Network + LAN Technologies 2 of 3

Data-Link Layer Opperations: LAN Sharing Basics

LANs have 3 Communication Methods:

• Multicast: Destination of transmitted frames is a group of clients
• Broadcast:  Frames are sent to all other devices
• Unicast: Frames are sent from a sigle host to a single destination

Collision Domain: A group of devices on a network that are directly connected by hubs.

Layer 2 Network devices (Switches and Bridges) use a process called segmentation to dedicate bandwith to users and reduce the size of collision domains.

CAM (Content Accessable Memory) Table: Same as a MAC Table – a record of MAC addresses used by that switch

Data-Link Layer Opperations: MAC Addresses

The Header (and trailer) of the MAC Sublayer of the Data-Link layer contains:

• MAC Address of the sending computer
• MAC Address of the destination computer
• A type feild containing a SAP (Service Access Point) to identify the type of protocol that is being carried and may give the length of the data part.

MAC Address are written in one of two ways:

• MM-MM-MM-SS-SS-SS
• MM:MM:MM:SS:SS:SS

(Hexidecimal numbers 12 Digits long – 42 bits in binary)

MAC Addresses made up of two parts:

1. Manufacturer’s OUI (Organiztionally Unique Identifyer) nubmer.
2. Adapter’s serial number

Data-Link Layer Opperations: Swithcing Fundamentals

Both Switches and Bridges:

• Connect different LAN segments
• Determine the network segment of a frame needs to be transmitted on by using a table of MAC address
• reduce network trafffic

Switches use Application-Specific Integrated Ciruits (ASICs). Optimized and embedded in the hardware. Results in shorted exectution time.

Switches support new functions like VLANs (Virtual LANs) that operate at higher speeds and at lower costs with greater port density than bridges.

Switches Support:

• Dedicated communication between devices. (Miscrosegmentation is a method of creating private or dedicated segments each of which only has one user.)
• Full-Duplex Communication: Only possible on dedicated connections
• Media Rate Allocation: Switches can translate between 10 and 100 Mbps
• Multiple Simotaneous conversations

Bridgeds loops can occurs when there are more than one path between the source and the destination. Frames travel the loop continuously.

The Spanning Tree Algorithm is used in bridged networks to help prevent bridge loops.

Two Switching Techniques are

1. Store-and-forward switching: Error-Checking
   • Switch must receive the complete frame before forwarding takes place.
   • Error checking is performed and frames that contain errors are discarded.
   • The entire frame is copied into the buffers onboard the LAN switch and the CRC (Cyclic Redundancy Check) is calculated.
   • If the frame contains a CRC error or its size is outside the rand of 64 byes to 1518 bytes, it is discarded.
   • If the frame contains no errors and is an acceptable size, the switch determines the outgoing port by looking up the destination address in the MAC table.
   • The switch then forward the frame to it final destination.
   • A frame less than 64 bytes is called a runt
   • A frame larger than 1518 bytes is called a giant
2. Cut-through switching: Reduces latency – begins forwarding frame as soon as header is received and destination interface is determined.
   • the LAN switch waits until the destination address of the frame is copied into the onboard buffers of the switch.
   • The destination address is indicated in the first six bytes following the preamble in the frame
   • The switch uses the MAC table to look up the destination address and determine the outgoing port.
   • It then forward the frame to its destination

Switches use 6 Steps to forward frames:

1. Switch receives a frame from a source on the network
2. The switch enters the switch port that received the frame and the MAC address of the source into the MAC table
3. If the switch does not know which port the destination address of the frame is on it floods the frame to all ports
4. When the destination device receives the broadcast, it replies. The destination device is the only device that replies
5. When the switch receives the reply from the destination device, it enters the switch port it received the reply on and the MAC address of the destination device into the MAC table
6. Now the switch can switch frames between the source and destination devices without broadcasting them on to the entire network.

Switching Frames Procedure: the process of a switch searching for a destination port and address before forwarding a frame.