Intro to Networks Chapter 5
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Logical Link Control 802.2 Handles the communication between the upper layers and the lower layers. LLC can be considered the driver software for the NIC
- The lower sublayer of the data link layer. 802.3 MAC is implemented by hardware, typically in the computer NIC
- 2 responsibilities
- data encapsulation
- media access control
process includes frame assembly before transmission and frame disassembly upon reception of a frame
Data encapsulation provides 3 primary functions
- Frame delimiting -
- Error detection
Frame delimiting -
The framing process provides important delimiters that are used to identify a group of bits that make up a frame.. Provides synchronization between the transmitting and receiving nodes.
The encapsulaiton process also provides for data link layer addressing.
Each ethernet frame contains a trailer with a cyclic redundancy check (CRC) of the frame contents. After reception of a frame, the receiving node creates a CRC to compare to the one in the frame. The the CRC calculations match it can be trusted
Media Access Control
Responsible for the placement of frames on the media and the removal of frames from the media
This sublayer communicates directly with the physical layer.
Carrier Sense Multiple Access CSMA
Detects is the media is carrying a signal. If it does it will wait and try again after a short time period. If no carrier signal is detected the device transmits its data
CSMA / Collsion Detection
The device monitors the media for the presence of a data signal. If no data signal, it will transmit, if signal is present will wait and retry
CSMA / Collision Avoidance
The device will examine the media for a data signal. If media is free the device sends a notification across the media of its intent to use it. The data is then sent Used by 802.11 wireless technologies
MAC Address Structure
a 48 bit value expressed as 12 hexadecimal digits. First the bytes (24 bits) are the vendors code, called the OUI Organizationally Unique Identifier
MAC address structure
Must be globally unique first 3 bytes are manufacturer last 3 bytes are vendor code or serial number
64 bytes and the max of 1518 bytes (includes all the bytes from the destination MAC address gield trhough the Frame Check Sequence Field (FCS)
Any frame less than 64 bytes in length
Automatically discarded by receiving stations
VLAN stands for
Virtual Local Area Network
Preamble and Start Frame Delimiter Fields
- Preamble (7 bytes)
- Start Frame Delimiter (1 byte)
used for synchronization between the sending and receiving devices. These 1st 8 bytes get tthe attention of the receiving nodes.
Destination MAC Address Field
6 byte field is the identifier for the intended receipt
Source MAC Address Field
This 6 byte field identifies the frame's originating NIC or interface
Used to describe which higher layer protocol is present
This field (46-1500) bytes contains the encapsulated data from a higher layer
Frame Check Sequence Field
4 bytes, is used to detect errors in a frame The sending device receives the frame and generates a CRC to look for errors. If the calculations match, no error occurred. If calculation do not match the frame is dropped
Unicase MAC address
A unique address used when a frame is sent from a single transmitting device to a single destination device
Contains a destination IP address that has all ones in the host portion.
all hosts on the local network will receive and process the packet.
allow a source device to send a packet to a group of devices. Devices that belong to a multicast group are assigned a multicast group IP address
Remote gaming, video conferencing
Physical address aka
the IP address
Domain Name Service - which an ip address is associated to a domain name
Address Resolution Protocol
ARP's have 2 functions
Resolving IPv4 addresses to MAC addresses
Maintaining a table of mappngs
2 ARP Issues
May be initially slow while the ARP address is being aquired
Relies on unidirectional data flow where sending and receiving data are no performed at the same time.
Typical of a hub slows data flow
data flow is bidirectional
- Point to point only-pc1 to switch, switch to pc 2
- requires full duplex suppoer on both ends
- collision free
A feature on the switch which detects the required cable type for copper Ethernet connections and configures the interfaces accordingly. You may use crossover or straight thru for connections
Store and forward switching
when the switch receives the frame it stores the data n buffers until the complete frame is received. During storage the switch determines destination and also preforms Cyclic Redundancy Check CRC.
Cyclic Redundancy Check uses a mathmetical formula based on the number of bits in the frame to determine whether the received frame has an error.
cut through switching
the switch acts upon the data as soon as it is received. The switch buffers just enough of the frame to read the dest. MAC address so that it can determine to which port to forward the data. NO ERROR CHECKING IS PREFORMED
Two types of cut through switching
Fast forward switching - immediately forwars a packet after reading the dest. address
Fragment free switching - stores the first 64 bytes of the frame before forwarding. Preforms a small error check on the first 64 bytes (where most errors occur)
Port Memory Buffering
Frames are stored n ques that are linked to specific incoming and outgoing ports. A frame is transmitted to the outgoing port only when all the frames ahead of it in the queue have been successfully transmitted.
Shared Memory Buffering
deposits all frames into a common memory buffer that all the ports on the switch share. This allows the packet to be received on one port and then transmitted on another port without moving it to a different que
allows for different data rates on different ports which allows more bandwidth to be dedicated to certain ports such as a port connected to a server.
PoE aka and what it does
Power over ethernet - supplies power from switch to device such as a phone over ethernet cable
Fixed configuration Swtiches
Fixed in their configuration, can't add features or options to the switch beyond those that originally came with the switch.
Typically come with different sized chassis that allow for the installation of different numbers of modular line cards. The line cards actually contain the ports.
Cisco Express Forwarding
cisco devices which support layer 3 switching
- Two main components of CEF
- Forwarding Information Base
- Adjacency tables
Forwarding Information Base
Used in Layer 3 switches
Similar to a routing table
maintain layer 2 next hop addresses for all FIB entries
Switch Virtual Interface
Logical interface on a switch associated with a virtual local area network VLAN
Physical port on a layer 3 switch configured to act as a router port
Layer 3 EtherChannel
Logical interface on a cisco device associated with a bundle of routed ports
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