diff --git a/content/notes/16-networks.md b/content/notes/16-networks.md
index cf4d8ada9..332a44661 100644
--- a/content/notes/16-networks.md
+++ b/content/notes/16-networks.md
@@ -16,11 +16,102 @@ packet swtiches - forward packets (chunks of data)
 communcation links - fiber, copper, radio, satellite
 	- transmission rate: bandwidthc
 networks -  collection of devices routers
-- links managed by organ
+- links managed by organisation
+
+
+## internet structure
+network edge
+- hosts: clients and servers
+	- servers (often in data centres)
+
+access networkds, physical media
+- wired inks
+- wireless communcation links
+
+network core
+- "backbone"
+- interconnected routers
+- network of routers
+
+internet is a network of networks
+
+## edge devices
+- sending function
+	- takes app message
+	- break into packers
+	- transmit them
+- recieving function
+	- recieve packets
+	- reasseble to create app message
 
 # communication Medias
+## physical media
+- wired links
+- twisted pair cable (TP)
+		- twist to the distance between the noisy source is roughly uniform 
+	- ![](https://i.imgur.com/NiL0cO7.png)
+	- two insludate copper wires twisted together
+	- one carries signal and the other is used as ground reference
+	- reciever uses the different between the two
+	- shielded vs unshielded
+		- sheld can resits internal and external interference
+		- more expensive but faster data rate
+	- abt 10MBPS to 1- Gbps
+	- max distance about 100 meters
+	- telephone, DSL, ethernet LAN
+
+Coaxial cable
+- central inner conductor
+- outer conductor as a shield
+- special insulation
+- long distance at high speed
+- cable TV, ethernet LAN
+- no grouding wire because the shield protects it and the signal is more stable
+- ![](https://i.imgur.com/UI6vx1i.png)
+
+Fibre-optic cable
+- glass fibre carrying light pulses
+- high speed 10 -100 GBps
+- low error rate
+- ![](https://i.imgur.com/fDMjJgs.png)
+
+## wireless media
+- wireless radio
+	- signal carried in electro magnetic spectrum
+	- broadcast, half-duplex
+	- propagation environment effecs
+		- reflection and obstruction
+		- interference/noise
+- radio link types
+	- wireless LAN (WiFi)
+	- wide area (e.g., 4g cellular)
+	- 10's Mbps over ~10km
+	- bluetooth  - short distance, limited rates
+	- satellite - up to 45 Mbps per channel
 
 # Packet and circuit switching
+- network core
+	- many routers connect to subnets
+	
+	Router
+	- two functions
+	- forwarding
+		- aka switching
+		- local action: move arriving packets from routers input link to appropriate output link
+		- looks at fowarding table and sends to appropriate output link depending on the header
+		- ![](https://i.imgur.com/4a6bTyz.png)
+	- routing
+		- global action: determine the source-destination paths taken by packets
+		- routing algorithms
+		- creates fowarding tables
+
+packet switching
+- host break application layer messages into packets
+- network fowards packets from one router to the next, across links on path from source to destination
+
+store and fowards: entire packet must arrive at trouter before it can be transmitted on next link
+packet queueing: occurs when work arrives faster than it can be serviced
+
 
 # network protocol