Chapter 1: Computer Networks and the Internet Flashcards
A Nuts-and-Bolts description of the internet
The Internet is a computer network that interconnects billions of computing devices throughout the world.
Internet Service Provider (ISP)
Provide internet access
Types of Host/End System
- Two types
- Client
- Server/Data centers
Server/Data centers
More powerful machines that store and distribute web pages
Communication link
Different links transmit at different transmission rate (bits per second, bps)
Packet
- Host divide data into segments, add header to each segment to generate a packet
- Destination reassemble packets into data
Packet switch
Receive and forward packets towards their destinations
Types of switch
- Router
- Link-layer switch
Router
Used in core networks
Link-layer switch
Used in access network
Route / path
End-to-end connections from hosts to destinations
- Comprised of links and switches
A service description of Internet
An infrastructure that provides services to applications
Protocol
A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.
Access Network
Connect hosts to edge router
Edge route
the first router in the global or regional ISPs
Two Types of the network core
- Packet switching
- Circuit switching
Requirement on end-to-end resource reservation(Packet switching)
No.
A source host wants to communicate with a destination host.
- It use resource from source host to destination host in an on demand manner.
- ex. Internet.
Requirement on end-to-end resource reservation(Circuit switching)
Yes.
A source host wants to communicate with a destination host
- It must reserve resources along a path (called circuit or dedicated end-to-end connection) from source host to destination host
- The resource must be reserved for the entire duration of the communication session.
- ex. Telephone networks
Main concept of packet switching
Store-and-forward transmission
- Packet switch mush receive (store) all bits of a packet, then only it can transmit (forward) the first bit of the packet.
Network performance of packet switching
- Queuing delay
- Packet loss
Routing protocol
Each router determine the shortest path to each destination and use the shortest path to configure its forwarding table.
Forwarding table
- Each host has IP address.
- Each router use forwarding table to map a destination IP address to one of its outgoing link.
Two types of circuit switching
- Frequency-Division Multiplexing (FDM)
- Time-Division Multiplexing (TDM)
Frequency-Division Multiplexing (FDM)
A link dedicate a frequency band to each connection
- The width of the frequency band indicate bandwidth
- ex. 4 kHz
Time-Division Multiplexing (TDM)
A link dedicate one time slot in every frame to each connection
- Time is divided into fixed duration frames.
- Each frame is divided into a fixed number of time slots.
Performance efficiency of packet switching
Higher
- No reservation
- More sharing of link capacity
Performance efficiency of circuit switching
Lower
- Required reservation
- Reserved resources may not be fully utilized
- Reserved resources may not be sufficient
- Underutilized reserved resources cannot be used for other packets
Do the packets in packet switching need to wait at queue?
Yes
- There is variable and unpredictable delay
- Not suitable for real-time service
Do packets in circuit switching need to wait at queue?
No
- Reserved resource provides guaranteed constant rate
Complexity of packet switching
Lower
Complexity of circuit switching
Higher
- Reservation requires end-to-end signaling protocol
Cost efficiency of packet switching
Higher
- No reservation
- Less cost involve
Cost efficiency of circuit switching
Lower
Popularity of packet switching
More popular
Popularity of circuit switching
Less popular
Formula of Node delay
Node delay = Processing delay + Queuing delay + Transmission delay + Propagation delay
d(node) = d(proc) + d(queue) + d(trans) + d(prop)
Processing delay
- Read packet header
- Determine the outgoing link
- Check bit errors
- Value: μs
Queuing delay
- Wait to be transmitted
- Value: ms
Transmission delay
- Push all bits of a packet into the link
- d(trans) = L/R
- Value: ms
Propagation delay
- Travel from one router to another across a link
- Speed depends on
- Propagation speed of physical media, s
- Distance between the routers, d
- d(prop) = d/s
s : 3 x 10^8 meters/sec
Traffic Intensity
- Ratio of bits arrival rate (bits/second) to transmission rate
- Traffic intensity = La/R
- L = packet length (bits)
- a = Packet arrival rate (packets/second)
- R = Transmission rate (bits/second)
When La/R(Traffic Intensity) ~ 0
queue size decress, and so queuing delay approaches zero
When La/R(Traffic Intensity) -> 1
queue size increases without bound, and so queuing delay approaches infinity
How queuing delay increase?
Queuing delay increases exponentially
- small increment in traffic intensity provide large increment in queuing delay
Packet loss
Packet arrive at a full queue will cause packet drop
- Lost packet may be retransmitted
Throughput
- Rate (bits/seconds) at which bits are transferred between source host and destination host
Types of throughput
- Instantaneous throughput
- Average throughput
Instantaneous throughput
Rate at a given point in time
Average throughput
Rate over a longer period of time
Bottleneck link
- Link on end-to-end path that constraints end-to-end throughput
Bottleneck bandwidth
- min { R(c), R(s) }
- Bottleneck bandwidth is transmission rate of client or servers for which is lower.
What is the end-to-end delay in bottleneck link?
F / min{ R(c), R(s) },
where F is file size.
Types of Protocol stack
- Internet Protocol Stack
- ISO Open System Interconnection (OSI) Reference Model
Each layer of protocol layers
- Perform certain actions
- Use services of the layer directly below it
Layering of Protocol layers
- Divide a complex system into layers
Advantage of protocol layers
Can maintain and update each layer without affecting the entire system
Disadvantage of protocol layers
Similar function in more then one layer
-eg. error recovery in link layer and network layer
Packet of Application layer
Message
Packet of Transport layer
Segment
Packet of Network layer
Datagram
Packet is Link layer (Ethernet, WiFi)
Frame
Packet is Physical layer
Bit
Function of Application layer
- Support network applications
- Provide Domain Name System (DNS)
- Translate address (e.g., www.ietf.org to 32-bit address)
- e.g.: HTTP, email
Function of Transport layer
- Break a long message into shorter segment
- Reduce source host transmission rate during congestion
- e.g.: HTTP, email
Function of Network layer
- Determine routers between source host and destination host
- e.g.: IP Protocol
Function of Link layer
- Transmit frame from a transmitting host to receiving host over one link
- e.g.: Ethernet, WiFi
Function of Physical layer
- Transmit bits on physical media (e.g., wireless, fiber optic)
Packet switch
- Link-layer switch
- Router
Link-layer switch
- Used in access networks
- Consist layer 1 and 2
Router
- Used in core networks
- Consist layer 1, 2 and 3
Encapsulation
At each layer, a packet consist
- Header
- Payload (a packet from the upper layer)
Segment consists
Segment = Message + Header of Transport Layer
Datagram consists
Datagram = Segment + Header of Network Layer
Frame consists
Frame = Datagram + Header of Link Layer
