Intro to Networks Chapter 5

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

• Frame delimiting - 
• Addressing
• Error detection

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

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.

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

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

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

a 48 bit value expressed as 12 hexadecimal digits.  First the bytes (24 bits) are the vendors code, called the OUI   Organizationally Unique Identifier

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

Virtual Local Area Network

• 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.

6 byte field is the identifier for the intended receipt

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

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

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

MAC address

the IP address

Domain Name Service  -  which an ip address is associated to a domain name

Address Resolution Protocol

Resolving IPv4 addresses to MAC addresses

Maintaining a table of mappngs

May be initially slow while the ARP address is being aquired

ARP spoofing/.poisioning

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

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.

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

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)

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.

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.

Power over ethernet   -  supplies power from switch to device such as a phone over ethernet cable

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 devices which support layer 3 switching

• Two main components of CEF
• Forwarding Information Base
• Adjacency tables

Used in Layer 3 switches

Similar to a routing table

maintain layer 2 next hop addresses for all FIB entries

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

Logical interface on a cisco device associated with a bundle of routed ports