English

Posted on Apr 1, 2020 in Other subjects

1-Impedance: it’s the total resistance that the cable presents to the electrical current. (Measure of the opposition to time) Attenuation: it’s the loss of intensity of the current. (Decibels per unit length)

Shielding: it’s the practice of reducing the electromagnetic field in a space. Capacitance: it’s the ability of a body to store an electrical charge. (Pico-farads)

 2-Pairs of copper: wires are encased in color-coded plastic insulation and are twisted together. An outer jacket protects the pairs. When an electricity flows through a copper wire, a magnetic field is created around the wireCoaxial cable: is a type of cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. The copper conductor is a layer of flexible insulation. The second layer can reduce the amount of outside interference. Optical fiber cable: it’s a cable containing one or more optical fibers. The optical fiber elements are coated with plastic layers and containing a protective tube where the cable will be deployed.

 3- UTP: (Unshielded twisted pair) it’s a cable that has 2 or 4 pairs of wires. 

STP: (Shielded twisted pair) each pair of wires is wrapped in metallic foil to shield the wires from noise. STP prevents interference better than UTP, but STP is more expensive. STP s more difficult to install because of the thickness, but STP cables also have a range of 100 m and it’s better to outside buildings.

 4- When crimping RJ-45 UTP cables, how can you create straight-through, crossover and loopback cables? Describe some situations in which they are used. 

Straight-through: is a type of twisted pair copper wire for LAN (local area network) use for which the RJ-45 connectors at each. It can be used to directly connect like hubs or switches.

Crossover cables: provide a network connection between two similar devices. (pc to pc) or (switch to router). It directly connects two networks devices of the same type to each other over Ethernet. Crossover cables have the 1st and 3rd wires crossed and the 2nd and 6th wires crossed.Two types: Pc to switch, Pc to Pc

5- Multimode fiber: Uses multiple light rays or modes currently, each at a different reflection angle. Single mode fiber: Uses a single ray of light. -Single-mode fiber :uses a thinner core than multimode fiber. -Multimode fiber uses a greater bandwidth than single-mode, but a single- mode fiber is better for longer distances than multimode fiber. -Multimode fiber can work with cheaper equipment while single-mode cannot. 

6- LAN: (Local Area Network) is a computer network that interconnects in a limited area such as home, school…MAN: (Metropolitan Area Network) is a computer network that spans a city or campus. (It usually interconnects a number of LAN) (Police station) WAN: (Wide Area Network) is a computer networks that covers a broad area using private or public networks transports. (Business and government) (Internet) PAN: (Personal Area Network) is the interconnection of information technology devices within a range of 10 meters (more or less) (Bluetooth) CAN: (Campus Area Network) is a computer networks made up of an interconnection of LAN within a limited geographical area. (Universities, airports)

7- Simplex: You can only transmit in one direction. (Signal TV)

 Half-duplex: it allows alternating transmission in two directions, but not simultaneously. (Walkie-talkie) Full-duplex: when data flows in both directions at the same time. (Mobile) 2

 8- Packets in a network Each piece of information transmitted on an Ethernet network is sent in a packet. A packet is a chunk of data enclosed in one or more wrapped. Three parts:

-HEADER: it contains instruction about the data carried in a packet. -Length of packet: is used to check if the packet arrived correctly. (The address can be IP or MAC) -Packet number: is used because you cannot receive the packets in order. -Frame: second layer (MAC) -Packet: third layer (IP)

 -DATA OF BODY: the actual data that the packet is delivering to the destination. 

-CRC: is used to hash function. It’s the most common error checking used in packets

 9- OSI model (7layers) -Physical: it’s a combination of software and hardware programming and may include electromechanical devices. It transmits information. (Hubs) -Data link: it provides the means to transfer data between networks entities and provides he means to detect and correct errors occurred in the physical layer. In this layer are uses MAC address. -Network: provides the functional and procedural means of transferring variable length data sequences from a source to a destination host via one or more networks while maintaining the quality of service functions. In this layer are uses the IP address (routers) -Transport: This layer is used to fragment the information and arrange in the correct order. – Session: provides the mechanism for opening, closing and managing a session between end-user application processes. -Presentation: compression and encryption the information. -Application: an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet Protocol (IP) 

 10- SMTP: Simple Mail Transfer Protocol is used to send e-mail to a remote e-mail server.POP3: Post Office Protocol is used to download e-mail from a remote mail server. IMAP: Internet Message Access Protocol is also used to download e-mail from a remote mail server. With IMAP4, clients cannot only retrieve messages from a server, but they can also manipulate the remote message folders in which the messages are stored. FTP: File Transfer Protocol provides services for file transfer and manipulation. FTP allows multiple simultaneous connections to remote file systems. HTTP: Hypertext Transfer Protocol governs how files such as text, graphics, sound, and video are exchanged on the World Wide Web (WWW). HTTPS: provides authentication of the web site and associated web server that one is communicating with, which protects against Man-in-the-middle attacks DNS: Is vital to the Internet, providing a mechanism for resolving host names into Internet Protocol (IP) addresses. 11

 12- INTERNET PROTOCOLS :

Address Resolution Protocol (ARP): is used to map the MAC address of a node on the network when IP address is known. (Routers) RARP (Reverse Address Resolution Protocol): is a protocol by which a physical machine in a LAN can request to learn its IP address. Internet Control Message Protocol (ICMP): is used for networks testing and troubleshooting. Internet Group Management Protocol (IGMP): is a communication protocol used by host and routers on IP networks to establish multicast group membership.

 13- IP ADDRESS: is a number that is used to identify a device on the network. Each device on a network must have a unique IP address to communicate with other network devices. (32 binary bits grouped into four 8-bit bytes called octets) MAC ADDRESS: (Media access control). Is a globally unique identifier assigned to network devices and referred to as hardware or physical address. 6-byte (48-bits) in length. 

14- How and why can you ping an IP address? You can ping the IP address to test IP connectivity. Ping works by sending an ICMP echo request to other network device, and the receiving device confirms the connectivity. (Four ICMP echo request, pings, are sent to the destination computer and the destination computer responds with four ICMP echo request)

 15- Are there reserved IP addresses within a given network? Which ones? What for? Yes, the IP address are X.X.X.0 and X.X.X.255. -The X.X.X.0 is used for the networks address and is an identifier for a node or network interface. -The X.X.X.255 is the broadcast address and is a logical address at which all devices connected are enabled to receive datagrams. 

 16- Compare subnet mask classes A, B and C. How do they relate with IP addresses? The subnet mask indicates the network portion of an IP address. Usually all hosts within a LAN use the same subnet mask. -255.0.0.0: Class A, which indicates that the first octet of the IP address is the network Portion. You have until two by 24 hosts possible. -255.255.0.0: Class B, which indicates that the first two octets of the IP address are the network portion. You have until two by 16 hosts possible. -255.255.255.0: Class C, which indicates that the first three octets of the IP address are the network portion. You have until two by 8 hosts possible. This class is usually used to LAN’s.

 17- Describe how TCP communication occurs: TCP 3-Way Handshake and TCP Windowing. HANDSHAKE: Establishing a normal TCP connection requires three steps. 1. The first host (Alice) sends the second host (Bob) a “synchronize” (SYN) message with its own sequence number, which Bob receives. 2. Bob replies with a synchronize-acknowledgment (SYN-ACK) message with its own sequence number and acknowledgement number, which Alice receives. 3. Alice replies with an acknowledgment message with acknowledgement number, which Bob receives, and doesn’t need to reply to. In this setup, the synchronize messages, act as service requests from one server to the other, while SYN-ACK return to the requesting server to let it know the message was received. The client first sends a segment with its own initial sequence number, then the server responds by sending a segment with its own sequence number and the acknowledgement number, and finally the client responds by sending a segment with acknowledgement number. The reason for the client and server not using the default sequence number such as 0 for establishing connection is to protect against two incarnations of the same connection. WINDOWING: is primarily used to avoid congestion in the traffic. It controls the amount of unacknowledged data a sender can send before it gets an acknowledgment back from the receiver that it has received it. You need confirm that the transmission is reliable. 

 18- Describe the main physical topologies (e.g. bus, star, ring and mesh) according to parameters such as cost, set-up, scalability, maintenance and performance. 

BUS: a bus topology connects all the devices using a single cable. The main cable segment must end with a terminator that absorbs the signal when it reaches the end of the wire. STAR: Is made up of a central connection point that is a device such as hub, switch, or router, where all the cabling segments meet. Although a star topology costs more to implement that the bus topology, is better because each host is connected to the central device with its own cable. RING: are connected in the form of a ring or circle. Unlike the physical bus topology, the ring topology has no beginning or end that needs to be terminated. MESH: Connects all devices (nodes) to each other for redundancy and fault tolerance. The advantage is that every node is connected physically to every other node, which creates a redundant connection. If any link fails, information can flow through many other links to reach its destination.

 19- Explain how communication takes place between two computers within an Ethernet network. Remember that Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection). Most of the Ethernet uses Carrier Sense Multiple Access (CSMA)/Collision Detection (CD). CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur. The CSMA/CD algorithm works like this: a) A device with a frame to send listens until Ethernet is not busy. b) When the Ethernet is not busy, the sender begins sending the frame. c) The sender listens to make sure that no collision occurred. d) Once the sender hears the collision, they each send a jamming signal, to ensure that all stations recognize the collision. e) After the jamming is complete, each sender randomizes a timer and waits that long. f) When each timer expires, the process starts over with Step 1. Jam signal: A signal sent by a device on an Ethernet network to indicate that a collision has occurred on the network. 20- Explain how communication takes place between two computers in a Token Ring network. In a token ring network individual host are arranged in a ring. A special data token circulates around the ring. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then places the token back in the ring, where it can be passed along, or seized, by another hostuí!