CS335 Networking & Network Administration Wednesday, April 7 PacketsPackets, Frames, and Error DetectionFramesError Detection

Packets We have been talking in previous classes about lowest levels of data transmission Computer networks do not transfer data as a string of continuous bits Send small blocks called packets Packet switching networks

Packets Sender and receiver need to coordinate to ensure that data arrives correctly If data is lost computers can determine which packets have arrived intact On a network computers share access Using small packets ensures fairness to the wire resource Computers take turns sending packets so one set of computers don’t monopolize the network resource

Time division multiplexing Breaking data into small parts ensures access to the resource

Packets and hardware frames Ex: RS-232 encodes bits to transmit an ASCII character, doesn’t allow sender to signal start and end of a block of characters, hence ASCII 01(soh) and 04(eot)

Byte stuffing If framing characters are in data

Byte stuffing Replace reserved characters in data with mapped escape characters

Transmission errors Interference causes data to be lost or changed Lightning, electro-magnetic interference, power surges Lost, changed, or spuriously appearing bits account for much of the complexity in networks

Parity bits At the end of each character (remember ASCII is 7 bit characters) Sender and receiver agree on odd or even parity Sum the total number of 1 bits in character and make the parity bit a 1 or 0 to make the sum odd or even Parity error checking can’t detect transmission errors that change an even number of bits

Error detection mechanisms Adds overhead (additional bits are added to data) Size of additional information Complexity of algorithm Number of bit errors that can be detected Error detection is approximate – a reasonable effort to produce low probability of corrupt data

Checksum Send a checksum along with each packet Treat the data as a sequence of binary integers and compute the sum

Checksum Advantage Size and ease of computation 16 or 32 bit checksum=small overhead Addition algorithm=small computation overhead Disadvantage Can’t detect all common errors

Checksum error detection

CRC Errors (Cyclic Redundancy Checks) or Better than checksum Exclusive or (xor) Shift register

Shift register, XOR,CRC A hardware 16 bit CRC

Common errors that CRC’s can detect Vertical errors First two bits of each character are set to 0 These errors appear in a vertical column if the characters are arranged in rows Burst errors Small set of bits in one location (from emi)da Account for many errors

Frame format Networks associate error detection at frame level Sender calculates checksum or CRC and transmits the extra data in the frame Receiver calculates the same value and compares it to the additional information sent with the frame

Network topologies Direct point-to-point Shared medium (LAN)

Locality of reference principle Computer communication follows two distinct patterns. First, a computer is more likely to communicate with computers that are physically nearby than with computers that are far away. Second, a computer is more likely to communicate with the same set of computers repeatedly

LAN topologies Star Ring Bus Logical connection vs. physical connection For instance a ring topology need not be in a circle, but could follow a hallway or rise vertically to another floor

Star All computers attach to a central point

Ring Entire ring network is disabled if one node is cut

Bus Single cable to which all computers attach Advantage/disadvantage

Ethernet History Coax, 500 meters, 3 meters between segments, operated at 10Mbps

Ethernet Transmission and Manchester Encoding Manchester Encoding Edge triggered Rising or falling edges encode data Falling edge encodes 0; rising edge = 1 Preamble allows for time synchronization Receiver uses preamble(64 alternating bits preceding the frame) to sync time slots

Sharing on ethernet Bus topology shares access to wire CMSA = Carrier Sense Multiple Access Checks for transmission on wire before sending data Wait until wire is empty to transmit

Collision detect Interference between two signals Simultaneous transmit Garbles data Computers use exponential backoff to choose which will proceed Random time delay before transmitting again