990s: Ethernet 2000s - Now: Switched LANNot scalable : Rarely used

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

Star Topology in Detail 1.P-A will compose the packet 2.P-A forwards the packet to NIC-A 3.NIC-A calculates the CRC, adds SOH, EOT 4.NIC-A modulates the packet  send out 5.After a while, NIC-1 will receive the m-carrier 6.NIC-1 demodulates the m-carrier and put the packet into a queue 7.P-S checks this packet and find destination is “C” 8.NIC-3 modulates the packet  send out 9.After a while, NIC-C will receive the m-carrier 10.NIC-C demodulates the packet and put it into its cache 11.NIC-C does two checkings; If failed, the packet will be discarded a)Length checking: the payload should be between 46 and 1500 bytes b)CRC checking 12.If the packet passes all checkings, NIC-C will strip off SOH, EOT, CRC, and forward “|C|A|I like you|” to P-C Steps for P-A to send “I like you” to P-C: CRC I like you SOHEOTCAI like youCA

14 Bus Topology 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 NIC-B P-D NIC-D P-A NIC-A P-C NIC-C

Bus Topology in Detail 1.P-A will compose the packet 2.P-A forwards the packet to NIC-A 3.NIC-A calculates the CRC, adds SOH, EOT 4.NIC-A modulates the packet  send out 5.After a while, NIC-C will receive the m-carrier 6.NIC-C demodulates the packet and put it into its cache 7.NIC-C does address checking; if the packet is not for P-C, discard it 8.NIC-C does length and CRC checkings; If failed, the packet will be discarded 9.If the packet passes all checkings, NIC-C will strip off SOH, EOT, CRC, and send “|C|A|I like you|” to P-C Steps for P-A to send “I like you” to P-C: CRCI like youSOHEOTCAI like you 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) 1.Before any packet is sent out, the sender will first senses its connecting point to see if the bus is idle. 2.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?

Bus Topology: The Collision Problem Advanced solution: CSMA/CD (CD = Collisions Detection) 1.During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus. 2.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?

Bus Topology: The Collision Problem Advanced solution: CSMA/CD (CD = Collisions Detection) 1.During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus. 2.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?

Bus Topology: The Collision Problem Advanced solution: CSMA/CD (CD = Collisions Detection) 1.During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus. 2.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?

Bus Topology: The Collision Problem Advanced solution: CSMA/CD (CD = Collisions Detection) 1.During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus. 2.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?

22 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.