Chapter questions 12 -17
consider a packet sent on a bridged LAN to a non-existent address. Across how many segments will bridges forward the packet?
All of them, bridges are cut through tech.
What type of multiplexing does ADSL use?
Asymmetric Digital Subscriber Line (ADSL) uses a simple form of Frequency Division Multiplexing
why would a provider choose Hybrid Fiber Coax instead of Fiber To The Premises?
FFTP (to premises) has FFTB and FFTH (to business and to home). Hybrid Fiber Coax has fiber from provider to the curb, then coax from the curb to the homes. It is cheaper.
What are SIFS and DIFS and why are they needed
SIFS - short inter-frame space: between CTS and RTS messages. DIFS - distributed inter-frame space: before sending a RTS are needed because of the CSMA/CA that 802.11 uses to avoid the hidden station problem. CSMA/CA requires a pair to exchange RTS (ready to send) and CTS(clear to send) messages before transmitting.
Broadcasting a copy of a video presentation, is a circuit or packet switching system preferable? Why?
Packet, circuit requires exactly two end points making broadcasting impossible; packet switching doesn't need to notify the underlying system when communication ends. Broadcast IEEE addressing specifies that each computer on the network should receive a packet.
Why does CSMA/CD use a random delay?
So the ethernet can recover quickly after a collision because each computer agrees to wait longer and the contention for the cable will be reduced after a few collisions (thanks to binary exponential back-off)
Given an IEEE MAC address, how can one tell if the address refers to unicast?
The least significant bit of the most significant byte is a multicast bit that specifies whether the address is unicast (0) or multicast (1). The next bit specifies whether it is globally unique (0) or locally assigned (1). Globally unique addresses are assigned by the IEEE
if two computers are connected on a bridged network, are changes required in addressing or in applications? explain
There are no changes needed as the device can't tell the difference between a LAN segment and a bridged LAN, and bridges are adaptive/learning because they learn locations of computers automatically.
Why is a splitter used with DSL
To distinguish between telephone and internet lines [so they can be used separately ]
What is a point-to-point network?
communication the connects between exactly two entities. [ opposed to a LAN that is multi-access]
How large is the maximum ethernet frame, including the CRC?
header + payload + CRC : Header: 6 byte destination addr+ 6 byte source addr + 2 byte type Payload: 46-1500 bytes CRC: 4-byte cyclic redundancy Check
give a precise statement of conditions under which an adaptive bridge will forward a packet
it uses the destination MAC address to determine whether to forward it, but if two messages are sent to the same device at the same time, both will be discarded.
In a packet switching system, how does a sender transfer a large file?
its divided into smaller blocks of data known as packets (usually 1500 bytes)
Figure 12.11
name: bit rate: voice circuits:where basic rate: .064Mbps : 1: N/A T1: 1.544 : 24 : North America T2: 6.312 : 96 : North America T3: 44.736 : 672 : North America E1: 2.048 : 30: Europe E2: 8.448 : 120 : Europe E3: 34.368: 480 : Europe [All Mbps]
Give Examples of Narrowband and Broadband technologies
narrow - up to 128Kbps aka dialup telephone connections, leased circuit using modems. broadband - anything with higher data rates than dialup aka DSL or Cable modem tech.
does it make sense to use IrDaA for applications such as file transfer? why?
no, they qualified as low-speed. They only allow short distance communication
Figure 12.12
page 213
Figure 15.8
page 261
Explain polling and the two general polling policies
polling uses a central controller which cycles through stations on a network and gives each an opportunity to transmit a packet. [there are both round robin and priority polling orders]
mesh topology: formula for the number of connections in a mesh network. How many connections are needed for 20 computers?
((n^2) -n) /2
in a reservation system, how does a controller form a list of stations that will transmit in a given round
(often used for satellite) - each potential sender specifies whether they have a packet to send, and the controller transmits a list of the stations that will be transmitting. The stations use the list to know when to send.
which ethernet designation is what we consider "gigabit ethernet"? (fig 15.8)
1000BaseT - page 261
In an 802.3 ethernet frame, what is the maximum payload size?
1492 bytes. figure 15.3 on 255
Name the four generations of cellular technology and describe each
1G: analog signals carry voice 2G/2.5G: digital signals to carry voice 3G/3.5G: higher-speed data services, 40Kbps to 2Mbps for web and photo sharing. Roaming across North America, Japan and Europe 4G: support for real-time multimedia like a tv program or high-speed video download utilizing both Wi-Fi and satellite at anytime choosing the better connection automatically
what are the 3rd generation of cellular technologies that use code division multiplexing
1xRTT, EVDO and EVDV combined CDMA and FDM to increase performance
Give the numeric prefix IEEE standards use for Wi-Fi networks
802.11
what are the three blocks of frequencies used by wireless LANs and PANs
902MHz to 928Mhz - 26MHz bandwidth 2.4GHz to 2.484GHz - 83.6 MHz bandwidth 5.725GHz to 5.850GHz - 125 MHz bandwidth
Two neighbors who live on the same street both use ADSL service, but measurements show that one can download at 1.5Mbps, and the other is at 2.0 Mbps. Why?
ADSL does not offer fixed data rates, only relative.
Explain CSMA/CD
Carrier Sense - each station monitors the cable to detect whether another transmission was already in progress. (improves network utilization) (Multi-Access with) Collision Detection: if the signal on the cable differs from the signal a station is sending, it means there is a collision. [ binary exponential back-off used]
Why is CSMA/CA needed in a wireless network?
Carrier sense multi-access with collision avoidance is needed in wireless lans due to "hidden stations" - figure 14.7 pg.249 - The idea is that if both the sender and the receiver transmit a message, all computers with in range of either will know a packet transmission is beginning and to wait.
list three spread spectrum techniques and give their general descriptions
DSSS - direct sequence spread spectrum FHSS - frequency hopping spread spectrum OFDM - orthogonal frequency division multiplexing see figure 16.5 for descriptions
Why has the use of ISDN declined?
It is narrowband technology. [Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network.]
Name the two sublayers of layer 2 protocols defined by IEEE, and give the purpose of each
LLC (logical link control) - addressing and demultiplexing. MAC (media access control) - access to shared media.
Why is byte stuffing needed?
Needed to transmit data outside of the SOH (201 start of header) and EOT(204 end of transmission) hexadecimal values without confusion. [Byte stuffing allows the control information to be distinguished from any data that might include bytes. The sender replaces each control byte with a sequence, and the receiver returns it to the original values]
in a circuit-switched network, can multiple circuits share a single fiber? Explain
Yes they can, they are multiplexed over shared media and the result is called a virtual circuit.
If a tv provides a wired extension for a remote infrared sensor, what technology is likely used?
a repeater: an analog hardware device used to extend a LAN, it amplifies and sends all incoming signals to the other side
According to figure 17.6 can two computers attached to a switched LAN transmit packets simultaneously? explain
a switch with N ports and N computers connected can do (N/2) transfers at the same time. 17.6 has 4 devices / ports so yes two can transmit at a time
why do most wireless LANs use an infrastructure approach rather than an ad hoc approach
ad hoc- wireless hosts communicate among themselves without a base station. infrastructure - hosts only communicate with an access point, and the access point relays all packets. Most uses the second because they can deploy several access points connected to a switch for better range.
Where is a head-end modem located?
at the internet service provider's site
Where is a tail-end modem located?
at the subscriber's location
how many satellites are used in GPS and how accurate is a GPS system
between 2 and 20 meter accuracy (military versions have higher accuracy) with 24 total satellites arranged in size orbital planes
explain the three basic approaches used to arbitrate access to a shared medium
random access - [reservation, polling, token passing] control access - [ALOHA, CSMA/CD, CSMA/CA] channelization - [FDMA/TDMA/CDMA].
why is a satellite dish shaped like a parabola
reflect incoming electromagnetic wavs back into the receiver's focus point
In the aloha protocol, what happens if two stations attempt simultaneous transmission on the inbound frequency, and how is that problem handled?
requires each sender to retransmit each lost packet after a randomized delay so repeated collision is less likely.
What are the four basic LAN topologies?
ring, bus, star, mesh
How can a receiver tell whether an Ethernet frame uses the 802.3 standards?
the extra header in figure 15.3 shows that the 802.3 standards interpret the original type field as a packet length instead - adding an extra 8-byte header to contain the packet type information. The overall frame size stays the same (1514 bytes) but the payload is reduced to 1492 instead of 1500 to distinguish between the two.
How did each computer attach to a thicknet ethernet?
the thicknet used a heavy coaxial cable (known as 10base5) that was divided into a Network interface card to hand digital aspects of communication and a transceiver that connected the ethernet cable/ handled carrier detection, conversion of bits into appropriate voltages for transmission, and conversion of incoming signals into bits. An AUI (attachment unit interface) was needed to connect a transceiver to a NIC in a computer. [thicknet -> transceiver > AUI cable > computer's NIC]
how does a computer attached to a shared LAN decide whether to accept a packet?
they all get a copy of the packet - check its metadata header for its MAC address and decide to ignore or accept it from there.
why must a wireless computer associate with a specific base station
to handle overlap between access points where a computer might be in range for multiple. Otherwise more expensive/complex access points coordinate to ensure smooth handoffs.
What is a token and how are tokens used to control network access
token is a control message shared between computers, if they have the token they can send - then they pass the token to the next computer in the assigned sequence
what is a VSAT satellite
very small aperture terminal: a dish that is less than 3 meters in diameter, and an antenna less than one meter in diameter
can the wires of a ring network be arranged in a straight line? (like down a hallway)
yes; with the last pathing back to the first.
