add osi model 2

cspell_dict_bash.txt

@@ -515,4 +515,7 @@ mysqldump
 ret
 mutex
 
-baseband
+baseband
+unicast
+multicast
+CSMA

docs/source/index.rst

@@ -129,6 +129,7 @@ The organization of this site is as following:
    reference/network/domain-name-and-dns
    reference/network/osi-model
    reference/network/osi-model-1
+   reference/network/osi-model-2
 
 
 .. toctree::

docs/source/reference/network/osi-model-2.md

@@ -0,0 +1,125 @@
+# OSI Model Layer 2
+
+Second Layer
+
+Data link layer
+
+Defines how to transfer data among network nodes, how data is formatted for transmission,
+how access to network is controlled.
+
+Also error detection.
+
+Data in this layer is called frames.
+
+## Devices
+
+* NIC
+* Bridge
+* Switch
+
+Switch is more common
+
+## LAN
+
+Local Area Network
+
+A network that is local. Usually network entities inside are close to
+each other.
+
+## Protocols
+
+There are many protocols that can be used for this layer.
+
+* PPP (Point-to-Point)
+* FR (Frame Relay)
+* Ethernet
+* ...
+
+Ethernet is the most used one.
+
+## Standards
+
+The standards for Ethernet and LAN and so on are defined in:
+
+* IEEE 802.2
+* IEEE 802.3
+* ...
+
+## Sub-layers
+
+Data Link Layer has 2 sub-layer
+
+* LLC (Logical Link Control Layer)
+* MAC (Media Access Control)
+
+## MAC Addressing
+
+Used for communication of a network segment.
+
+There are many nodes in one network
+
+An identification for specific ones
+
+See [MAC address - Wikipedia](https://en.wikipedia.org/wiki/MAC_address)
+for more details (about 48-bit structure)
+
+Each MAC address of one hardware is (supposed to be) unique and cannot be changed.
+
+It has 48 bits, 2 parts.
+
+* OUI
+  * Organizationally Unique Identification
+* Vendor Assigned
+
+MAC Address also has some special bits indicating
+unicast vs multicast and globally unique vs locally administered.
+
+Since the number of devices is much larger than 48 bits, MAC address is hard to be unique any more.
+
+It's usually represented in hexadecimal.
+
+## Ethernet Frame Structure
+
+Each layer will "attach" its HDR to the data is OSI model.
+
+And the HDR for this layer with Ethernet as the protocol is
+called Ethernet Frame Structure.
+
+[Learn more](https://en.wikipedia.org/wiki/Ethernet_frame#Structure)
+
+FCS (Frame Check Sequence) is used for error detection.
+
+Devices will check the destination address when receiving a frame.
+
+IEEE 802.3.
+
+## Communication Within LAN
+
+* Unicast
+  * 1 to 1
+* Broadcast
+  * 1 to all
+  * Use FF:FF:FF:FF:FF:FF as the destination address
+* Multicast
+  * 1 to more
+  * [Wikipedia](https://en.wikipedia.org/wiki/Multicast_address#Ethernet)
+
+## Collisions
+
+Collision happens when multiple devices try to access a medium at the same time.
+
+### CSMA/CD
+
+Wired network
+
+Carrier Sense Multiple Access Collision Detection
+
+[Wikipedia](https://en.wikipedia.org/wiki/Carrier-sense_multiple_access_with_collision_detection)
+
+### CSMA/CA
+
+Wireless network
+
+Carrier Sense Multiple Access Collision Avoidance
+
+[Wikipedia](https://en.wikipedia.org/wiki/Carrier-sense_multiple_access_with_collision_avoidance)