We are still looking at internetworking basics. We will look at what is Ethernet networking and network cabling. You will need this knowledge for your day-to-day role as a network engineer.
Ethernet Concepts – What is Ethernet?
Ethernet has become the de facto standard in modern networking.
You already know that Ethernet operates at layer 2 of the OSI model and consists of the two sublayers—LLC and MAC. Moreover, you should already be familiar with Ethernet from previous studies, such as CompTIA Network+ (because, again, the CCNA is not a suitable exam for internetworking beginners).
Ethernet networking is actually a group or family of networking technologies. It was originally developed by Bob Metcalfe in 1972 while he was working at Xerox. It became commercially available in the early 1980s with the IEEE forming the 802.3 subcommittee. The 802.3 standard gradually replaced vendor-centric technologies such as Token Ring and FDDI. Ethernet is known as a contention-based access method, meaning all of the hosts in the network segment are fighting to be heard. This brings us to CSMA/CD.
In earlier versions of Ethernet networks, there were many workstations sharing a single wire. It was quickly discovered that frames would often collide on the wire, causing errors in the data and lost frames. In order to control all the traffic, an algorithm known as carrier sense multiple access with collision detection (CSMA/CD) was created. This ensures that when a network interface card (NIC) wants to transmit data, it first listens to the wire to determine whether any other host is transmitting. It works in much the same way as a formal board meeting, where only one member can talk at a time (and the others need to remain quiet until their turn has come).
The very first implementation of Ethernet networking used coaxial cables, which are only capable of sending or receiving a signal, but not at the same time. This is why CSMA/CD was required.
Ethernet using CSMA/CD transmits frames in this order:
- Carrier sense – checks the cable to ensure that it is not in use (signal on the wire)
- Transmit if quiet – sends the frame if the line is clear
- Collision – both frames are destroyed if they collide on the wire; the collision is measured by the amount of voltage on the cable
- Jam signal – if the sending device detects the collision, it stops transmitting and sends a jam signal to destroy any fragmented frames
- Back off algorithm – a random timer to prevent all stations from retransmitting at the same time
The entire CSMA/CD process can be explained through several steps:
- The device wanting to transmit a frame listens for a carrier signal on the wire. No signal means that it is clear to send the frame.
- The frame is put onto the wire.
- The sending device listens to discover whether a collision has occurred.
- If there was a collision, all sending devices send a jamming signal to announce the fact that there was a collision.
- A random timer runs and no frames can be sent by the original devices until it expires.
- The device starts back at step 1.
Figure 2.1 below gives you a visual representation of the process:
FIG 2.1 – CSMA/CD
Modern implementations of Ethernet no longer require CSMA/CD because different wires are used to send and receive, so there are no collisions. Despite this fact, Cisco wants you to understand this concept for the exam. You can also read the Wikipedia Ethernet pages.
The traditional method for Ethernet to operate was to use half-duplex mode. This works in the same way as walkie-talkies: a host can either transmit or receive—it can do either but not both at the same time. Half-duplex is a technology that originally ran on coaxial cables. This method is still used today if there are hubs in the network.
Full-duplex mode allows the host to send and receive frames at the same time. This is achieved by using UTP cables (which we will cover later), allowing a dedicated connection between devices on a LAN and providing separate paths for the data to be transmitted on. Because a wire is not needed to listen for collisions, it is free to be used to send or receive frames. Full-duplex is equivalent to Ethernet minus CSMA/CD.
IN THE REAL WORLD: A very large amount of late collisions showing up on your Ethernet interface is a sure sign of duplex mismatch settings on your Ethernet ports. We will cover this situation in the troubleshooting section.
FIG 2.2 – Duplex operations
Bear in mind that a network running half-duplex at 100 Mbps won’t work at 200 Mbps when you enable full-duplex. Instead, you will be able to transmit 100 Mbps both upstream and downstream at the same time. These are theoretical speeds that are dependent on other factors, like the limitations of the hardware used in the network.
Duplex and CSMA/CD issues are generally a thing of the past because most devices have a dedicated connection to the network switch, and different wires are used to send and receive traffic, so there will not be any collisions on the wire. Moreover, there is no need to check whether the wire is free to transmit or to check for collisions, and there will be no retransmissions, which are all good news for software drivers and CPU cycles.
Network engineers originally had to determine which devices could transmit at 10 or 100 Mbps, as well as those working at full- and half-duplex. This made configuring devices, switches, and routers very time intensive, as well as increased the likelihood of configuration errors.
Autonegotiation uses fast link pulses (FLPs) to allow the devices to agree to send data at the highest available rate. As the network administrator, you can let devices autonegotiate or you can use commands to set the speed and duplex to be used. You are bound to encounter autonegotiation issues in the real world at some point and possibly in the CCNA exam.
Mini-lab – Configuring Ethernet Speed and Duplex Settings
You should configure speed and duplex settings on a router Fast Ethernet interface rather than let it autonegotiate. Your default settings may differ from mine, and your interface name may also differ. Use the show ip interface brief command to see which interface types you have available. You don’t need to add an IP address, but I already have one on my router.
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.1.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
R1#show interfaces FastEthernet0/0
FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is c200.06f4.0000 (bia c200.06f4.0000)
Internet address is 192.168.1.1/24
MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, Loopback not set
Keepalive set (10 sec)
Half-duplex, 10Mb/s, 100BaseTX/FX
Enter configuration commands, one per line. End with CNTL/Z.
10 Force 10 Mbps operation
100 Force 100 Mbps operation
auto Enable AUTO speed configuration
auto Enable AUTO duplex configuration
full Force full-duplex operation
half Force half-duplex operation
R1#show int f0/0
FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is c200.06f4.0000 (bia c200.06f4.0000)
Internet address is 192.168.1.1/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, Loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, 100BaseTX/FX[END OF MINI-LAB]
In my installation experiences over the years, best practice has been to always hard-set the interfaces to 100 or 1000, if available, with full-duplex, but check with your network manager for your policy. If you had the interface operating at half-duplex, then CSMA/CD would be in effect, regardless of the speed.
When LAN devices transmit information to each other, they must use an agreed method to format the data so that all devices understand what the fields signify. LAN data encapsulated at layer 2 is referred to as frames. If you inspected the data, it would be encapsulated with a layer 2 header and trailer. If the data was encapsulated with layer 3 information, such as an IP address, it would be referred to as a packet.
Ethernet has four different frame types available. They have been improved over the years but some have become obsolete.
- Ethernet 802.2 SAP
- Ethernet 802.2 SNAP
- Ethernet 802.3
- Ethernet II
The 802.2 standard is not used today, while 802.3 was eventually superseded by Ethernet II, which is also known as Ethernet 2 or DIX Ethernet (named after DEC, Intel, and Xerox, who were the original designers).
FIG 2.3 – Ethernet II frame
The IEEE Ethernet II frame consists of specific fields that have been determined by the IEEE committee:
- Destination Address – destination MAC address; can be unicast, broadcast, or multicast
- Source Address – MAC address of the sending host
- Ether Type – identifies upper-layer protocols such as IPv4 (0x0800) and IPv6 (0x86DD)
- Data – payload in the frame; the data being transferred
Not shown in Figure 2.3 above are two fields: the Preamble, which starts the frame, and the Frame Check Sequence (FCS), which ends it.
- Preamble – allows devices to synchronize their receiver clocks
- Frame Check Sequence (FCS) – provides a cyclic-redundancy check (CRC) on all data in the frame to check for corruption
Ethernet addresses can be one of three types (IPv6 differs but we will cover this later):
- Unicast (one-to-one) – a single LAN interface is the recipient
- Broadcast (one-to-all) – all devices on the LAN are the recipients
- Multicast (one-to-many) – a subset of all devices are the recipients
FIG 2.4 – IPv4 Ethernet traffic types
Unicast traffic is transmitted from one host on the LAN to another host on the LAN. The switch forwards a unicast frame out of the interface the destination address is associated with. If there was no interface associated with the frame, the switch will forward the frame out of all interfaces (as a broadcast) except the interface the frame was received on.
In the output below, if the switch receives a frame addressed to host 0001.42dd.eca2 from the host connected to F0/1, it will broadcast out of all ports except F0/1, but including all the ports it currently has a MAC address recorded.
Switch#show mac address-table
Mac Address Table
Vlan Mac Address Type Ports
—- ———– ——– —–
1 0001.42dd.eca1 DYNAMIC Fa0/1
1 0001.c7d2.4eb1 DYNAMIC Fa0/2
1 00e0.f9de.e036 DYNAMIC Fa0/3
Broadcast traffic is exclusive to LANs because broadcasts are not forwarded by routers. A broadcast is sent by a device on the LAN and heard by all other devices on that segment of the LAN. A LAN can be broken into segments by routers or VLANs (more on VLANs later). A broadcast frame appears as FF:FF:FF:FF:FF:FF in hexadecimal.
A switch will forward a broadcast frame out of all interfaces except the interface the frame was received on. All hosts receive the broadcast frame. Each host must use CPU resources to check the destination frame to see whether it is the intended recipient. If it isn’t, it will discard the frame.
Multicast traffic is transmitted from a host and listened for by a subset of hosts in the network. Although all devices hear the frame, it is ignored by all but the devices waiting to receive the multicast. An example of a multicast is a Hello packet from the OSPF routing protocol, which is forwarded on address 126.96.36.199 (more on OSPF later).
A switch will forward a multicast in the same way that it forwards a unicast frame with an unknown destination port, unless there are explicit configurations to do otherwise.
You can see a packet capture of an OSPF multicast packet in Figure 2.5 below:
FIG 2.5 – OSPF multicast packet capture
The Institute of Electrical and Electronics Engineers (IEEE) is a non-profit group of professionals and, among other things, they are the leading authority in the field of computer engineering.
The IEEE publishes standards for networking and one such standard is called the 802 project. These standards are constantly changing to keep pace with emerging technologies and developments in the industry. IEEE 802.3 is actually a working group as well as a collection of standards that defines wired Ethernet at layers 1 and 2. This is mostly concerned with LAN networking, with some limited WAN applications. The connections between devices under 802.3 are copper or fiber cables. Also defined under 802.3 is CSMA/CD.
802.3 standards include 802.3u Fast Ethernet (100BASE-TX, 100BASE-FX), 802.3z Gigabit Ethernet (1000BaseX) over Fiber at 1 Gbps, and 802.3ab Gigabit Ethernet (100BASE-T) over copper wiring. IEEE 802.3bs™ “Standard for Ethernet Amendment: Media Access Control Parameters, Physical Layers and Management Parameters for 400 Gb/s Operation” is currently under development by the IEEE P802.3bs 400 Gb/s Ethernet Task Force.
FOR THE EXAM: Be very familiar with the common 802 standards. You will be expected to know them for the exam.
Table 2-1: IEEE standards
|802.2||Logical Link Control (LLC)|
|802.6||Metropolitan Area Networks (MANs)|
|802.9||Integrated Voice and Video|
|802.11||Wireless Networking (Wi-Fi)|
The Cisco approach to designing and building networks is to break networking into three distinct layers: Core, Distribution, and Access. Each layer has very specific responsibilities to perform. You will find yourself referring to these layers in your day-to-day job as a network engineer and, if your network is big enough, each layer may well have its own support team.
Cisco Hierarchical Networking Model
For the past couple of CCNA exam iterations, Cisco has expected students to have an appreciation for their network design model. Network design is a field of expertise in its own right, and in my humble opinion, it is one of the most rewarding and interesting careers available. Of course, the added incentive is that it rarely involves on-call, weekend, or shift work.
Network design is covered in great detail in the Designing Cisco Enterprise Networks (ENSLD) exam which is a CCNP elective. For CCNA students, Cisco expects you to have some understanding of their design methodology; for example, the current exam syllabus asks to “1.2 Describe characteristics of network topology architectures.”
Cisco Modular Design
Cisco has taken a modular approach to the construction of their network. The main advantage of this approach is that each module can be built by specialist teams (e.g., security, voice, routing, etc.) and supervised by the network architect. This (according to Cisco) is the key to simplifying the network.
There are many benefits to using the Cisco network design methodology, and it is worth making a note of them in case they appear as options in an exam question:
- Ease of understanding and implementation
- Cost savings
- Easily modified
- Easy network growth
- Facilitates summarization (small routing tables and network stability)
- Fault isolation
In the Cisco Hierarchical Model, the layers of the network are broken down into the following:
- Core (backbone) layer
- Distribution layer
- Access layer
These layers are illustrated in Figure 2.6 below:
FIG 2.6 – The layers of the Cisco Hierarchical Model
These layers map the Cisco Model to the OSI Model, as shown in Figure 2.7 below:
FIG 2.7 – Mapping the Cisco Model to the OSI Model
Collapsing the Layers
For any number of reasons, companies may not want to use the three-layer model because of cost issues, convenience, or the need to run a small network. In these instances, Cisco can simplify the three-layer model into a two-layer model.
This process is referred to as collapsing the design or collapsing the core, which only merges the core and distribution layers, leaving the access layer intact. Figure 2.8 below shows three layers and a collapsed distribution/core layer:
FIG 2.8 – Three layers and a collapsed distribution/core layer
Cisco Network Layers
Because students are expected to understand the concept of a collapsed core, it is worth spending a little time studying the features and role of each specific layer.
The core layer is the backbone of the network (see Figure 2.9 below). Depending on the network size and budget, it contains Layer 2 or Layer 3 switches, or high-speed routers. The main features of the core layer are gigabit bandwidth and low overhead (i.e., wire speed). Cisco has inverted the usual 80/20 network traffic rule so that only 20% of core traffic is local. Also known as the backbone, this is the heart of the network and it’s responsible for switching huge amounts of traffic very reliably. In order to carry out this role, certain characteristics are associated with the core layer. It must offer high reliability, provide redundancy (i.e., if one part breaks, another can pick up where it left off), provide fault tolerance, and avoid slowing traffic down by filtering it in any way.
You will find core switches used in the largest campus LANs. Campus simply refers to LANs created to support larger buildings or a number of buildings in close proximity to one another. Core switches are very expensive and are used to connect distribution layer switches. They are designed to forward traffic at very high speeds.
FIG 2.9 – The core layer
The distribution layer allows access to remote resources on other networks or the Internet, and it must be as fast as possible (usually Layer 2 switches) (see Figure 2.10 below). Also known as the workgroup layer, the distribution layer allows communication between the core and access layers. Packets are manipulated at this layer. The distribution layer should perform a range of services such as security, address summarization, workgroup access routing between LANs, traffic filtering, and redistribution.
Distribution layer switches are found in larger networks and are used to forward traffic between access layer switches. Distribution layer switches usually connect to one another with two or more uplink cables, ensuring redundancy and reducing the amount of cabling on the LAN because you aren’t connecting the access layer switches together. They are not used by end-user devices for network access.
FIG 2.10 – The distribution layer
The access layer connects end-users to the network, and it consists of layer 2 or layer 3 switches (see Figure 2.11 below). Also known as the desktop layer, this is the point at which end-users connect to the network. The access layer allows user access to local segments in the network. Functions performed here include the creation of separate collision domains (segmenting the network into smaller chunks), sharing bandwidth, and local policies for network traffic control.
Cisco access layer switches connect directly to end-users’ PCs, network printers, and servers. The access layer switches can connect to one or more distribution layer switches. You will hear me refer to these three layers throughout this manual.
FIG 2.11 – The access layer
It’s well worth visiting the Cisco.com website to review the models and features available for their core, distribution, and access layer switches.
FIG 2.12 – Cisco device model options
Spine-Leaf Network Topology
Network topology designs reflect changing trends in data usage.
Also referred to as Leaf-Spine, the Spine-Leaf design is aimed at data centers that need to quickly move data on a massive scale. The three-tier model cannot serve this need and so is being replaced to match evolving needs.
Devices in a spine-leaf design are always exactly the same number of segments away and provide consistent latency for traffic. The two-layer design allows for this with the Leaf layer consisting of access switches that provide network access to servers, firewalls, and edge routers. The Spine layer is made up of layer 3 switches and are the backbone of the network.
Every Leaf switch is connected to every Spine switch in a full-mesh, and dynamic routing is used to interconnect the two layers. This type of data center design supports an East-West data flow where traffic is designed to travel inside the data center and not outside to an ISP or another network.
An advantage of using routing as opposed to layer 2 switching is that all links can be used and loops are avoided whereas a Spanning Tree Protocol (STP) would necessarily involve closing links to traffic in order to prevent loops. Should a Spine switch fail for any reason, traffic performance would only be slightly degraded.
FIG 2.13 – Spine-leaf network topology
Cabling the Network
PCs, hubs, switches, routers, and all other types of networking equipment use different connector types. There are many reasons for this but the main two are:
- Different networks can send data in different formats, requiring the use of different cables.
- As speeds have improved, certain cable types and network interfaces have become obsolete and have been replaced by newer, more efficiently designed components.
LAN cabling can use several types of cables, including coaxial and fiber optic; however, the industry standard for LAN is referred to as unshielded twisted pair (UTP) cable. UTP cable is broken down into different categories (CAT) based on what speed the cable is capable of passing data.
A UTP Ethernet connection consists of the UTP cable, an attachment for the end of the cable known as an RJ-45 male connector, and an RJ-45 female connector on the device the cable connects to.
FIG 2.14 – RJ-45 connector
Inside the cable are eight smaller wires that are color-coded so they can be identified. You should not be expected to remember which color is associated with which pin, but you will be expected to know what you can do with these wires to make different types of UTP cables.
Each standard or category is rated for a different purpose. Table 2-2 below shows the IEEE speeds for the various cable specifications.
Table 2-2: CAT 1 to 7 characteristics
|Category of Cable||Transmission Speed||Standards|
|CAT 1||N/A||Voice only (telephone cable)|
|CAT 2||4 Mbps||Token Ring|
|CAT 3||10 Mbps||Token Ring/10BASE-T|
|CAT 4||16 Mbps||Token Ring|
|CAT 5||100 Mbps||Token Ring/10BASE-T/100BASE-TX|
|CAT 5e||1 Gbps||10BASE-T/100BASE-TX/1000BASE-T|
|CAT 6||10 Gbps||1000BASE-T/1000BASE-TX/10GBASE-10|
|CAT 6a||10 Gbps||1000BASE-T/1000BASE-TX/10GBASE-10|
|CAT 7||10 Gbps||1000BASE-T/1000BASE-TX/10GBASE-10|
Another commonly used network cable type is FO (fiber optic) cable, which usually ensures higher transmission rates and distances than copper-based cables. FO technology sends the signal on the wire using light beams, as opposed to the electrical signals used by copper cables. Another difference between copper and FO cables is the connectors used to terminate the cable. While the standardized approach for copper cables is using RJ45 connectors almost exclusively, FO cables commonly use multiple connector types, including LC, SC, MTRJ, ST, and FC (see Figure 2.14 below).
FO cables can be built using several technologies, including single-mode and multi-mode optics. Single-mode FO cables offer higher transmission distances compared with multi-mode cables, but they are also more expensive.
Figure 2.15 – FO patch cord with LC connectors on both ends
IEEE and Cabling Standards
LAN cabling is a fairly detailed subject, and for historical reasons there are many cable types. Each cable type has certain limitations and can be used only on certain topologies.
Ethernet works at 10 Mbps, as referred to by the IEEE 802.3 standard. Ethernet can be either a star or a bus topology. The cabling used can be coaxial, CAT 3 or higher, using twisted pair. Twisted pair cable is referred to as XBase-T, where X refers to the speed at which the media operates, Base denotes that baseband transmission is used, and T refers to the fact that the wires are twisted into pairs to reduce crosstalk between the wires and radio frequency interference (please read any Network+ guide for more information on this).
The bus topology was the only choice in early Ethernet networks because the cable used was coaxial. With the move to network switches and UTP cables, the common topology is now star.
802.3u Fast Ethernet
The 802.3u standard is used by most modern networks. Fast Ethernet is a vast improvement on standard Ethernet. It uses a star topology and either CAT 5 twisted pair or fiber optic cable. It overcame technologies such as FDDI, 100VG AnyLAN, and ATM to become the leading solution.
Fast Ethernet provides an effective and inexpensive upgrade path from 10 Mbps Ethernet, as well as being backward compatible. It is able to operate over CAT 3, CAT 5, fiber optic, and even USB cables.
802.3z (or 802.3ab) Gigabit Ethernet
Gigabit Ethernet is an improvement on Fast Ethernet. It uses the same 802.3 frame format as Fast Ethernet in a star topology with twisted pair (802.3ab/1000Base-T) or fiber optic/shielded copper (802.3z/1000Base-X) cable.
Table 2-3: Ethernet cable specifications
|Cable||Bandwidth (Mbps)||Max Cable Distance (meters)||Cable Construction|
|1000BASE-SX||1000||220 or 550||Short-wavelength laser/ MM fiber|
|1000BASE-LX||1000||550 (MM), 5 Km (SM), or 10 Km (SM)||Long-wavelength laser/ MM fiber/SM fiber|
|1000BASE-ZX||1000||70 Km||Extended-wavelength laser/SM fiber|
NOTE: MM is multi-mode fiber and SM is single-mode fiber
Although the CCNA isn’t a general networking exam, you may see questions about cabling specifications such as which ones use fiber. It’s worth learning the standards as well as reading a good quality Network+ study guide, even if you don’t plan to take the actual exam.
When the color of each wire matches on both sides of the cable, then it is known as a straight-through cable. In modern networks, hosts and devices can autosense the types of cables connected and communicate over the cable, whether it’s straight-through or crossover (see next section). Historically, straight-through cables were used to connect dissimilar devices, so if you were connecting a PC to a hub/switch, you would have needed a straight-through cable.
Note the cable specifications of 568A and 568B in Figure 2.16 below. Specific colors are assigned to each of the eight internal wires, but these won’t show in the printed book so do check them on Google. You won’t be expected to name the colors in the CCNA exam.
FIG 2.16 – Straight-through cable
|568A Color Codes||568B Color Codes|
|Pin #||Wire Color||Pin #||Wire Color|
FIG 2.17 – Straight-through cable from a PC to a switch
You can compare cable ends by holding them together with the retaining clip facing down.
As mentioned earlier, the main use of a crossover cable is to connect two similar devices together, for example, connecting PCs together without having to buy a hub or switch. You will need to remember these for the CCNA exam. An easy way to remember this is that like-to-like devices use a crossover cable.
FIG 2.18 – Crossover cable
If you take the cable from pin 1 and put it into pin 3 at the other end, and then pin 2 into pin 6, you will have made a crossover cable connection.
FIG 2.19 – Crossover cables in use
You should be familiar with the details of crossover and straight-through cables for the exam. The specifications are defined by the EIA/TIA. A straight-through cable has matching wires on both ends, whereas a crossover cable has T568A on one end and T568B on the other. Typically, a network card in a PC transmits on pairs 1 and 2 and receives on pairs 3 and 6. This is why you would need a crossover cable if you were connecting two PCs together without using a switch.
Many Cisco switches support a feature known as Auto-MDIX, which stands for automatic medium-dependent interface crossover. This feature can detect an incorrectly attached cable and swap the pairs of wires on the cable used to transmit and receive. The Cisco 2960 Switch supports Auto-MDIX, but it must have its speed and duplex settings configured for autonegotiation in order for the feature to work.
The last type of cable using UTP is referred to as a rollover (or sometimes called a flat or console) cable. This cable is only used to connect to a special port on routers and switches known as the console port. Connecting to the console port allows you to configure the networking equipment when you first install it or if you cannot reach it over the network.
FIG 2.20 – Rollover cable
On a rollover cable, pin 1 on one side goes into pin 8 on the other, pin 2 goes into pin 7, and so on. As you hold the ends of the cables together with the retaining clips facing down, you can see that each wire is reversed.
You will be expected to know which type of cable to use between which devices in the exam (and in the real world). Table 2-4 below summarizes what you have learned so far:
Table 2-4: Which cable to use?
When using fiber connectivity within a network infrastructure, the digital signals sent by network devices are converted into light beams, sent across the fiber link, and converted back into digital signals on the other side. One of the major advantages of using this approach instead of other technologies (e.g., RF signals) is that nobody can easily tap into that connection without being noticed. However, if this does happen, the intruder can be easily identified by monitoring systems because the signal will drop significantly. The major advantages of using fiber cabling include the following:
- It offers high transfer rates (more than 10Gbps)
- It operates over long distances (up to 100 km)
- It is immune to radio interference
Fiber signals experience very slow degradation, so fiber connections can be used over very long distances, unlike copper connectivity. Fiber cabling is also immune to radio interference that can come from microwaves, wireless networks, mobile phones, or other devices (because light signals do not interfere with radio signals). For these reasons, fiber cabling is recommended in industrial environments that are electrically noisy.
Fiber connections can operate based on two technologies:
- Multi-mode fiber
- Single-mode fiber
Note: There is a direct compatibility requirement between the type of fiber used and the type of device interface/transceiver in which you plug the fiber. You cannot use single-mode fibers with multi-mode transceivers or multi-mode fibers with single-mode transceivers.
Multi-mode fiber technology involves the light beam travelling across multiple paths inside the fiber, bouncing off the fiber walls, as presented in Figure 2.21 below. This technology is called multi-mode because the light beam can travel in “multiple modes” across the cable.
Figure 2.21 – Multi-Mode Fiber
Multi-mode fiber is often used for communication over short distances, either inside a building or outside if the distance is less than 2 km. If you try to use it on longer distances, you will experience signal degradation because of the light beams that bounce off the fiber walls. Because the light does not have to travel over very long distances, multi-mode fiber connections can use inexpensive light sources based on LED technologies, unlike the case with single-mode fibers. Using inexpensive components allows you to keep the cost down for multi-mode fiber infrastructures.
While multi-mode fiber allows the light to travel on multiple paths, the main characteristic of single-mode fiber is that it offers a single light path across the fiber, as shown in Figure 2.22 below:
Figure 2.22 – Single-Mode Fiber
When using a single-mode fiber, the light follows a direct path (a “single mode”) to the destination, without the light beam bouncing off the cable walls like in the case with multi-mode fiber. Single-mode fiber is generally used for communications over very long distances (up to 100 km). Communication can even exceed 100 km, but in that case, you would have to use special devices to regenerate the signal.
The light source in single-mode fiber is more expensive than in the case of multi-mode fiber because laser beams are used instead of LED sources. Laser beams offer a very strong light that can reach the other side of the cable, even over long distances, without major attenuation. Because of this, equipment using single-mode fiber will be a lot more expensive than equipment using multi-mode fiber. In addition, the actual single-mode fiber cables are more expensive than multi-mode fiber cables because the core of the fiber must be manufactured with different technologies in order to be very thin. For this reason, single-mode fiber is generally used only in special cases and not over short distances.
There are many types of WAN services available, including ISDN, Frame Relay, ATM, ADSL, and others. Each type of service can have different connector types associated with it.
Most Cisco routers use a 60-pin D-shaped connector for Serial interfaces. This usually connects to a slot containing a WAN interface card (WIC). The connector for the other side of the cable can vary, depending on who is providing you with the WAN connection and what type of service you have asked for.
It’s important to discuss with your Cisco reseller what your requirements are and what type of presentation you have from your service provider to ensure that you are buying the correct modules and cables for your WAN connection.
You can see in Figure 2.23 below that the cable end has “DTE” printed on it. This indicates the Data Terminal Equipment end that connects to the Serial interface on your router. If the end has “DCE” printed on it, this would indicate that it is the Data Communication Equipment end, and you need to add a clock rate to this interface in order for it to become operational.
You can buy cheaper imported cables (and Serial cards) from China but you will find that non-Cisco products tend to have a high failure rate. I found this out at my cost when I started running my Cisco classroom training courses.
FIG 2.23 – DTE Serial cable end
The DTE cable inserts into the WIC-1T card (see Figure 2.24 below), which will allow you to configure Serial connections for your router, including PPP and Frame Relay (we will cover this later in this guide).
FIG 2.24 – WIC-1T
End of Chapter Questions
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