2.1 – Select the appropriate media, cables, ports and connectors to connect switches to other network devices and hosts
2 – Implement a Small switched Network
2.1 – Select the appropriate media, cables, ports and connectors to connect switches to other network devices and hosts
network cables attach to LAN devices via the network interface card (NIC) or network interface port, such as found on a switch.
Cabling
The two most popular types of LAN cabling: twisted pair and fiber optic.
Twisted-Pair Cabling
Twisted-pair cable is a thin-diameter copper wire used for voice and data network cabling. The wires are twisted around each other to minimize interference from other twisted pairs in the cable. Twisted-pair cabling, enables the use of less bandwidth than required for coaxial cable or optical fiber.
Two types of twisted-pair cabling are found in LANs: shielded twisted-pair (STP) and unshielded twisted-pair (UTP).
The cable category indicates the number of twists per inch. The more twists in the cabling, the more immune the cable from interference, the faster the cable can transmit, and the greater the bandwidth.
Fiber-Optic Cabling
An optical fiber is a thin glass or plastic strand designed for light transmission and capable of transmitting trillions of bits per second. Optical fiber offers many advantages over copper wire because the light pulses carried by fiber are not affected by random radiation in the environment, and its error rate is significantly lower. Fiber enables longer distances to be spanned before the signal has to be regenerated by repeaters, as required for the electrical signal carried by copper wire. Fiber is also more secure than copper because wire taps in the fiber line can be detected.
the two primary types, or modes, of fiber used in optic transmission: multimode and single mode.
Single-mode fiber is used to span longer distances, and multimode fiber is common for short distances.
Single-mode fiber (SMF) is an optical fiber used for high-speed transmission over long distances. SMF provides a higher-quality cable that allows for a cleaner, stronger signal, and therefore provides more bandwidth than multimode. However, the smaller core of SMF makes it more difficult to align the light source at the receiver.
Multi-mode fiber (MMF) is an optical fiber with a larger core than single-mode fiber and is the most common fiber used for short distances, such as for LANs. Light can enter the core at different angles, making it easier to transmit light from the source to a broader receiver. This broader scope permits the use of a light emitting diode (LED) rather than the precise laser required by single-mode fiber. This is comparable to the difference between using a flashlight and a laser pointer as a pointing device during a lecture; the flashlight is somewhat broad in its coverage, whereas the laser pointer is more precise.
**Read about cables on part 1.2 of this series
Cable Termination
Cabling between two devices serves no purpose if there is no way to attach the two together and although duct tape certainly has its purposes in this world, this is not one of them. Cables, whether copper or fiber optic, are clamped at the ends with a jack connection, known as a registered jack, or RJ.
Several types of RJ connectors are used in networking today, and each type is identified by a number. For example, most telephone handset and wall ports use RJ-11 connectors. Ethernet uses RJ-21 and RJ-45 jack types, and T1 lines use RJ-48.
The RJ-21 (Registered Jack-21) is an Ethernet cable using a 50-pin Telco connector on one end. On the other end, the cable branches out to 12 RJ-45 (Registered Jack-45) connectors. The RJ-45 is a connector that holds up to eight wires.
These RJ-45 plugs and sockets (jacks) are used in Ethernet and Token Ring devices.
The NIC found inside the user’s desktop computer or other network device, such as a mail server or network printer, is connected via the network cable to the network interface jack.
Wall Plates and Wall Boxes
Wall plates and wall boxes serve the aesthetic purpose of hiding holes in the wall and visible wires. Wall plates and wall boxes help protect the cable from being pulled out, cut, or damaged. They also help with providing a place to label cables. Further, a box can be organized to centralize multiple services for a user in a single location (for example, phone and data together).
The wall plate is mounted onto the wall with an opening for the RJ connection, and a wall box is a freestanding box that can be, but is not always, mounted to a wall. Behind these wall plates and wall boxes is the cabling that runs back to the LAN switch, often sitting in a communications closet somewhere within the building.
Network Interface Card (NIC)
Much as your driveway is an interface to the main road, the network interface card is your interface to the network. With one end of the network cable connected to a port in the wall, the other end needs to connect to a device to complete the circuit. This device is the network interface card, or NIC. NICs are circuit boards that plug into your desktop, laptop, or network servers, such as a web or e-mail server. The NIC controls the sending and receiving of data across the physical Open System Interconnection (OSI) model Layer 1 and data link OSI model Layer 2.
**As explainned in part 1.9 previously on this series
Local Area Networks (LANs)
The topology of the LAN is determined by the technology. (For example, a ring topology is implemented by Token Ring or FDDI, and a star or tree topology is implemented by Ethernet.)
Token Ring
Token Ring is a technology developed by IBM and standardized by the Institute of Electrical and Electronics Engineers (IEEE) 802.5 committee for implementation in a LAN environment. Token Ring uses a special frame, called a token, to designate the authoritative speaker for that LAN segment. This technology can connect up to 255 nodes in a physical star or ring connection that can sustain 4 or 16 Mbps. Each node on a Token Ring LAN connects to a central wiring hub called the multi-station access unit (MAU) using a twisted wire cable, such as UTP.
Token Ring is more deterministic than Ethernet, which means that it ensures that all users get regular turns at transmitting their data. With Ethernet, all users have to compete for network access to get on to the network. In a Token Ring network, a token is passed around the network from one workstation to the next, giving each workstation equal access to the network. Unlike an Ethernet workstation, which can send data if the line is idle, a Token Ring workstation cannot send data across the network unless it is in possession of the token.
FDDI
Fiber Distributed Data Interface, or FDDI (pronounced “fiddy”), is a LAN and metropolitan-area network (MAN) access method. It is a token-passing network, similar to Token Ring, and uses optical fiber cabling to transmit at 100 Mbps up to 10 kilometres. FDDI provides network services at the same OSI model layers as Ethernet and Token Ring (Layer 1 and Layer 2).
FDDI provides the option of a dual counter-rotating ring topology. This dual-ring topology is used for redundancy so that if one ring fails the other ring carries the traffic. Traffic on these rings travels in opposite directions: The traffic on one ring travels clockwise, whereas the traffic on the other ring travels counter clockwise.
Ethernet
Ethernet is the most widely deployed LAN access method, defined by the IEEE as the 802.3 standard. Ethernet has become popular such that a specification for a LAN connection or network card implies the use of Ethernet even if not explicitly stated. A 10/100 Ethernet port supports both 10BASE-T at 10 Mbps and 100BASE-T at 100 Mbps.
Ethernet is often considered to be a shared-media LAN, which means that all stations on the segment share the total bandwidth?10 Mbps (Ethernet), 100 Mbps (Fast Ethernet), or 1000 Mbps (Gigabit Ethernet). When Ethernet is deployed in a switched environment, it is no longer considered to be shared. Therefore, each sender and receiver pair has the full Ethernet bandwidth available for use.
Ethernet uses carrier sense multiple access collision detect(CSMA/CD) technology, broadcasting each frame onto the physical medium (wire, fiber, and so on). All stations attached to the Ethernet listen to the line for traffic, and the station with the matching destination MAC address accepts the frame and checks for errors before doing anything further with the frame. If the frame is error free, it is handed to the network layer (Layer 3) of the OSI model and ultimately the data is presented to the user, such as an e-mail. If the frame has errors, however, it is discarded.
Media Access Control (MAC) Addressing
The MAC address is the unique serial number burned into each network adapter that differentiates the network card from all others, just as your house number is unique on your street and identifies your home from all others. To be a part of any network, you must have an address so that others can reach you. Two types of addresses are found in a network: the logical (OSI model Layer 3, network) and the physical (OSI model Layer 2, data link). For this discussion of LAN environments, the physical address (also known as the Media Access Control [MAC] address) is relevant.
A MAC address is the physical address of the device. It is 48 bits (6 bytes) long and is made up of two parts: the organizational unique identifier (OUI) and the vendor-assigned address.
The MAC address on a computer might look like this: 00-08-a1-08-c8-13. This MAC address is used for the Fast Ethernet adapter on the computer in question. The OUI is 00-08-a1, and the vendor-assigned number is 08-c8-13.
The OUI is administered by the IEEE and identifies the vendor of the network adapter. The vendor-assigned portion of the MAC address is just that, the alphanumeric identifier assigned by the vendor. It is the combination of the OUI and the vendor-assigned number that ensures that no two network adapters have the same MAC address.
| note - | MAC addresses are represented as hexadecimal (hex) numbers. |
With the hexadecimal numbering system, each half byte (4 bits) is assigned a hex digit, which is listed in Table 5-1, with its decimal and binary equivalents. Hex values are identified with an h or dollar sign, so $3E0, 3E0h, and 3E0H all stand for the hex number 3E0.
| Hexadecimal (Base 16) | Decimal (Base 10) | Binary (Base 2) |
| 0 | 0 | 0000 |
| 1 | 1 | 0001 |
| 2 | 2 | 0010 |
| 3 | 3 | 0011 |
| 4 | 4 | 0100 |
| 5 | 5 | 0101 |
| 6 | 6 | 0110 |
| 7 | 7 | 0111 |
| 8 | 8 | 1000 |
| 9 | 9 | 1001 |
| A | 10 | 1010 |
| B | 11 | 1011 |
| C | 12 | 1100 |
| D | 13 | 1101 |
| E | 14 | 1110 |
| F | 15 | 1111 |
At the heart of LAN operation is the MAC address. The MAC address is the unique network adapter serial number distinguishing that network card from all others on the network. The MAC address is made up of two parts: the OUI and the vendor-assigned serial number.
Half-duplex Ethernet uses CSMA/CD as the LAN access method. When an Ethernet device wants to gain access to the network, it checks to see whether the network is quiet; if the network is not quiet, the device waits a random amount of time before retrying. If the network is quiet and two devices access the line at exactly the same time, their signals collide. When the collision is detected, they both back off and each waits a random amount of time before retrying
On the figure bellow is lists LAN hardware and the layer in the OSI model at which each piece of hardware operates.
LAN Hardware |
|
| Device | OSI Layer |
| Repeater | Layer 1 (physical) |
| Hub | Layer 1 (physical) |
| Bridge | Layer 2 (data link) |
| Switch | Layer 2 (data link) or Layer 3 (network) |
| Router | Layer 3 (network) |
Repeaters regenerate signals in the cable line and are used in both local-and wide-area networking environments to extend the distance a signal can reach. Ethernet hubs are multiport repeaters because each signal that is received by the hub is repeated out all hub ports and is received by any device connected to the hub.
Ethernet bridges are essentially multiport hubs. Instead of repeating the incoming signal out all ports, however, the bridge maps the MAC address to a port. This map keeps track of the MAC addresses of each node that resides on each network segment and allows only necessary traffic to pass through the bridge, such as traffic destined for a segment other than the source. If the frame’s source and destination network segments are the same, the frame is filtered; if the segments differ, the frame is forwarded by the bridge to the appropriate segment.
A LAN switch is a network device that cross-connects stations or LAN segments. Network switches replace shared media hubs, increasing network bandwidth. Each port on the switch can give full bandwidth to a single server or client station or each can be connected to a hub with several stations. The switch also forwards a frame out all ports if the destination MAC address is unknown.
Routers are basically computers with two or more NICs supporting one or more network protocols, such as the Internet Protocol (IP). A switch receives frames and makes filtering and forwarding decisions based on the hardware MAC address, whereas the router opens these frames and examines the packets contained therein. The router looks at the destination network address in these packets and makes a forwarding decision based on this address. If the router does not know how to reach the destination network, the packet is dropped. The router then forwards the packet to the appropriate LAN or WAN network segment.
Layer 3 and Above
The logical topology at Layer 3 (network) is made possible by the logical topology at Layer 2 (data link) and the physical topology at Layer 1 (physical) underneath it all. A packet has to and from addresses (destination and origination), much as a letter has sending (return) and receiving addresses. The letter does not concern itself (as much as a letter is “concerned”) with how it gets from sender to receiver because it has a logical “straight line.” The letter, or packet, is not aware of the lower logical and physical layers that comprise the line of direction, just that the letter has a path to get to its intended destination, see bellow.
Logical and Physical Topology of a Letter’s Travels
The physical topology is illustrated by the roads between the house and the post offices. This physical topology is broken down into segments by the traffic lights at various points along the way. The logical topology here is the straight line from the house to the post office, unaffected by the roads travelled or the traffic signals along the way. The letter’s transmission from house to post office is affected here when there is no physical path at all, such as all available roads closed or blocked.
Packets
Because packets and frames work at different layers (Layer 3 and Layer 2 respectively), they involve different aspects of the network. Think of a frame as a train engineer? He needs to know where to go and how to get there and is not concerned with where the train has just left. A packet needs to know where it is going and from where it came, much as a letter needs to have the recipient’s address and the sender’s address. The recipient in turn uses the return address to send a reply.
|
NOTE |
A packet is a fixed block of data sent as a single entity across a network. Commonly when LANs are discussed, the terms frame and packet are used synonymously. However, packets are found in the network layer (Layer 3 of the OSI model), and frames are at the data link layer (Layer 2 of the OSI model). |
Packets are only affected by the underlying physical and logical topology if a failure results in the path being broken. For example, suppose you have three roads between home and work and at any time you can take any one of those roads. One morning one of those roads is closed for construction; the physical path is unavailable for use. The physical topology for your drive has changed because now two roads are available rather than the original three. You are not concerned here because you still have a way to get from home to work. Your logical path has not changed; it is still home to work, but the physical topology has changed in that now you have to take a different road. Network packets work in the same way. It is the routers and Layer 3 switches that decide over which path the packets move, making the decisions just as you would behind the wheel of the car.
Hardware
Hubs and repeaters are found at Layer 1, bridges and switches and found at Layer 2, and routers are found at Layer 3. A router is a network device that receives and forwards data packets along a network. A router connects two or more networks together; often these are WANs, but routers can also be used to connect two or more LANs. The most common placement of a router is between a LAN and a WAN, such as the Internet, see bellow.
Router Connecting a LAN and the Internet
Routers work at Layer 3 of the OSI model to examine the header of each packet. From the header the router determines the path on which the packet must be forwarded. This is similar to the decision you make when you look at an arrival and departure board in the train station to determine on which track your train departs. Routers determine pathways for packets based on routing tables.
The common theme here is that you make a determination based on a table of information, and routers make a determination based on a similar table of information, called a routing table.
The physical (OSI model Layer 1) topology of a network represents how each device is interconnected by media or equipment. The logical (OSI model Layers 2 and 3) topology of a network represents the conceptual view of how devices are interconnected, often, but not always, bearing a resemblance to the physical topology.
Hubs carry bits, switches carry frames, and routers carry packets. They all connect physical segments together to create a larger network. Frames are moved around the network by Layer 2 hardware, such as bridges or switches. Bridges and switches use the frame header to determine to which network segment the frame must be forwarded. Bridges and switches determine forwarding decisions for frame movement based on a forwarding table in a MAC table.
The packet, a Layer 3 data unit, is carried by the frame inside its payload section. Packets are the concern of Layer 3 hardware, such as routers. The difference is that whereas a bridge or switch just forwards the frame out a specified port, routers decide the disposition of the packet, such as through which port to forward the packet and if the router is to forward the packet at all. A router can make a more intelligent decision because it knows the source and destination and has capacity to make a decision about paths that are several hops downstream from the router.
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