COMPUTER NETWORKS Flashcards
(114 cards)
Protocols
TCP/IP
General Network (WAN or LAN)
nodes, aka switches) inside the subnet, connected by
communication links;
(external devices, e.g., computers, servers, terminals, and so
on) outside the subnet, connected via the subnet
Messages
originated at these external devices,
pass into the subnet,
from node to node on the links,
to the external recipient.
: a string of bits
Topology
placement of links between
nodes
LAN
Local Area Networks
on the order of a square kilometer or less.
restricted topology – nodes distributed on a bus, a ring, or a
star
MAN
Metropolitan Area Networks)
WAN
more than a metropolitan,
comparatively arbitrary topology.
Packet
transmitting messages long in length can be harmful – delay,
congestion, complex buffer management…,
solution: messages broken into shorter strings, i.e., packets,
packets transmitted through the subnet as individual entities,
and reassembled into messages at the destination
Session
a sequence of messages. e.g.:
Interactive: short messages, small delay, high reliability,
File: long messages, moderate delays
Shared Media Broadcast Network
defn: terminals share a common channels (for example,
Ethernet, Token Ring);
all will receive the sender’s message, but only the receiver is
interested;
cheap, limited in scalability
Switched Point-to-point Network
defn: information travels over several points from one terminal
to the other;
circuit switched - separate channel for session, e.g., telephone.
packet switched/store-and-forward - several sessions share
channel
Circuit Switching
Packet Switching
virtual circuit routing (generally used in practise, connection
oriented)
a fixed path,
but it is virtual - link capacity shared by sessions, and link use
based on demand.
dynamic routing (connectionless)
packets may be missing,
packets might arrive destination in different order.
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Circuit Switching
assign a rate rs to the session s,
create a path
allocate/reserve rs on each link of the path
condition: sum of all rates cannot exceed the total
transmission capacity of the link
if no such path can be found, the session is rejected,
once a session is setup, it has guaranteed transmission rate rs
through the network
example: telephone network
Packet Switching concepts
circuit Switching: Inefficient Use of Links
Typical data sessions tend to have short bursts of high activity
followed by lengthy inactive periods;
but circuit switching wastes the allocated rate during these
inactive periods.
A quantitative view:
let λ be the message arrival rate for a given session. (e.g.,
λ = 3 messages/min)
1/λ, the expected interarrival time between messages (1/3
minutes, 20 sec.)
̄X , expected transmission time of a message (4 sec).
Delays
Queueing Delay: the time the packet is assigned to a queue
for transmission and the time it starts being transmitted.
During this time, the packet waits while other packets in the
transmission queue are transmitted.
Propagation Delay: the time between the last bit is
transmitted at the head node of the link and the time the last
bit is received at the tail node. This is proportional to the
physical distance between transmitter and receiver
Packet Switching: Control of Queueing Delay
packets queued are from many different sites, when delay
excessive (or buffer full) need to slow down.
hard to control,
largely nonexistent in circuit switching
Circuit Switching vs. Packet Switching
Complexity of Networks
Many issues to address
synchronization, encoding, addressing, error control, ordering
messages, flow control, message segmentation, framing,
routing, scheduling, multiplexing, security, billing, compression,
code conversion.
useful method of dealing with complexity is through the use of
functional modularity.
break complex problem into simple sub-problems,
use “black box” abstraction of sub-problems.
example:
computer: processor, memory, bus, …
processor: control unit, arithmetic unit, I/O Unit …
arithmetic unit: adders, accumulators
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Layered Network Architectures
Hierarchical Layering, a type of functional modularity useful in
communication networks
Peers of Peer Processe
members of the same layer at
different locations
Protocol
set of rules for how peers interac
Service
function performed by layer N for layer N+1 across
an interface
