Network Introduction

Data Networking continues to evolve. The demand for a High-Speed Network Infrastructure has been growing at an alarming rate. Just a few short years ago, 4 Mbps  (Million bits per second) Token Ring and 10 Mbps Ethernet shared networks were the norm. Now they can’t keep up with the growing demands from end-users. End-user applications and files are growing in size and number.

This is an introduction to networking terminology and network infrastructure. For an introduction to Local Area Networks (LANs), please see What is a LAN?

One of the early types of networking topologies was Token Ring. Token Ring uses a token passing access method. In token passing, collisions between packets are prevented by assuring that only one station can transmit at any given time. This is accomplished by passing a special packet, called a token, from one station to another in a ring topology. (To learn more about ring topologies, please see What is a LAN?) When a station gets the token, it can transmit a packet, which travels in one direction around the ring. When the packet passes by the station it is addressed to, it is copied. The packet continues to travel around the ring until it returns to the sending station, which removes it and sends the token on to the next station on the ring.

Another popular network technology is Fiber Distributed Data Interface (FDDI). FDDI provides data transport at 100Mbps. Originally, FDDI networks required fiber-optic cable, but today they can accommodate twisted-pair cable as well. Fiber-optic cable is still preferred in many FDDI networks because it can be used over much greater distances than twisted-pair cable. FDDI uses a token passing access method and is usually configured in a ring topology. FDDI is used primarily as a backbone, a segment of network that links several individual LANs together in building or campus environments.

One of the most common types of networks is Ethernet. Originally, Ethernet cables were coaxial (similar to cable TV wires), but now twisted-pair cables (similar to telephone wires) are more common. Fiber-optic cables are used for higher speeds and greater distances than copper wires are capable of providing. One version of Ethernet, called 10BASE-T, consists of twisted-pair cables and can transmit data at 10Mbps (10 million bits per second). Another version, Fast Ethernet is 10 times faster than 10 Base-T.

Two of the common types of Fast Ethernet are 100Base-T and 100Base-FX. 100BASE-T consists of twisted-pair cabling similar to 10Base-T except the cables have to be of a higher quality to transmit the data reliably, and the distance limitation is 100 meters. 100Base-FX is Fast Ethernet that runs over fiber-optic cabling. The fiber-optic cable extends the distance to 2,000 meters over Multi-mode cable.

The newest version of Ethernet is called Gigabit Ethernet and it can transmit data at 1000Mbps. Gigabit Ethernet can use either Category 5 twisted-pair cable or fiber-optic cable to transmit the signals. Soon, 10 Gigabit Ethernet will be available. It will only use fiber-optic cables. 10 Gigabit Ethernet will primarily be used for backbone applications.

Ethernet can support both bus and star topologies. (To learn more about bus and star topologies, please see What is a LAN?) The most popular is the star topology, which makes use of a central hub or switch through which all information is passed. All stations on the network are connected to the hub or switch and can "sense" packets as they are sent across the wire. Because each station can send a packet at any time, collisions between packets do occur. These are common and are corrected instantaneously.

Ethernet networks are replacing Token Ring and FDDI networks daily. Ethernet with its speed, simplicity and low cost has won the war of the LAN. Many sites are migrating their legacy networks to Ethernet.

Hubs (also referred to as repeaters or concentrators) are a very important part of the networking process. Hubs are wiring concentrators, which make use of structured wiring to connect stations on a LAN. They contain user ports into which each station's cable is connected. Many hubs are called intelligent, or manageable, which means that each of the ports on the hub can be configured, monitored, enabled, and disabled by a network operator from a hub management console.

There are three different types of hubs:

• A stand-alone hub is, as the term implies, a single unit with a fixed number of ports. Stand-alone hubs usually include some method of linking them to other stand-alone hubs.
• A stackable hub looks and acts like a stand-alone hub except that several of them can be stacked together, usually joined by short lengths of cable. When they are linked together, they can be managed as a single unit.
• A modular hub consists of modules that each act like a stand-alone hub, but like the stackable hubs they can be managed as a single unit. Modular hubs consist of a chassis and the modules that go into the chassis. The chassis links the modules together with an internal backplane and handles the management of individual modules.

All of these hubs can be linked together to broaden the network. In order to create an internetwork (linking LANs together), other more complex devices are used.

• Bridge - device that connects two or more networks and forwards packets among them. Usually, bridges operate at the physical network level. For example, an Ethernet bridge connects two physical Ethernet cables and forwards from one cable to the other exactly those packets that are not local. Bridges differ from repeaters because bridges store and forward complete packets while repeaters forward electrical signals. They differ from IP Gateways or IP Routers because they use physical addresses instead of IP addresses.
• Routers - use Network Layer Protocol Information within each packet to route it from one LAN to another. This means that a router must be able to recognize all of the different Network Layer Protocols that may be used on the networks it is linking together.
• Switches - devices with multiple ports, each of which can support an entire Ethernet, FDDI or Token Ring segment. With a different segment connected to each of the ports, it can switch packets between them as needed. In effect, a switch acts like a very fast multi-port bridge because packets are filtered based on the destination address. A new network technology, called Asynchronous Transfer Mode (ATM), is based on the switch concept.

Switches are starting to replace hubs and routers in many installations. Switching technology is increasing the efficiency and speed of networks. This technology is making current systems more powerful, while at the same time facilitating the migration to faster networks. Switching directs network traffic in a very efficient manner. It sends information directly from the port of origin to only its destination port. Switching increases network performance, enhances flexibility and eases moves, adds and changes. Switching establishes a direct line of communication between two ports and maintains multiple simultaneous links between various ports. It proficiently manages network traffic by reducing media sharing, traffic is contained to the segment for which it is destined, be it a server, power user or workgroup.

There are many different types of Switches, some common examples are:

• Unmanaged switch: These switches come in many port varieties. Anywhere from 4 to 24 ports. Unmanaged switches are inexpensive, but lack features for management. Comparable to an unmanaged hub, except they have the speed of a switch.
• Workgroup switch: Similar to unmanaged switch, except provide management of the unit. Sometimes provide Gigabit ports to uplink to larger backbone switches.
• Stackable switch: Usually has a proprietary cable to interconnect them together. It allows a stack of switches to only use one IP address for management. Some use Gigabit links to interconnect them and to uplink them to backbone switches.
• Chassis Switch, Backbone Switch or Core Switch: Usually support Layer 3 switching, along with Layer 2 switching and many high level protocols. The Chassis have blades similar to high-end routers. So you can mix and match different interfaces for connecting different types of networks together.

Layer 2 switches (The Data-Link Layer) operate using physical network addresses. Physical addresses, also known as link-layer, hardware, or MAC-layer addresses, identify individual devices. Most hardware devices are permanently assigned this number during the manufacturing process. Switches operating at Layer 2 are very fast because they’re just sorting physical addresses, but they usually aren’t very smart—that is, they don’t look at the data packet very closely to learn anything more about where it’s headed.

Layer 3 switching (The Network Layer) and all of its related terms (e.g. multilayer switching, IP switching, routing switches, etc.) was introduced as the router-killer. Layer 3 switching attempts to reduce the performance bottlenecks associated with traditional routers. Layer 3 switches use network or IP addresses that identify locations on the network. They read network addresses more closely than Layer 2 switches—they identify network locations as well as the physical device. A location can be a LAN workstation, a location in a computer’s memory, or even a different packet of data traveling through a network. Switches operating at Layer 3 are smarter than Layer 2 devices and incorporate routing functions to actively calculate the best way to send a packet to its destination. But although they’re smarter, they may not be as fast if their algorithms, fabric, and processor don’t support high speeds.

Layer 4 (The Transport Layer) of the OSI Model coordinates communications between systems. Layer 4 switches are capable of identifying which application protocols (HTTP, SMTP, FTP, and so forth) are included with each packet, and they use this information to hand off the packet to the appropriate higher-layer software. Layer 4 switches make packet-forwarding decisions based not only on the MAC address and IP address, but also on the application to which a packet belongs. Because Layer 4 devices enable you to establish priorities for network traffic based on application, you can assign a high priority to packets belonging to vital in-house applications such as Smartstream, with different forwarding rules for low-priority packets such as generic HTTP-based Internet traffic. Layer 4 switches also provide an effective wire-speed security shield for your network because any company- or industry-specific protocols can be confined to only authorized switched ports or users. This security feature is often reinforced with traffic filtering and forwarding features.

Hubs vs. Switches

Traditional Ethernet LANs run at 10Mbps over a common bus-type design. Stations physically attach to this bus through a hub, repeater or concentrator, creating a broadcast domain. Every station is capable of receiving all transmissions from all stations, but only in a half-duplex mode. This means stations cannot send and receive data simultaneously. Nodes on an Ethernet network transmit information following a simple rule: they listen before speaking. In an Ethernet environment, only one node on the segment is allowed to transmit at any time due to the CSMA/CD protocol (Carrier Sense Multiple Access/Collision Detection). Though this manages packet collisions, it increases transmission time in two ways. First, if two nodes begin speaking at the same time, the information collides; they both must stop transmission and try again later. Second, once a packet is sent from a node, and Ethernet LAN will not transfer any other information until that packet reaches its endpoint. This is what slows up networks. Countless hours have been lost waiting for a LAN to free up.

When a single LAN station is connected to a switched port it may operate in full-duplex mode. Full-duplex does not require collision detection, there is a suspension of MAC protocols. A single device resides on that port, and therefore no collisions will be encountered. Full-duplex switching enables traffic to be sent and received simultaneously. (Hubs between a workgroup and a switch will not run full-duplex, because the hub is governed by collision detection requirements. The workgroup connected to the hub is unswitched Ethernet).

The bottom line is a 24 port 100Mbps hub is only capable of sharing the full 100Mbps with all 24-ports, which averages out to 4.16Mbps for each port. While at the same time a 24-port 100Mbps Switch has 24 individual 100Mbps ports. The switch is capable of 2400Mbps or 2.4 Gigabits per second. Also a switch can operate in full-duplex mode, so it has a theoretical throughput of 4800Mbps or 4.8 Gbps.

Virtual LANs (VLANs)

When something is virtual it appears to be real, but it is not. A virtual LAN, or VLAN, appears to be one large network.  It is actually a collection of multiple networks. While these networks are physically connected, logically they are separate. The protocol of each can be different. A switch can control and regulate traffic of a number of networks (creating a virtual LAN), but it cannot connect a user on one VLAN with a user on another. A router is required for that kind of connection.

A switched virtual LAN is a broadcast domain connecting a group of LANs at wire speed. Ethernet switches have evolved from creating VLANs based on port assignment. They can now create VLANs based on MAC addressing and network addressing. This enables VLANs to be divided into closed logical user groups, called subnets, determined by administrative controls. An Ethernet VLAN can be established through software, allowing a network administrator to group a number of switch ports into a high bandwidth, low-latency switched workgroup. For network management identification purposes, each virtual LAN gets a unique network number. VLANs function on a bridge architecture, switching and transmitting data by media access control (MAC) source and destination addresses. Traffic between virtual LANs is filtered, secured and managed by a router at the software level, separate from the virtual LAN switching logic.

To learn more about the NCI-Frederick network, please see Network Infrastructure.