Chapter 9: WANs Flashcards
Compare and contrast dedicated-circuit services and packet-switched services.
Dedicated-circuit services are circuits that can be leased by users through common carriers. Dedicated-circuits are point to point connections that can only be used by the two devices at each end of the circuit. In contrast, packet-switched services act more like Ethernet and IP networks used in LANs and BNs where multiple connections can exist simultaneously.
Compare and contrast ring architecture, star architecture, and mesh architecture.
Ring architecture, as the name suggests, is in the shape of a ring. Each computer is connected to the next computer in a closed loop system. Since the circuits can be either full or half duplexed, communications can flow in both directions. However, if one computer or circuit goes down, the flow of information does not stop, but all the communications that would have flowed through that computer or circuit will be rerouted in the opposite direction which will cause network slowdowns. Star architecture is setup with a single hub computer that receives and transmits all communications on the network. If any one computer or circuit is lost, it only effects that one computer or circuit. However, if the central hub computer is lost, the entire network is affected since all traffic must flow through that hub. A full mesh network is very expensive and not commonly used but essentially, every computer is connected to each computer. More commonly used it the partial mesh network. This network combines the ring and star topologies by having some, but not all computers connected. This creates several hubs that connect to each other (ring) and have several circuits that extend out to individual computers (star).
What are the most commonly used T carrier services? What data rates do they provide?
The two most commonly used T carrier services are the T1 and T3 circuits. The T1 circuit provides rates of 1.544 Mbps, whereas, T3 circuits provide up to 44.736 Mbps.
Distinguish among T1, T2, T3, and T4 circuits.
T1 can transmit data and voice communications using 24 64 Kbps circuits. A T3 circuit is the equivalent of 28 T1 Circuits allowing data rates at 44.736 Mbps. Although T2 and T4 are defined standards, they are not available commercially.
Compare and contrast frame relay, MPLS, and Ethernet services.
Frame relay for WANs, like wired Ethernet for LANs, does not provide error checking and simply discards any packets with errors leaving the responsibility of retransmission to the sending and receiving computers. Frame relays use T carrier networks for physical transmissions. Most LANs and BNs use Ethernet services, but here WANs are currently using layer-2 protocols. This means that their transmissions must be translated and retranslated from the sender to the receiving computer. The emergence of Ethernet WANs will resolve this and allow companies to install Ethernet Services. Multiprotocol Label Switching (MPLS), commonly called layer 2.5 technology since it is designed to work with layer-2 protocols, is another newer WAN technology. With MPLS, the user sends the packets using common layer-2 technology to a switch were the packet is reviewed and converted to a layer-3 packet. The layer-3 packet contains a 4-byte header between the layer-2 packet and the layer-3 packet. This is used to move the packets through the network until it reaches the receiving edge where the layer-3 packet is removed and the layer-2 packet is transmitted to the receiving computer.
Compare the three types of VPN.
The three types of VPNs are Intranet, Extranet, and Access. Intranet is used to connect internal offices over the internet. Extranet is the same on intranet, but is used for other organizations outside of the company (such as vendors/suppliers). Access VPN is used by remote employees that need to access the companies network while offsite.
How can you improve WAN performance?
WAN performance can be improved by upgrading devices to increase the speed or by simply using better routing protocols. Using analysis of the network to identify circuits that need to be increased or decreased along with identifying the need to lease additional circuits can also help to improve performance.
What is a common carrier?
Private companies such as AT&T, Bell Canada, Sprint, and BellSouth that provide communication services to the public.
How are voice and data communications regulated I the US?
They are regulated by a government agency (Federal Communications Commission (FCC)).
What is CSU/DSU?
Channel Service Unit/Data Service Unit: Is a WAN equivalent of a NIC in a LAN.
Describe a packet-switched network
With packet-switched services, the user buys a connection into the common carrier cloud. The user pays a fixed fee for the connection into the network and is charged for the number of packets transmitted. The user’s connection into the network is a packet assembly/disassembly device (PAD), which can be owned and operated by the customer or by the common carrier.
What is a PVC?
The connections between the different locations in the packet network are called permanent virtual circuits (PVCs), which means that they are defined for frequent and consistent use by the network.
What is a Frame Relay?
Is one of the most commonly used WAN services in the United States. Like wired Ethernet LANS, it is an unreliable packet service because it does not perform error control. Frame relay checks for errors but simply discards packets with errors. It is up to the software at the source and destination to control for lost messages.
What is a VPN Tunnel
VPN Gateways enable you to create PVCs through the internet that are called tunnels.
How does VPN Work
The VPN gateway at the sender takes the outgoing packet and encapsulates it with a protocol that is used to move it through the tunnel to the VPN gateway on the other side. The VPN gateway at the receiver strips off the VPN packet and delivers the packet to the destination network. The VPN is transparent to the users; it appears as though a traditional packet-switched network PVC is in use.
