first commit

Semester 1/Computer Systems Internals & Linux/Week 7/Week 7 Computer Systems.md

@@ -0,0 +1,93 @@
+## Lecture 1 (12:00)
+
+### Flow Control
+
+If sender sends data faster than recipient can consume it, the recipient must store the data in a buffer until it is ready - in RAM or storage.
+If this happens for a sustained period, the buffer gets full, and any excess data is discarded.
+Flow Control is the mechanism that ensures the sender stops transmitting before the buffer is full to prevent wasted bandwidth.
+
+- Full Duplex (Two way negotiation)
+	- Destination has received all frames sent so far
+	- Destination is ready to receive more data.
+	- Data and control info can flow simultaneously
+- Simplex (one way) transmission system, we cant have two-way negotiation
+
+### Packets
+
+- Frame / Packet is a chunk of data with a fixed or max size.
+	- May stand alone or be fragments of larger items
+- Dividing data gives us chance to send management info between frames.
+	- Extra overhead = Slight loss of efficiency.
+- Propagation Time vs Transmission Time
+	- Propagation Time is the amount of time it takes for one particular signal to get from one point to another.
+	- Transmission Time is the total amount of data that can be sent from one place to another in a given period of time.
+
+#### Sliding Windows Flow Control
+
+- Every packet is numbered in sequence.
+- **Window Size** defines the number of packets that can be sent by the sender without needing an acknowledgement from the recipient
+	- Amount of space recipient reserves for buffer space to be used for conversations with this particular sender.
+- Recipient will periodically tell the sender the start of the window
+	- This way , the sender can send packets up to (windowStart + windowSize - 1)
+- Used in the TCP protocol on the internet.
+![](Pasted%20image%2020231030122822.png)
+![](Pasted%20image%2020231030122739.png)
+
+### Error Correction
+
+- Fix the Error (FEC)
+- Ask for the frame containing the error to be re-transmitted - Automatic Repeat ReQuest / Query (ARQ)
+- Throw away the frame containing the error
+	- Usual, and ARQ requires it
+- Ignore the error
+	- Useful for low quality streaming multimedia data
+	- Dangerous in networking, as it often matters where the error is.
+
+#### Forward Error Correction (FEC)
+
+- Represent something using more bits than needed. An error results in an invalid value that can be rounded to the nearest valid one.
+- The more extra bits, the better redundancy gained.
+- Simple ex.
+	- Majority Voting:
+		- Transmit each bit 3+ times (000, 111)
+		- For the recipient, 001/010/100 are rounded to 0
+		- 110/101/011 are rounded to 1
+
+#### Re-transmission
+
+- If we cannot fix the error, ask for retransmission
+	- We need a return channel to ask, so we cannot do this over simplex channels ( ex. TV )
+	- Must be unicast ( cannot be performed over broadcast / multicast data )
+	- Data must be time independent ( cannot be used for realtime data such as multimedia )
+
+#### Go-Back-N ARQ
+
+- Pairs with Sliding Windows Flow Control.
+- Recipient discards a packet if it contains an error
+	- You notice you have discarded the packet when you receive the next packet with an out of sequence number.
+	- When noticed, discard packet and request sender to backpedal to last correct packet.
+	- Sender re-transmits all packets started from missing packet.
+- Sender polls for a response if no acknowledgement is received (assumed lost)
+	- Recipient re-sends last acknowledgement.
+
+## Lecture 2 (15:00)
+
+### Sliding Windows Flow Control
+
+#### Parameters of Communication
+
+- **A** will send 8 packets to **B**
+- Transmission time (time to put on network) for a packet is 1 "time unit"
+	- Transmission time for acknowledgement is 0 "time units"
+- Propagation Time (time to travel through network) is 2 "time units"
+- **B**'s packet processing time is 1 "time unit" (then acknowledges)
+- Initial **Window Size** is 3
+
+#### Legend
+
+- Thick border (yellow bg) = Sliding Window
+- Black number = in process of send
+- Red number = sent but no acknowledge
+- Green number = sent and acknowledged
+
+	![](Pasted%20image%2020231030152517.png)