From Mycomputer Notes

Ethernet is a shared Media meanning that only one node can transmit data at any given time. The more nodes are added the more the demand of the bandwith will increase. The more nodes are added to the the media(wire) the higher the probability of collisions to occur and retramissions will increase.

A solution when adding more nodes is to break large segments into parts and separate them into different collision domains. This can be accomplished through Bridges and swithces. With a bridge this is possible because the device keeps a table of MAC addresses and the associated ports. Normally a bridge has only 2 ports and divides a collision domain into to parts.

A Bridge will:

• Divide a collision domain NOT effect on logical or broadcast domain
• Creates morcollision domains NOT add broadcast domains.

To accomplish this a bridge keeps a table of MAC addresses and the associated ports. The bridge then forwards or discards frames based on the table entries.

Contents 1 Switching Operation 2 Network Latency 2.1 Switching Modes 2.2 Spanning Tree Protocol 2.3 CSMA/CD

Switching Operation

A switch is a fast multi port bridge that will create more collisions domain but smaller collision domains. Each port on the switch is it own collision domains. A switch dynamically builds and maintains a content-addressable memory (CAM) table, which holds all of the necessary MAC information for each port. Each port on the switch that connect to a host IS creating it own segment or collision domain. These physical segment is called microsegment.

When only two nodes are connected to the switch (i.e 1 host --> to the switch) other possibilities emerges. With a UTP cable one pair is used to carry the transmitted signal from one node to the other node. A separate pair is used for the return or received signal. Since only two nodes are communicating at the same time it is possible for signals to pass through both pairs simultaneously.

The ability to communicate in both directions at once is known as full duplex. Most switches are capable of supporting full duplex, as are most NICs. In full duplex mode, there is no contention for the media. A collision domain no longer exists. In theory, the bandwidth is doubled when full duplex is used. A switch also has a memory called CAM that allows it to find the port that is associated with a MAC without search algorithms.

1. Fowards packet based on MAC address in fowarding table.
2. Operates at OSI Layer 2
3. Learns a station's location by examining source address stored in CAM

Network Latency

Network Latency ;

Represenet the itime it takes a fram to travel from is source device to its final destination onthe network. Latency can also de described as the delauy between the time a device request access to a network and the time it is granted permission to transmit. A variety of conditions can cause delays:

• Media delays may be caused by the finite speed that signals can travel through the physical media.
• Circuit delays may be caused by the electronics that process the signal along the path.
• Software delays may be caused by the decisions that software must make to implement switching and protocols.
• Delays may be caused by the content of the frame and the location of the frame switching decisions. For example, a device cannot route a frame to a destination until the destination MAC address has been read.

Trasnmission time;

It is not Network Delay. It is the time necessary to move a packet from the data link layer to the physical layer.

Switching Modes

Switching a frame to the destination port is a trade-off of latency and reliability. Three modes of switching are available:

Store-and-forward switching

• Switch reads the entire frame of data.
• Checks the frame for errors and decides where it needs to go.
• Sents the frame to its way.

Cut-through switching

• Switch reads the beginning of the frame up to the destination MAC Address as trafic flows.
• Cuts-through ot its destination without continuing to read the rest of the frame.

Fragment-free switching

• Modified form of cut-thorough.
• Fragment-free switching filters out collision fragments (majority of packet errors) befor fowarding begins.
• Waits until the received packet has been determined no the be a collision fragment before fowarding the packet.

Symmetric Switching

provides switched connections between ports with the same bandwidth (10/10 Mbps or 100/100 Mbps).(can cause bottlenecks as users try to access servers on other segments)

Asymetric Switching

Reduces the likelihood of potential bottleneck at the server by attaching the segment with the server to a higher bandwith port 100Mbps. Asymetric switching requires memory buffering in the switch.

Memory Buffering

Are of memory in a switch where destination and trasmission data are stored until it can be switched out to the correct port. Two types of memory:

• Port-based memory buffering.
• Shared memory buffering.

Spanning Tree Protocol

When building a network with several switches switching loops are unlikelly to occur. But switched networks are often designed with redundant paths to provide for fault tolerance and reliability. Having a redundancy strategy is good but it can have undesirable effects (ie. switching loops and broadcast storms)

STP is a standard protocol that is used to avoid switching loops. Each switch on the LAN that used STP sends messages called Bridged Protocol Data Units (BPDU). STP will sent out the message to all its ports to let other switches know about its existence. The switches use the spanning-tree algorithm (STA)to resolve and shut down the redundant paths.

Each port on a switch that uses STP exist in one of the following states:

• Blocking (Receives BPDUs only)
• Listening (Builidng an active topology)
• Learning (Building a bridging table)
• Forwarding (Sending and Receiving userdata)
• Disabled (Administratively down)

A port in a switch moves through five states as follows:

• INITIALIZATION --> Blocking
• Blocking --> Listenning OR Disabled
• Listening --> Learning OR Disabled
• Learning --> Forwording OR Disabled
• Forwarding --> Disabled

States are stablished by configuring each port according to policy. Then the protocol (STP) modifies the states based on traffic patterns and potentials loops

Poperties of half duplex - Only one host can transmit at a time becayse the NIC needs to listeen for collisions.

The NIC provides several circuit.ost important are:

• received (RX)
• transmit (TX)
• Collision Detection
• Banwith usage = 50% to 60%

CSMA/CD

The first device to detect the collision will generate a jam signal (colliding devices continue to transmit so that all devices will hear the collision)

• Network Congestions:
• Relieving congestion requires
• Increasing the amount of bandwidth and/or
• Using available bandwidth more efficiently