By far the most common type of cabling that you’ll come across today, unshielded twisted pair (UTP) cabling originates it the world of voice, being the same type of wiring used for telephone connections. UTP provides a number of advantages over coaxial cable, the main being the flexibility it provides in wiring a network. Instead of having to connect devices along a single length of cable, individual systems can be connected to switches or hubs using a patch cable with RJ45 connectors. UTP is most commonly used to wire Ethernet networks.
UTP cables are made up of pairs of copper wires twisted together. The twisting serves an important purpose – it helps to eliminate electromagnetic interference (EMI). EMI is a common problem on networks using copper wire. Signals from one wire pair might interfere with another (referred to as crosstalk), while powerful external electrical devices may also impact transmission capabilities. When using UTP cables, a common mistake is to unravel the twisting too far – this will certainly degrade signal strength and make the wires more prone to interference.
The category of the cabling defines how many wire pairs you’ll find in a given cable. Voice grade cable, also known as Category (or simply ‘Cat’) 3, uses only two pairs and is used for telephone service and 10Mb Ethernet. Cat 5 wiring, on the other hand, uses 4 wire pairs and is the minimum required for 100Mb Fast Ethernet. For the most part, buildings today are usually pre-wired for Cat 5, although Cat 3 may still be found in older environments. You may also come across what is known as Cat 5E – this version of Cat 5 simply has more twists per inch of wiring, providing better resistance to EMI and higher transmission capabilities.
While network cabling may not be the most exciting topic in the world, you’ll definitely need to understand the basics of different LAN media. The three main types of media found on LANs include coaxial cable, twisted pair, and fiber optics. In this section we’ll look not only at the properties of each but also maximum distances, signaling, and how cables are wired.
It really wasn’t all that long ago that coaxial cabling was the defacto standard for wiring LANs. On Ethernet LANs that used coaxial cable, individual computers were connected to the cable using devices such as BNC-T connectors or external transceivers. These connected to the system’s network card, and then to a run of cable that went to computers in either direction. At either end of this long segment, you were required to use a terminator that would absorb signals and prevent them from bouncing back down the wire and creating collisions.
Coaxial networks had a major downside. Since every system was connected to this same run of cable, a break or disruption at any point could bring down the network. Even as other wiring cabling standards became popular, coaxial cable was often used as backbone cabling between LANs. Its ability to span longer distances made it useful, even though the speeds at which data could be passed were fairly limiting by today’s standards.
While it’s not commonly used anymore, it is still important to know something about the two most popular types of coaxial cabling:
ThickNet. This type of coaxial cabling is used with Ethernet 10Base5 networks and is able to span distances of up to 500 meters. Originally used to directly connect computers, it eventually became popular in backbone implementations between LANs. Systems connected to the cable using an external transceiver unit than actually tapped directly into the wire. The transceiver was then connected to a network card using an Attachment Unit Interface (AUI) cable. You’ll learn more about external transceivers and AUI connections when we look at Cisco router ports.
ThinNet. A much thinner and more flexible type of coaxial cable, ThinNet is used on Ethernet 10Base2 networks and can span distances of up to 185 meters. This was usually the media of choice for connecting computers on a LAN. In ThinNet networks, computers connect to the network via a BNC-T connector attached to the network card.
When traffic is passed between hosts on a network, three different transmission mechanisms are possible. These include unicasts, multicasts, and broadcasts.
A unicast is the most simple network transmission. As the name suggests, it is a direct transmission from one system to one other system only. As such, the destination address will always uniquely identify a single host for whom the data is meant. In a shared Ethernet environment (where a system might be exposed to all frames), systems would check to see whether the destination MAC address matched their own. If it did, it would process the frame. If not, it would discard the frame. On an IP-based network, the address 192.168.1.24 represents a unicast address.
Unlike unicasts, which are meant for a single host, a multicast is meant for a group of systems. Think of multicasts as a one-to-many transmission method. Multicasts are generally used when traffic such as video needs to be passed to many hosts at the same time. In this way, a sender would transmit a single stream of data, which would in turn be picked up by many different hosts. On IP networks, a special group of addresses is reserved for multicasting, those in the Class D range. When multiple hosts need to receive a multicast, they are all configured with an identical multicast IP address. When they receive traffic destined for this shared address, they process it. Do not confuse a multicast address with a regular IP address. In this example, all systems still have a unique IP address, but also “listen in” on a configured multicast address.
The final type of network transmission is a broadcast. Quite simply, a broadcast is a transmission destined for all hosts. A special destination address designates a broadcast – in Ethernet, the broadcast address is FF-FF-FF-FF-FF-FF. When a host sees frames with this destination MAC address, it knows it has to process the frames. While excessive broadcasts on a network are generally undesirable, many network services depend on this type of transmission.
Whether you’re preparing for the CCNA or CCDA, the most critical requirement is a solid understanding of networking theory. While you personally may be more worried about router configuration or the related commands, do not underestimate the emphasis placed on general networking concepts. For all intents and purposes, your knowledge of these concepts will likely be the difference between passing and failing the exams, so be prepared.
There are a variety of elements involved in how communication happens between systems across an internetwork. These not only include equipment and cabling, but also topologies, transmission methods, and technologies. Take the time to understand these different elements and their responsibilities, and you’ll find both exams much less challenging, hopefully even simple.
The material to be covered in this chapter includes:
- An overview of LANs versus WANs
- An overview of basic network equipment including repeaters, hubs, bridges, switches, and routers
- Network transmission methods
- Network cable types and wiring standards
- Media access methods
- Network topologies
The primary reason for looking at any network model is to better understand how systems communicate. In real-life, network communication requires that data be encapsulated by the sender, transmitted over the network, and then de-encapsulated by the receiver. This is best illustrated by looking at what happens when one system running TCP/IP sends data to another. The list below outlines 5 simplified steps in a typical TCP/IP data transfer over an Ethernet network. Note that each layer considers whatever has been passed down to it from an upper layer as “data”. It doesn’t concern itself with what was added by the upper layers.
- Data is created by an application such an FTP client program. Let’s assume that a file transfer is being initiated with a local FTP server.
- The data is passed to the Host-to-host (Transport) layer, where it is encapsulated to include source and destination port numbers. These uniquely identify the applications that the data should be passed between. For example, if this data were being sent to an FTP server, the destination port would be TCP 21. The data is now considered to be a segment.
- The data is passed to the Internet (Network) layer, where it is again encapsulated to include information such as the source and destination IP addresses. The data is now considered to be a packet.
- The data is passed down to the Network Interface (Data Link) layer, where it is encapsulated for Ethernet to include source and destination MAC addresses, as well as the an error-checking mechanism known as a cyclic redundancy check (CRC). The data is now considered to be a frame.
- The data is converted to a series of bits, and transmitted across the network.
Tip: A CRC is also often referred to as a Frame Check Sequence (FCS).
Figure: TCP/IP Data Encapsulation Process
Note that upon reaching the destination host, the entire process happens in reverse, with each layer de-encapsulating the data by striping away the information that was added at each layer. Eventually, the required data is passed to the FTP server as intended by the FTP client application. Consider the frame captured below using Ethereal, a network protocol analyzer. Notice that each heading area directly corresponds to the encapsulation process just defined (with the exception that the program shows the layers in reverse order).
Internet Protocol, Src Addr: 192.168.0.1 (192.168.0.1), Dst Addr: 192.168.0.135 (192.168.0.135)
Transmission Control Protocol, Src Port: 4653 (4653), Dst Port: ftp (21), Seq: 2739356837, Ack: 204742999
File Transfer Protocol (FTP)
The Data Link Layer of the OSI model acts as an interface between the Network and Physical layers. The main responsibilities of the Data Link layer include:
- Data framing and physical addressing. When data is passed to the Data Link layer, it is framed for transmission using various LAN and WAN protocols. This allows network protocols to be transmitted over different network technologies including Ethernet, Token Ring, and Frame Relay as examples. Hardware or Media Access Control (MAC) addressing is used to uniquely identify hosts at the Data Link layer. Since they make forwarding decisions based on MAC addresses, bridges and switches are examples of equipment found at this layer.
- Flow control, error checking, and frame sequencing. Data Link layer devices are capable of transmitting flow control codes that identify whether upper layer protocols are capable of receiving data at the current rate. Error checking is provided in the form of a Cyclic Redundancy Check (CRC), a simple mathematical calculation performed on each frame to ensure it hasn’t been corrupted in transit. Frame sequencing reorders frames that were received in a different order than they were sent.
Interacting with Network layer protocols. When a host receives a frame, the frame header contains information on which Network layer protocol the data will be passed to. The Data Link layer helps to make network technologies independent of the upper layer protocols in use.
Examples of Data Link layer protocols:
- Ethernet (802.3): Contention-based LAN technology
- Token Ring (802.5): Token-passing LAN technology
- Wireless LAN (802.11): Wireless LANs
- Frame Relay: Packet-switched WAN technology
- ISDN: Digital dial-up connections
Tip: Remember that the protocol data unit (PDU) of the Data Link layer is referred to as a frame.
The Data Link layer is actually comprised of two sub-layers (defined by the Institute of Electronics and Electrical Engineers – the IEEE), called Media Access Control (MAC) and Logical Link Control (LLC).