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School/Analyse/Periode 1/Layers/2. Datalink Layer.md

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+# 2. Data link Layer
+The Data Link Layer is responsible for handling the data transfer between two devices on the **same** network. (so a bit like the Network Layer)
+
+The Data Link Layer also splits up packets into smaller chunks called a frame. The reverse happens on the receiving device (reassembling frames into packets).
+
+Like the Network Layer the Data Link Layer is also responsible for flow control and error checking.
+
+## Error Detection
+Transmissions are not always error free. In case of an error, it should be detected and the message should be re transmitted (whenever possible)
+
+Detecting errors are handled by different algorithms:
+- Parity bits
+- CRC
+
+## Data Framing
+Data is split up in smaller chunks called frames. For each frame we add some extra data:
+
+- Who is sender?
+- Who is receiver?
+- Length of chunk
+- Type of chunk (normal message, error message, control message)
+- Error checking values (ex. CRC code)
+- Data and metadata are put in single message called frame
+
+## Ethernet Protocol
+Ethernet is a protocol used at the data link layer of wired networks.
+
+Ethernet uses specific hardware addresses to identify hosts on a network.
+These 6-byte-integer addresses are names Medium Access Control (MAC Address)
+
+Ethernet frame has the following format:
+![[ethernet-frame.png]]
+
+## Wireless networks
+Wireless networks do not use a physical medium for data transmission. The data is send through radio waves.
+
+- Each member of a wireless network is called a station.
+
+Networks are implemented in different sizes:
+- Wireless local area networks (WLAN)
+- Wireless Metropolitan Area Networks (WMAN)
+- Wireless Wide Area Networks (WWAN)
+
+### BSS Network
+A BSS is a network with a specific station called Access Point.
+
+All transmissions from a station to another are trough the Access Point.
+
+Access Points also connect the network (BSS) other networks, hence the name internet. 
+
+### AD-HOC Networks
+AD-HOC networks do not have a central node (access point).
+
+Every station can send data directly to a other station.