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# ACIT 2620

## Week 3

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# Ethernet

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<div>
<img src="https://hackmd.io/_uploads/SJaa6kTtT.png"
alt="layer 2" style="width: 70%; " />
</div>

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## Data Link Layer Sublayers

<dl>
<dt>Logical Link Control (LLC)</dt>
<dd>Used to facilitate multiple upper layer (i.e. network ) protocols</dd>
<dd>Provides common interface to upper layers</dd>
<dd>Supplies multiplexing and flow control services</dd>
<dd>Provides error checking</dd>
</dl>

----

<dl>
<dt>Media Access Control (MAC)</dt>
<dd>Provides addressing and channel access control mechanisms (i.e. CSMA CD, CSMA CA)</dd>
<dd>Appends physical address of destination computer onto the frame</dd>
</dl>

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## Frames


- Preamble
- Marks beginning of entire frame
- Start of Frame Delimiter (SFD)
- Indicates beginning of addressing fields
- Destination Address
- Contains destination node address

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- Source Address
- contains address of sender node
- Length (LEN)
- indicates length of data/payload
- Data (payload)
- contains data, or segmented part of data, transmitted from originating node

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- Pad
- Used to increase size of the frame to its minimum size requirement of 46 bytes
- Frame Check Sequence
- provides algorithm to determine whether data were correctly received
- most commonly used algorithm is Cyclic Redundancy Check (CRC)

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## Ethernet Addressing

- MAC address: Media Access Control (MAC) sub-layer
- 48 Bits
- Number uniquely defining a network node
- Generally rendered as Hex: __`00:1e:33:ba:87:c1`__
- Doesn't contain any data regarding network location –just an ID

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<div>
<img src="https://hackmd.io/_uploads/Syypbl6Kp.png"
alt="mac address" style="width: 80%; " />
</div>

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- first three bytes
- Either Manufacturer hard coded
- Or Reserved Addresses (common ones)
- Broadcast Address __`FF:FF:FF:FF:FF:FF`__
- Spanning Tree Multicast: __`01:80:C2:00:00:00`__
- IANA reserves all address starting with __`00:00:5E`__ see Ethernet Numbers(this includes IPv4 multicast -and inserts the low 23 Bits of the multicast IPv4 Address into the Ethernet Address)
- __`33:33:XX`__ is reserved for IPv6 Multicast

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## Switching

- Making Forwarding decisions
- Transparent bridging

<div>
<img src="https://hackmd.io/_uploads/HJhTGe6Y6.png"
alt="transparent bridging" style="width: 70%; " />
</div>

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## Broadcast Loop and STP

<div>
<img src="https://hackmd.io/_uploads/HJ8x7epYT.png"
alt="broadcast loop" style="width: 80%; "/>
</div>

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### Spanning Tree Protocol

<div>
<img src="https://hackmd.io/_uploads/BytG7xpKa.png"
alt="spanning tree protocol" style="width: 80%; "/>
</div>

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## VLANs

- Virtual Local Area Networks
- A logical network within a physical network
- Achieved by grouping some of the switch ethernet ports into a logical broadcast domain
- Can span multiple switches

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<div>
<img src="https://hackmd.io/_uploads/S1Zg4e6Kp.png"
alt="vlans" style="width: 80%;"/>
</div>

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### VLAN port types

- Access ports
- assigned VLAN ID
- for connecting end hosts/nodes
- nodes connected to ports with same VLAN ID are in the same broadcast domain

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- Trunk ports
- typically for switch to switch or switch to router connection
- carry "tagged" frames, i.e modified ethernet frames with VLAN markers

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### Tagged frames

<div style="height: 200px; margin-bottom: 1em;">
<img src="https://hackmd.io/_uploads/BJd8Hx6Ka.png"
alt="tagged frame" style="height: 100%;"/>
</div>

- 4-byte tag header inserted between Source MAC and EtherType fields
- 2-byte tag protocol identifier (TPID)
- a fixed value of 0x8100 that indicates the frame carries tag information.

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- 2-byte tag control information (TCI)
- Three-bit user priority (used to prioritize traffic
- Drop Eligible Indicator (DEI) (in congestion is frame “dropable”
- Twelve-bit VLAN identifier (VID)-Uniquely identifies the VLAN to which the frame belongs

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## Link Access Methods

- Manage shared medium access contention (collision)
- Two methods:
- CSMA/CD: for wired Ethernet
- CSMA/CA: for wireless Ethernet

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### CSMA/CD


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### CSMA/CA

<div style="height: 600px; text-align: center;">
<img src="https://hackmd.io/_uploads/rJZoPxaFp.png"
alt="CSMA-CA" />
<div>

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## Reading List

- [IPv4 Addressing (video)](https://www.linkedin.com/learning/networking-foundations-ip-addressing/introducing-ipv4-and-the-ipv4-addressing-structure?u=2097252)
- [Internet Protocol Version 4](http://intronetworks.cs.luc.edu/current2/html/ipv4.html)