Computer Network - Network Overview

Features

Interconnection - No Master-Slave Relationship

Autonomy - Interconnectivity

Exchange Network interconnects hosts via Switching Nodes to complete data communication.

Network Protocols

Network Protocol, is a rule, standard, or convention established for data exchange in progress.

Three Elements

Syntax: Protocol structure or format

Semantics: What control information needs to be sent, error control

Timing: Event sequence, speed matching

Authoritative Internet Protocol RFC

Network Edge Access Core Network

Access Network Three Elements:

• Computing devices accessing the network
• Communication links
• Packet switching (Routers, Switches)

Access Network Methods:

• Residential
• Institutional Access Network
• Mobile Access Network

Access Method	Simple Understanding	Main Features
Fiber Access (FTTx, e.g., FTTH Fiber To The Home)	Fiber directly to community/building/home	Large bandwidth, low latency, many home broadband services are now FTTH
Cable Modem	Use cable TV coaxial cable to access internet	Once popular, faster than ordinary telephone lines, now gradually replaced by fiber
DSL (Digital Subscriber Line) Access (ADSL/VDSL)	Phone line can also access internet (ADSL Modem)	Once common, now gradually eliminated (slow speed, poor upload)
Ethernet Access	Plug in network cable at home/company (RJ45 interface)	Highest popularity, low cost, fast speed (now generally 100M/Gigabit)
LAN (Local Area Network) Access	Internal network, school/company uses switches/routers to connect	Local internal high-speed connection, usually combined with Ethernet

Network Core Interconnection

• Routing: Routing Algorithm
• Forwarding: Switch packet from router's input port to correct output port

ISP (Internet Service Provider) Interconnection

Peering Link: Uses BGP protocol, BGP stands for Border Gateway Protocol, a protocol for exchanging routing information between different Autonomous Systems (AS).

Peering Link is not used within the same AS

Upstream Link (Transit Link)

IXP (Internet Exchange Point)

                   🌍 International Internet Backbone Network (Tier 1)
        ┌────────────────┬────────────────┐
        │                │                │
┌───────▼───────┐  ┌─────▼─────┐  ┌───────▼───────┐
│  Level 3 (US) │◄►│  NTT (JP) │◄►│  Telia (EU)   │
86: └───────────────┘  └───────────┘  └───────────────┘
87: 		▲                ▲                ▲
88: 		│ Private Peering │ Private Peering│
89: 		▼                ▼                ▼
90: ┌────────────────────────────────────┐
91: │     National/Regional Backbone (e.g. China Telecom)    │
92: └─────────────────┬──────────────────┘
93: 									│
94: 							┌────▼────┐
95: 							│   IXP   │ ← Domestic IXP: CHN-IX, HKIX etc.
96: 							└────┬────┘
97: 		┌───────────────┼───────────────┐
98: 		│               │               │
99: 		┌─────▼─────┐   ┌─────▼─────┐   ┌─────▼─────┐
100: 		│   ISP A   │◄──►   CDN     │◄──► Content Platform   │
101: 		└───────────┘  Peering Links └──────────────┘
102: 		│
103: 		┌─────▼─────┐
104: 		│  User PC  │
105: 		└───────────┘

Large Interconnected Networks:

Tier-1 ISP (Unicom, Telecom)

Content Provider Networks (Google: Private network, its data centers connect to Internet, usually bypassing Tier-1 ISPs and regional ISPs)

Small operators access global network via IP Transit to access the entire Internet.

Dynamic Switching

Multiplexing

• FDM Frequency Division Multiplexing
• TDM Time Division Multiplexing
• WDM (Actually FDM) Wavelength Division Multiplexing
• CDM Code Division Multiplexing Encoded Signal = Original Data X Chip Sequence

1. Circuit Switching

• Establish Connection (Call/Circuit Setup)
• Communication
• Release Connection (Teardown Circuit)

Exclusive Resources (Multiplexing)

One fiber is greater than resources needed for one phone call

2. Message Switching

Must send the entire packet, too slow, also occupies switch memory

3. Packet Switching (Mainstream)

Allow more users to use the network simultaneously;

Store-and-forward switching mode

Suitable for bursty data transmission networks

Layered Network Structure

OSI Model

Open System Interconnection: Heterogeneous network system interconnection

Layer Name	Function	Description
Physical Layer	Interface Characteristics, Bit Verification, Data Rate, Bit Synchronization, Transmission Mode	Bit Transmission
Data Link Layer (Non-end-to-end)	Framing, Physical Addressing, Flow Control, Error Control, Access Control	Data transmission between adjacent network elements (hosts, switches, routers, etc.)
Network Layer (Non-end-to-end)	Routing, Packet Forwarding	Data packet routing and forwarding from source host to destination host
Transport Layer	Segmentation and Reassembly, SAP Addressing, Connection Control, Flow Control, Error Control	Ensure message submitted to correct process, e.g., port number
Session Layer	Dialogue Control, Synchronization	Generally does not exist alone (very thin layer)
Presentation Layer	Data Representation Conversion, Encryption/Decryption, Compression/Decompression	Generally does not exist alone. Handles syntax and semantics issues of transaction information between two systems
Application Layer	FTP, SMTP, HTTP	Support users using network via user agent or network interface

TCP/IP Model

Five-Layer Model

Layer Name	Description	Protocol	Data Format
Physical Layer	Bit Transmission	None	Bit
Data Link Layer	Data transmission between adjacent network elements	Ethernet, 802.11(Wifi), PPP	Frame
Network Layer	Data packet routing and forwarding from source to destination	IP Protocol, Routing Protocols	Datagram
Transport Layer	Ensure message submitted to correct process	TCP, UDP	Segment
Application Layer	Support users using network	FTP, SMTP, HTTP	Message

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