Computer Networks 2

encoding

translating bits from sending node into form that can be send via network

framing problem

representation of bits transmitted over the link, so they can be delivered to the end node

media access control (MAC)

how to share medium efficiently among nodes

Direct Links Networks

Wireless network, Last-mile links networks, Local Area Networks (LAN)

network adaptor

hardware that connects a node to a link, it encodes bits into signals at the sending node and decodes signals into bits at the receiving node

Encoding methods

Non-return to zero, Non-return to zero inverted, Manchester, 4B5B

Non-return to zero approach

when bit is 0 then signal is low and vice versa, sequences of consecutive 0s/1s makes singnal stays low/hgih for long

Non-return to zero inverted approach

bit 1 change the signal frequency (low/high)

Manchester approach

0 being encoded as low-to-high transition and 1 being encoded as high-to-low transition

4B5B encoding

insert extra bits into the bit stream so as to break up long sequences of 0s or 1s

Byte oriented framing methods

Usage of special characters (sentinel characters), Include number of bytes in the frame

HDLC

High Data Level Control Protocol- used for transmission of bit data, ensures reliable data transfer and can operate without a continuous connection

Bit Stuffing

when a specific sequence of bits appears in the data an extra bit is added to break the pattern, prevents the receiver from mistaking the data for control information

Error Detection solving

Notify the sender, to get the message again. Reconstruct the correct message by recipient

error-detecting codes

redundant information bits to help receiver determine if error occurs

reliable delivery

achived using acknowledgments and timeouts

Stop and wait algorithm

after transmitting one frame, the sender waits for an acknowledgment before transmitting the next frame. If ACK do not arrive retransmit message

Sliding window algorithm

sender assigns a sequence number to each frame and the sender maintains three variables: send window size, last ACK received and last frame send

repeater

device that forwards digital signal but do not understand it

spread spectrum

spread the signal over a wider frequency band to minimize the impact of interference from other devices

spreaad spectrum techniques

frequency hopping, direct sequence spread

Frequency Hopping

transmition of the signal over a random sequence of frequencies

Direct Sequence Spread

adds redundant bits for greater tolerance of interference

Endpoints in wireless networks

Base station, Mesh networks

Base station

has no mobility but has a wired connection to the Internet or other networks

Mesh network

nodes are peers, there is no base station nodes