Computer Networking QE
What is a transceiver?
(a small device directly attached to the tap) detects when the line is idle and drives signal when the host is transmitting.
What three layers are implemented on all network nodes ?
the Lower three layers are implemented on all network nodes including _________________ | Network | | Data link | | Physical | --------------
What part of the Ethernet frame serves as the demultiplxing key to identify the higher level protocol?
the Type field
802.11 addresses the exposed node problem and the hidden nodes problem by doing what?
The Multiple Access with Collision Avoidance algorithm
Any node that sees the RTS frame but not the CTS frame.
-is not close enough to the receiver to interfere with it, and so is to transmit.
Key idea of MACA
1) Sender and receiver exchange control frames with each other before the sender actually transmits any data. 2) This exchange informs all nearby nodes that a transmission is about to begin 3) Sender transmits a Request to Send (RTS) frame to the receiver. 4) Receiver replies with a Clear to Send (CTS) frame.
Max number of host that an Ethernet can connect ?
1024
Each frame contains up to how many bytes of data ?
1500 bytes
How many bits is the type field ?
16 bits
Mbps to bps
1e^-6 (0.000001)
How many bits is the CRC in an Ethernet frame ?
32
How many bits are both the dest addr and the src addr ?
48 bits
CSMA/CD (Carrier Sense, Multiple Access with Collision Detect )
A functionality of network hardware. "Carrier sense multiple access" means that multiple stations can listen to the link and detect when it is in use or idle; collision detect" indicates that, if two or more stations are transmitting on the link simultaneously, they will detect the collision of their signals. Ethernet is the best-known technology that uses CSMA/CD.
If two host is on the complete opposites of the Ethernet and has collided, what's the least amount of bits needed to send?
512 bits (64 bytes)
Ethernet Frame Format
64 48 4 8 16 32 Preamble | Dest addr |src addr | Type | Body | CRC
How many bits is the Preamble ?
64 bits
What is 802.11 suitable for ?
802.11 is suitable for an ad-hoc configuration of nodes that may or may not be able to communicate with all other nodes.
SONET (Synchronous Optical Network)
A clock-based framing standard for digital transmission over optical fiber. It defines how telephone companies transmit data over optical networks.
What is a broadcast address?
A computer network is transmitting a message which does not require a response to all users of the network.
What is the reason for the minimum frame size for Ethernet frames ?
A frame must contain at least 46 bytes of data even if it means the host has to pad the frame before transmitting it so that it must be long enough to detect a collision.
in an 802.11 distributed system, what plays the same role as a base station ?
APIs
What does AP stand for
Access point
CRC (cyclic redundancy check)
An error-detecting code computed over bytes composing a packet and then appended to the packet by the network hardware (e.g., Ethernet adapter). Provides stronger error detection than a simple checksum.
Congestion control
Any network resource management strategy that has , as its goal, the alleviation or avoidance of congestion. A congestion-control mechanism may be implemented on the routers (switches) inside the network, by the hosts at the edges of the network, or by a combination of both.
Example of a Layered network system
Application programs Process-to-process channels Host-to-host connectivity Hardware
The general strategy of using acknowledgments and timeouts to implement reliable delivery is sometimes called ?
Automatic Repeat Request (ARQ)
an Ethernet address consisting of all 1s is treated as a what?
Broadcast address
How does the stop-and-wait prototypical prevent the receiver from thinking a repeat frame is a new frame ?
By including a 1-bit sequence number, that is the sequence number can take on the values 0 and 1 and the sequence numbers used for each frame alternate.
What does the Data Link Layer do ?
Collects a stream of bits into a larger aggregate called a frame. Network adapter along with device driver in OS implement protocol in this layer Frames are actually delivered to hosts Frames are actually delivered to hosts
What does the Presentation Layer do?
Concerned about the format of data exchanged between peers.
TCP (Transmission Control Protocol)
Connection-oriented transport protocol of the internet architecture. TCP provides a reliable, byte-stream delivery service.
The OS Socket interface defines operations for what functions?
Creating a socket Attaching a socket to the network Sending and receiving messages through the socket Closing the socket
When will an adapter send only 96 bits and what is it called ?
Its called a runt frame, and it sends if the two hosts are close to each other. had the two host have been farther apart, they would have had to transmit longer and thus send more bits, before detecting the collision.
What formula does the receiver always hold in the sliding window algorithm?
LAF - LFR ≤ RWS
ms to sec
Divide by 1000
How is it ensured that every adapter gets a unique address,?
Each manufacturer of Ethernet devices is allocated a different prefix that must be prepended to the address on every adapter
Placing binary data on a signal is called what ?
Encoding
Draw the TCP/IP Architecture
FTP HTTP NV TFTP \/ \ / TCP UDP \ / IP Net1 Net2 ...Netn
What does the Network Layer do?
Handles routing among nodes within a packet-switched network Unit of data exchanged between nodes in this layer is called a packet
What does the Physical layer do?
Handles the transmission of raw bits over a communication link
802.11
IEEE wireless network standard.
What is the focal point of the internet architecture ?
IP
When an adapter has a frame to send and the line is busy what does it do?
It waits for the line to go idle, and then transmits immediately.
What does the Transport Layer do?
Implements a process-toprocess channel unit of data exchanges in this layer is called a message
What is flow control ?
In data communications, flow control is the process of managing the rate of data transmission between two nodes to prevent a fast sender from overwhelming a slow receiver.
What is the main shortcoming of the stop-and-wait algorithm?
It allows the sender to have only one outstanding frame on the link at a time, and this may be far below the links capacity.
Why is layering important?
It decomposes the problem of building a network into more manageable components.
What happens when an adaptor detects that its frame is colliding with another ?
It first makes sure to transmit a 32-bit jamming sequence and then stops the transmission. Thus a transmitter will minimally send 96 bits in the case of a collision: 64-bit preamble plus 32-bit jamming sequence.
In the transmitter Algorithm, when the adapter has a frame to send and the line is idle what does it do?
It transmits the frame immediately; there is no negotiation with the other adapters.
With the Sliding Window Algorithm, what is set when the receiver acknowledges the received SeqNumToAck, even if the packet with higher number has been taken?
LFR = SeqNumToAck & LAF = LFR + RWS
What formula does the sender always hold in the sliding window algorithm?
LFS - LAR ≤ SWS
what is LFS in the sliding window algorithm ?
Last frame received
The algorithm that controls access to a shared Ethernet link.This algorithm is commonly called the Ethernet's what ?
Media Access Control (MAC)
In regards to IEEE 802.11 distribution system all APs within reach reply with a what ?
Probe response frame
Latency = ?
Propagation + transmit + queue
ARP (Address Resolution Protocol)
Protocol of the Internet architecture, used to translate high-level protocol addresses into physical hardware addresses. Commonly used on the internet to map IP addresses into Ethernet addresses.
What does the Session Layer do?
Provides a name space that is used to tie together the potentially different transport streams that are part of a single application.
What dominates when there is infinite bandwidth ?
RTT
What does the reeving adapter do with the Preamble and CRC?
Removes them.
When do we consider that the frame is not in receiver's window and discard the frame?
SeqNum ≤ LFR or SeqNum ≥ LAF
Protocols define what two different interfaces ?
Service Interface & Peer Interface
What does the Application Layer do?
Standardize common type of exchanges.
Terabyte to bits
Terabyte (1024) Gigabyte (1024) Megabyte (1024) Kilobyte (1024) Byte (1024) bits (8)
What does the upper bound of 1500 bytes in the message mean for the adapter in the transmitter algorithm ?
That the adapter can occupy the line for only a fixed length of time.
What is one of the most important observations people have made about mulit-access Ethernets ?
That they work best under lightly loaded conditions. Heavy loads result in too much of the network's capacity being wasted by collisions.(anything over ~30%)
a-bit oriented framing protocol
The Ethernet
What are protocols?
The abstract objects that make up the layers of a network system. They provided a communication service that higher-level objects use to exchange messages.
in an 802.11 distributed system, what layer does the distribution network run at ?
The distribution network runs at layer 2 of the ISO architecture
An address that has the first bit set to 1 but the broadcast address is called a what?
multicast address
what is SeqNumToAck ?
The largest sequence number which is still not acknowledged, such that all frame with SeqNum ≤ SeqNumToAck has been accepted.
What is a socket ?
The point where a local application process attaches to the network.
What does the sending adapter attach to the frame before transmitting?
The preamble and CRC
What transmission algorithm would you use if you would like the sender to be ready to transmit the ninth frame at pretty much the same moment that the ACK for the first frame arrives?
The sliding window
What is scanning in regards to IEEE 802.11 distribution system
The technique for selecting an AP (access point)
Where is the transceiver located ?
The transceiver is connected to an Ethernet adapter which is plugged into the host.
What layers typically run only on end-hosts?
The transport layer and higher typically run only on end-hosts and non on the intermediate switches and routers.
Exponential Backoff
This strategy of doubling the delay interval between each re-transmission attempt.
What is the Basic Idea of Error Detection?
To add redundant information to a frame that can be used to determine if errors have been introduced
Why are collisions possible with Ethernet transmitter algorithm ?
There is no centralized control so it is possible for two (or more) adaptors to begin transmitting at the same time, either because both found the line to be idle or because both had been waiting for a busy line to become idle.
What is the formula for Transfer Time
Transfer Time = (A)mount of data needed to transfer/(S)peed at which we can transfer
UDP (User Datagram Protocol)
Transport protocol of the Internet architecture that provides a connection-less datagram service to application-level processes.
The relative importance of bandwidth and latency depends on the application, for large file transfer what is critical?
bandwidth
what is important with Large bytes transmission?
bandwidth is important
What are frames?
blocks of data
Propagation = ?
distance/speed of light
We think of the channel between a pair of processes as a what ?
hollow pipe
When does the receiver window accept the frame in sliding window algorithm ?
if the following conditions are NOT met: SeqNum ≤ LFR or SeqNum ≥ LAF
what is a runt frame?
is an Ethernet frame that is less than the IEEE 802.3's minimum length of 64 octets.
What is a unicast address?
is an address that identifies a unique node on a network
Any node that sees the CTS frame knows what ?
it is close to the receiver, therefore: - Cannot transmit for the period of time it takes to send a frame of the specified length.
what is LAR in the sliding window algorithm ?
last acknowledgment received
The relative importance of bandwidth and latency depends on the application, for small messages (HTTP, NFS, etc.) what is critical?
latency
Formula for approximate number of packets transferred ?
packets_number = Bandwidth x round trip delay
what is important with One bit transmission?
propagation
Transmit = ?
size/bandwidth
What is the exposed node problem?
when although two nodes are exposed to each other's signals, there is no interference if B transmits to A while C transmits to D. ( A and D's reaches are not shown.)
Define the hidden nodes problem in regards to IEEE 802.11 collision avoidance.
when two nodes are unaware of each other since their signals do not carry that far. These two frames collide with each other at B, but unlike an Ethernet, neither A nor C is aware of this collision. The "Hidden Node" Problem. Although A and C are hidden from each other, their signals can collide at B.
What is the 802.11 frame format?
|Control (16) | Duration (16) | Addr1 (48) | Addr2 (48) | Addr3 (48) | SeqCtrl (16) | Addr4 (48) | Payload (0-18,496 | CRC (32) | (BITS)
