CS 438 - Computer Networks - Xiao - Ch1 - Exam 1

Computer Network

interconnected collection of autonomous computers via wired media (copper, fiber etc) or wireless (microwave, radio, etc).

Distributed System (DS) vs Computer Network (CN)

DS: transparent to users, higher layer over CN

CN: explicit

Client-Server Model

client explicity requests information from server that hosts that information

Peer-to-peer system

no fixed clients and servers

Types of computer networks

broadband (mobile) access networks, data-center networks, transit networks, enterprise networks

Metcalfe's Law

the value of a network is proportional to the square of the number of connected users in the network

CDN (Content Delivery Network)

collection of servers geographically distributed so content is as close as possible to the users requesting it

Transit Network

carries traffic between content provider and the ISP when not directly connected. Also called backbone networks

VPNs

connect individual networks at different physical sites into one logical network

Broadcasting vs Multicasting vs Unicasting

Unicasting: one to one

Broadcasting: one to all

Multicasting: one to a group/subset

Taxonomy by Scale

PAN < LAN < MAN < WAN < Internet

Personal, local, metropolitan, country, global

Internetwork

Network of interconnected networks. Combines subnets and hosts.

ARPANET

research network sponsored by DoD. Eventually became the Internet.

Why the decline trend in transit networks?

Consolidation of many CDNs, the likes of Amazon, Cloudfare, Limelight etc. (pg. 69 Ch1 notes)

802.16

WiMAX

LTE

Long Term Evolution

5G

up to 10 Gbps. Faster speeds.

4G LTE parts

E-UTRAN, EPC, UMTS,

Broadcast Link

Single channel shared by all the machines. Packets sent by one are received by all. Upon getting a packet, machine checks if it is for itself. Ex analogy) airport announcement

IXP

Internet eXchange Points

E-UTRAN

Evolved UMTS Terrestrial Radio Access Network. Protocol used over the air between mobile device and cellular base station (eNodeB)

UMTS

Universial Mobile Telecomm System. Formal name for cell phone network. Air interface is based on CDMA. 4GLTE

RNC

Radio Network Controller. Controls how the spectrum is used.

Core network

carries traffic for the radio access network and is packed switched, called EPC (evolved packet core).

S-GW

Serving Network Gateway

PGW

Packet Data Network Gateway

802.11 network modes

Ad hoc, access point (AP)

WEP

Wired equivalent privacy

WPA

replaced WEP. Wifi Protected Access

WPA2

replaced WPA. Uses 802.1x

CSMA

Carrie Sense Multiple Access, a method used by Ethernet networks to allowed shared usage of a baseband network, and avoid collisions

Layer n protocol

Layer n in one machine carries on a conversation with layer n on another machine.

Protocol stack

a list of protocols used by a certain system, one protocol per layer

Connection-Oriented vs Connectionless Services

CO requieres a connection to be negotiated and established before communication. Similar to making a phone call. TCP.

Connectionless sends data without dedicated connection. UDP. ICMP.

Six core service primitives

Listen, Connect, Accept, Receive, Send, Disconnect

Services and Protocols Relationship

Services are what, protocol is how. Similar to an abstract data structure in CS, where methods are available but the implementation is handled behind the scenes (ie protocol).

Layer 1

Physical

Layer 2

Data Link

Layer 3

Network

Layer 4

Transport

Layer 5

Session

Layer 6

Presentation

Layer 7

Application

Layer 1 unit exchanged

bit

Layer 2 unit exchanged

data frame

Layer 3 unit exchanged

Packet

Layer 4 unit exchanged

TPDU

Layer 5 unit exchanged

SPDU

Layer 6 unit exchanged

PPDU

Layer 7 unit exchanged

APDU

Three central concepts of OSI model

Services, Interfaces, Protocols

Physical Layer

transmits raw bits

Data Link Layer

break message into data frames (hundreds to thousands bytes) and transmit them sequentially. Error control (ACK), flow control (traffic reg and buffer size).

Network Layer

controls operation of subnet, routing, load balance, and addressing.

Transport Layer

Slit data into smaller units if needed, pass these to network layer, ensure all pieces arrive correctly at other end.

Session Layer

Allows users on different machines to establish sessions between them

Presentation Layer

syntax and semantics of information transmitted

Application layer

HTTP, WWW, FTP, Email etc.

TCP/IP reference model

Application, Transport, Internet (Network), Data Link

Problems with TCP/IP reference model

service, interface, and protocol not distinguished. Model is not at all generalizable to other protocol stacks.

Model used in Book

Application 5, Transport Layer 4, Network Layer 3, Data Link 2, Physical 1