Basic Networking – Part 2

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Collision domain explained

The term collision domain is used to describe a part of a 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 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.

To better understand the concept of collision domains, consider the following example:

collision domain example

In the picture above you can see a network of seven computers, two hubs, a bridge, a switch, and a router. The collision domains created by these devices are marked in red. Remember, all devices connected to the hub are in the same collision domain. Each port on a bridge, a switch or router is in a separate collision domain. That is why there are seven collision domains in the network pictured above.

Broadcast domain explained

The term 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.

To better understand the concept of broadcast domains, consider the following example:

broadcast domain explained

In the picture above we have a network of six computers, two hubs, a bridge, a switch, and a router. The broadcast domains are marked in red. Remember, all devices connected to a hub, a bridge, and a switch are in the same broadcast domain. Only routers separate the LAN into multiple broadcast domains. That is why we have four broadcast domains in the network pictured above.

Ethernet broadcasts are usually used by Address Resolution Protocol (ARP) to translate IP addresses to MAC addresses.

CSMA/CD explained

Half-duplex Ethernet networks use an algorithm called Carrier Sense Multiple Access with Collision Detection(CSMA/CD). This algorithm helps devices on the same network segment to decide when to send packets and what to do in case of collisions. CSMA/CD is commonly used in networks with repeaters and hubs because these devices run in the half-duplex mode and all of their ports are in the same collision domain.

Packet collisions occur when packets are transmitted from different host at the same time. To prevent this, CSMA/CD forces a transmitting station to check for the presence of a digital signal on the wire. If no other hosts are transmitting packets, the sender begins sending the frame. The sender also monitors the wire to make sure no other hosts begin transmitting. However, if another host begins transmitting at the same time and a collision occur, the transmitting host sends a jam signal that causes all hosts on the network segment to stop sending data. After a random period of time, hosts retransmit their packets.

Consider the following example:

how csma/cd works

In the picture above we have a network of four hosts connected to a hub. Since hubs work in the half-duplex mode and each port on a hub is in the same collision domain, packet collisions can occur and CSMA/CD is used to prevent and detect them. Host A detects that there are no other signals on the network and decides to send a packet. However, Host B also assumes that no other station is transmitting and sends a packet as well. A collision occurs and it is detected by Host A and Host B. The sending stations send a jamming signal telling all hosts on the segment that a collision occured. After a random period of time, Host A and Host B resend their packets.

Since switches have replaced hubs in most of today’s LANs, CSMA/CD is not often used anymore. Switches work in full-duplex mode and each port on a switch is in a seperate collision domain, so no collisions can occur.

IEEE Ethernet standards

Ethernet is defined in a number of IEEE (Institute of Electrical and Electronics Engineers) 802.3 standards. These standards define the physical and data-link layer specifications for Ethernet. The most important 802.3 standards are:

  • 10Base-T (IEEE 802.3) – 10 Mbps with category 3 unshielded twisted pair (UTP) wiring, up to 100 meters long.
  • 100Base-TX (IEEE 802.3u) – known as Fast Ethernet, uses category 5, 5E, or 6 UTP wiring, up to 100 meters long.
  • 100Base-FX (IEEE 802.3u) – a version of Fast Ethernet that uses multi-mode optical fiber. Up to 412 meters long.
  • 1000Base-CX (IEEE 802.3z) – uses copper twisted-pair cabling. Up to 25 meters long.
  • 1000Base-T (IEEE 802.3ab) – Gigabit Ethernet that uses Category 5 UTP wiring. Up to 100 meters long.
  • 1000Base-SX (IEEE 802.3z) – 1 Gigabit Ethernet running over multimode fiber-optic cable.
  • 1000Base-LX (IEEE 802.3z) – 1 Gigabit Ethernet running over single-mode fiber.
  • 10GBase-T (802.3.an) – 10 Gbps connections over category 5e, 6, and 7 UTP cables.

Notice how the first number in the name of the standard represents the speed of the network in megabits per second. The word base refers to baseband, meaning that the signals are transmitted without modulation. The last part of the standard name refers to the cabling used to carry signals. For example, 1000Base-T means that the speed of the network is up to 1000 Mbps, baseband signaling is used, and the twisted-pair cabling will be used (Tstands for twisted-pair).

Cisco three-layered hierarchical model

Because large networks can be extremely complicated, with multiple protocols and diverse technologies, Cisco has developed a layered hierarchical model for designing a reliable network infrastructure. This three-layer model helps you design, implement, and maintain a scalable, reliable, and cost-effective network. Each of layers has its own features and functionality, which reduces network complexity.

Here is an example of the Cisco hierarchical model:

cisco hierarchical model

Here is a description of each layer:

  • Access – controls user and workgroup access to the resources on the network. This layer usually incorporates Layer 2 switches and access points that provide connectivity between workstations and servers. You can manage access control and policy, create separate collision domains, and implement port security at this layer.
  • Distribution – serves as the communication point between the access layer and the core. Its primary functions is to provide routing, filtering, and WAN access and to determine how packets can access the core. This layer determines the fastest way that network service requests are accessed – for example, how a file request is forwarded to a server – and, if necessary, forwards the request to the core layer. This layer usually consists of routers and multilayer switches.
  • Core – also referred to as the network backbone, this layer is responsible for transporting large amounts of traffic quickly. The core layer provides interconnectivity between distribution layer devices it usually consists of high speed devices, like high end routers and switches with redundant links.
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