Routing & Switching: Network Traffic Properties | Network+ N10-007 | 1.3a

In this video you will learn about the properties of network traffic such as: Ethernet, network switching, broadcast & collision domains, collision detection & avoidance, maximum transmission units, and broadcast vs. multicast vs. unicast.

Properties of Network Traffic

What is Ethernet?

Ethernet is a family of wired computer networking technologies commonly used in wired local area networks (LAN), metropolitan area networks (MAN), or wide area networks (WAN), enabling them to communicate with each other via a protocol — a set of rules or common network language.  Ethernet describes how network devices can format & transmit data so other devices on the same local or campus area network (CAN) segment can recognize, receive and process the information.  An Ethernet cable is the physical, encased wiring over which the data travels.

Ethernet Cable

What is Network Switching?

Network switching is the process of channeling data received from any number of input ports to another designated port that will transmit the data to its desired destination.  The device through with the input data passes is called a switch.  Data entering a port is referred to as ingress, while data leaving the port is referred to as egress.  The switch represents the medium through which the data is routed to its final destination.

A network switch is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device.  A network switch is a multiport network bridge that uses MAC addresses to forward data at the data link layer (layer 2) of the OSI model.  Some switches can also forward data at the network layer (layer 3) by additionally incorporating routing functionality.  Such switches are commonly known as layer-3 switches or multilayer switches.

Networking Switch

Packet switching is a method of grouping data that is transmitted over a digital network into packets.  Packets are made of a header and a payload.  Data in the header is used by networking hardware to direct the packet to its destination, where the payload is extracted and used by application software.  Packet switching is the primary basis for data communications in computer networks worldwide.

Packet Switching

Broadcast Domains

A broadcast domain is used to describe a group of devices on a specific network segment that can reach each other with Ethernet broadcasts.  Broadcasts sent by a device in one broadcast domain are not forwarded to devices in another broadcast domain.  This improves the performance of the network because not all devices on a network will receive and process broadcasts.  Routers separate a LAN into multiple broadcast domains (every port on a router is in a different broadcast domain).  Switches (by default) flood Ethernet broadcast frames out all ports, just like bridges and hubs.  All ports on these devices are in the same broadcast domain.

Broadcast Domain

Collision Domain

A collision domain is used to describe a part of the network where packet collisions can occur.  Packet collisions occur when two devices on a shared network segment send packets simultaneously.  The colliding packets must be discarded and sent again, which reduces network efficiency.  Collisions occur often in a hub environment because all devices connected to the hub are in the same collision domain.  Only one device may transmit at a time, and all the other devices connected to the hub must listen to the network in order to avoid collisions.  Total network bandwidth is shared among all devices.  In contrast to hubs, every port on a bridge, switch, or a router is in a separate collision domain.  This eliminates the possibility of collisions and enables the devices to use the full-duplex mode of communication, which effectively doubles the maximum data capacity.

Broadcast vs. Collision Domain

CSMA/CD

Carrier-sense multiple access with collision detection is a media access control (MAC) method used notably in early Ethernet technology for local area networking.  It uses carrier-sensing to defer transmissions until no other stations are transmitting.  This is used in combination with collision detection in which a transmitting station detects collisions by sensing transmissions from other stations while it is transmitting a frame.  When this collision condition is detected, the station stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

CSMA/CD

How CSMA/CD Works?

The following procedure is used to initiate a transmission. The procedure is complete when the frame is transmitted successfully or a collision is detected during transmission:

  1. Is a frame ready for transmission? If not, wait for a frame.
  2. Is a medium free? If not, wait until it becomes ready.
  3. Start transmitting & monitor for collision during transmission.
  4. Did a collision occur? If so, go to collision detected procedure.
  5. Reset retransmission counters & complete frame transmission.

CSMA/CA

Carrier-sense multiple access with collision avoidance in computer networking, is a network access method in which carrier sensing is used, but nodes attempt to avoid collisions by beginning transmission only after the channel is sensed to be “idle”.  When they do transmit, nodes transmit their packet data in its entirety.  This method was developed to decrease the chances of collisions when two or more stations start sending their signals over the data link layer.  Carrier-sense multiple access requires that each station first check the state of the medium before sending.

CSMA/CA

Collision avoidance is used to improve the performance of the CSMA method by attempting to divide the channel somewhat equally among all transmitting nodes within the collision domain.

  1. Carrier Sense:  prior to transmitting, a node first listens to the shared medium (such as listening for wireless signals in a wireless network) to determine whether another node is transmitting or not.
  2. Collision Avoidance:  if another node was heard, it’ll wait a period of time (usually random) for the node to stop transmitting before listening again for a free communications channel.
    1. Request to Send/Clear to Send (RTS/CTS):  may optionally be used at this point to mediate access to the shared medium.  This goes some way to alleviating the problem of hidden nodes because, for instance, in a wireless network, the access point only issues a Clear to Send to one node at a time.  However, wireless 802.11 implementations do not typically implement RTS/CTS for all transmissions; they may turn it off completely, or at least not use it for small packets.
    2. Transmission:  if the medium was identified as being clear or the node received a CTS to explicitly indicate it can send, it sends the frame in its entirety.  Unlike CSMA/CD, it is very challenging for a wireless node to listen at the same time as it transmits.

MTU (Maximum Transmission Unit)

A maximum transmission unit is a term used in networking and operating systems.  It defines the largest size of the packet that can be transmitted as a single entity in a network connection.  The size of the MTU dictates the amount of data that can be transmitted in bytes over a network.  The larger MTU results in more data transmission during a single connection, therefore, reduces the overhead.  On the other hand, the smaller MTU can be transferred faster, because of its size, thus reducing delay in the network.  Therefore, the size of the MTU should be adjusted to optimize both the requirements.  Another way to visualize this is to imagine it being like a height limit for freeway underpasses or tunnels:  cars & trucks that exceed the height limit cannot fit through, just as packets that exceed the MTU of a network cannot pass through that network.  However, unlike cars & trucks, data packets that exceed MTU are broken up into smaller pieces so that they can fit through.  This process is called fragmentation.  Fragmented packets are reassembled once they reach their destination.  MTU is  measured in bytes:  a “byte” is equal to 8 bits of information, meaning 8 ones and zeroes.  1,500 bytes is the maximum MTU size.

MTU

Broadcast

In computer networking, broadcasting refers to transmitting a packet that will be received by every device on the network.  In practice, the scope of the broadcast is limited to a broadcast domain.  Broadcasting is the most general communication method, and is also the most intensive in the sense that many messages may be required and many network devices are involved.  Broadcasting may be performed as “all scatter” in which each sender performs its own scatter in which the messages are distinct for each receiver, or “all broadcasts” in which they are the same.

Broadcasting

Multicast

Multicast is a group communication where data transmission is addressed to a group of destination computers simultaneously.  Multicast can be one-to-many or many-to-many distribution.  Multicast enables distribution of content such as internet TV and other types of streaming media.

Multicast

Unicast

Unicast is a one-to-one transmission from one point in the network to another point; that is, one sender and one receiver, each identified by a network address.  Unicast is in contrast with multicast and broadcast which are one-to-many transmissions.  Internet Protocol (IP) unicast delivery methods such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are typically used.

Unicast