diff --git a/content/notes/22-LAN-physical-layer.md b/content/notes/22-LAN-physical-layer.md
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+---
+title: "22-LAN-physical-layer"
+aliases: 
+tags: 
+- cosc203
+- lecture
+---
+
+# LAN vs WAN
+Local area network
+- a network connecting computers within a limited geographic area
+- e.g., building, school, home etc
+- wired or wireless
+
+Wide Area network
+- a network that extends over a large area such as town, country, region or the world
+- used to connec LANs and other types of networks together
+- often established with leased telecommunication circuits (e.g., chorus)
+
+## ethernet
+dominant wired tech
+- first used LAN tech
+- simple cheap
+- IEEE 802.3 standard
+- kept up with speed race: 10 Mbps - 400 Gbps
+- physical topology
+	- bus: all nodes in the same collision domain can collide (old)
+	- switched: active link-layer switch in center (new)
+
+frame structure
+- encapsulates IP datagram (or other network layer protocol packet) in ethernet frame
+- ![ethernet frame diagram](https://i.imgur.com/ElTUCEi.png)
+- preamble: used to sync reciever and sender clock rates
+	- 7 bytes of 10101010 followed by one byte of 10101011
+- addresses: 6 bytes source, destination MAC addr
+	- if adapter recieves frame with matching MAC destination address, it passes data in frame to network layer protocol, otherwise adapter discards frame
+- type
+	- most IP but other possible: e.g., Novell IPX, Appletalk
+	- used to demultiplex up at reciever  
+- CRC: cyclic redundancy check at reciever
+	- error detected: frame is dropped
+	- it can correct but doesn't because just dropping and resending is faster
+
+# ARP 
+Address resolution protocol
+
+arp table: each IP node on lan has table
+- IP/MAC mappings for some LAN nodes
+- mapping is forgotten after TTL is exceeded (usually 20min)
+- 
+
+# Switch vs Router
+
+# All layers together
\ No newline at end of file
diff --git a/content/notes/cosc-203.md b/content/notes/cosc-203.md
index 1356b0c2a..13a080367 100644
--- a/content/notes/cosc-203.md
+++ b/content/notes/cosc-203.md
@@ -49,8 +49,8 @@ tags:
 - [18-transport layer](notes/18-transport-layer)
 - [19-network layer data plane](19-network-layer-data-plane)
 - [20-network layer control plane](20-network-layer-control-plane)
-- [21-data link layer and LAN](21-data-link-layer-LAN)
-- [22-LAN and physical layer]
+- [21-data link layer and LAN](21-data-link-layer)
+- [22-LAN and physical layer](22-LAN-physical-layer)
 
 # Archive