Part-1
1. What are common carriers, local exchange carriers, and interexchange carriers? A common carrier is a private company that sells or leases communication services and facilities to the public. Common carriers are profit-oriented, and their primary products are services for voice and data transmissions, both over traditional wired circuits as well as cellular services. Common carriers often supply a broad range of computer-based services, such as the manufacturing and marketing of specialized communication hardware and software. A common carrier that provides local telephone services (e.g., BellSouth) is commonly called a local exchange carrier (LEC), while one that provides long distance services (e.g., MCI) is commonly called an interexchange carrier (IXC). As the LECs move into the long distance market and IXCs move into the local telephone market, this distinction may disappear.
3. Explain how cloud architecture works. Cloud architecture is represented as a cloud with connection points. Users of circuit switched services lease the connection points (e.g., telephone lines) into the common carrier’s network, which is called the cloud. This architecture is very flexible and hides its internal details. Circuits can be established as needed between any computers attached to the cloud at any point. However, data can only be transmitted while a circuit is established, and only to the one location it connects to. These designs are simpler for the organization because they hide the common carrier’s burner of network design and management inside the cloud. Network managers do not need to worry about the amount of traffic sent between each computer, they just need to specify the amount of traffic entering and leaving each computer and buy the appropriate size and number of connections into the PSTN. Cloud-based designs can be more expensive because users must pay for each connection into the network and pay based on the amount of time each circuit is used. Cloud-based designs are often used when network managers are uncertain of network demand, particularly in a new or rapidly growing network. 9. Compare and contrast circuit-switched services, dedicated circuit services, and packet-switched services. For both circuit switched and dedicated circuit networks, a circuit is established between the two communicating computers. This circuit provides a guaranteed data transmission capability that was available for use by only those two computers and is assigned solely to that transmission. No other transmission is possible until the circuit is closed. In contrast, packet switched services enable multiple connections to exist simultaneously between computers over the same physical circuit or even over different physical circuits.
In the POTS and ISDN circuit switched networks each connection goes through the regular telephone network on a different circuit. These circuits may vary in quality, meaning that while one connection will be fairly clear, the next call may be noisy. The data transmission rates on these circuits are usually low. Usually, transmission rates range from 28.8 Kbps to 56 Kbps for dialed POTS circuits to 128 Kbps to 1.5 Mbps for ISDN circuits. You usually pay per use for circuit switched services. With a dedicated circuit network, you lease from common carriers for their exclusive use twenty-four hours per day, seven days per week. All connections are point to point, from one building in one city to another building in the same or a different city. The carrier installs the circuit connections at the two end points to the circuit and makes the connection between them. The circuit still runs through the common carrier’s cloud, but the network behaves as if you have your own physical circuit running from one point to another. Dedicated circuits are billed at a flat fee per month and the user has unlimited use of the circuit. Once you sign a contract, making changes can be expensive because it means rewiring the building and signing a new contract with the carrier. Therefore, dedicated circuits require more care in network design than switched circuits both in terms of locations and the amount of capacity you purchase. With packet switched services, the user again buys a connection into the common carrier cloud. The user pays a fixed fee for the connection into the network depending upon the type and capacity of the service and is charged for the number of packets transmitted.
13. Distinguish among T1, T2, T3, and T4 circuits. T1 circuit is sometimes called a DS-1 circuit. It provides a data rate of 1.544 Mbps. T1 circuits can be used to transmit data, but often are used to transmit both data and voice. In this case, a time division multiplexer (TDM) provides 24 64-Kbps circuits. Digitized voice using pulse code modulation (PCM) requires a 64 Kbps circuit so T1 circuit enables 24 simultaneous voice channels. T2 circuit transmits data at a rate of 6.312 Mbps. Basically; it is a multiplexed bundle of four T1 circuits. T3 circuits allow transmission at a rate of 44.376 Mbps, although most articles refer to this rate as 45 megabits per second. This is equal to the capacity of 28 T1 circuits. T3 circuits are becoming popular as the transmission medium for corporate MANs and WANs because of their higher data rates. At low speed, these T3 circuits can be used as 672 different 64 Kbps channels or voice channels. A T4 circuit transmits at 274.176 Mbps, which is equal to the capacity of 178 T1 circuits. Obviously, an organization using either T3 or T4 circuits must have a tremendous need to transmit very large quantities of data.
15. How do packet-switching services differ from other WAN services? Packet switched services are quite different from the other type of network services. For each of these three, dialed circuit services, dedicated circuit services, and circuit switched services, a physical circuit was established between the two communicating computers. This circuit provides a guaranteed data transmission capability that was available for use by only those two computers. In contrast, packet switched services enable multiple connections to exist simultaneously between computers. With packet switched services, the user again buys a connection into the common carrier network. The user pays a fixed fee for the connection into the network and charged for the number of packets transmitted.

22. Compare and contrast X.25, frame relay, ATM, SMDS, and Ethernet services.
X.25:
It is the oldest packet switched service.
A standard developed by ITU-T
Offers datagram, SVC and PVC services
Uses LAPB and PLP protocols at the data link and network layers, respectively
- Requires protocol translation at PADs ( for those users who use different protocols at their LANs)
- A reliable protocol (It performs error control and retransmits bad packets)
- Widely used in Europe
- Not in widespread use in North America
- Low data rates (64 Kbps)
Frame Relay: Another standardized technology Faster than X.25 but slower than ATM Encapsulates packets
- Packets delivered unchanged through the network
Unreliable, like ATM
- Up to the end-points to control the errors NO QoS support (under development) Common CIR speeds: -56, 128, 256, 384 Kbps, 1.5, 2, and 45 Mbps ATM: Uses fixed length, 53 byte “cells”
- 5 bytes of overhead and 48 bytes of user data
- More suitable for real time transmissions
Provides extensive QoS information
- Enables setting of precise priorities among different types of transmissions (i.e. voice, video & e-mail) Data Rates
- Same rates as SONET:51.8, 466.5, 622.08 Mbps
- New versions: T1 ATM(1.5 Mbps), T3 ATM(45 Mbps
SMDS:
A non-standardized technology
- Developed by Telcordia for local phone companies
Unreliable, like ATM
Encapsulates packets
Originally developed for MANs, but could be used for WANs as well
Transmission speed offered:
- 56 Kbps to 45 Mbps Uncertain future
- Not standardized; competition from FR, ATM, and others Ethernet services: Most organizations use Ethernet and IP in the LAN and BN Ethernet services differ from MAN/WAN packet services like X.25, ATM, Frame Relay or SMDS Currently offer CIR speed from 1 Mbps to 1 Gbps at ¼ the cost of more traditional services
No need to translate LAN protocol (Ethernet/IP) to the protocol used in MAN/WAN services
- X.25, ATM, Frame Relay and SMDS use different protocols requiring translation from/to LAN protocol
Emerging technology; expect changes
25. How do VPN services differ from common carrier services? A new type of VAN, called a virtual private network (VPN) has recently emerged. VPNs provide circuit that run over the Internet but appear to the user to be private networks. Internet access is inexpensive compared to the cost of leasing dedicated circuits, circuit switched services, or packet switched services from a common carrier. Different VPNs provide different services, but most offer packets switching hardware that will communicate via the Internet, or VPN services which you lease from the VPN in much the same way as leasing a service from a common carrier. These VPN hardware (or services) take your data, encrypt it, and send it through the Internet through a series of “tunnels”, a virtual circuit through the Internet which constrains the source and destination to only those within the VPN.
29. Describe five important factors in selecting WAN services. Five important factors in selecting WAN services are vendor capabilities, network capacity, flexibility, control, and reliability. The best vendors provide high quality service, quickly respond to network problems, adapt to changing customer needs, and provide useful network management services along with the data transmission services.
33. It is said that packet-switched services and dedicated circuit services are somewhat similar from the perspective of the network designer. Why? Network managers build packet-switched networks using PVCs, most packet-switched networks behave like dedicated circuit networks. At first glance, the basic architecture looks very similar to the cloud mesh of switched-circuit services, and in fact, they are very similar because data can move from any computer attached to the cloud to any on the cloud. However, because virtually all data-intensive networks use PVCs, this means that the network is actually build using virtual circuits that are the software equivalent of the hardware-based dedicated circuits.