The Network Edge
The network edge is made up of the following devices:
- Hosts - Clients and servers.
- Servers - Often in data centres.
The hosts are connected to the internet via access networks through physical media:
- Wired links
- Wireless links.
The hosts are then connected to the network core which are a set of interconnected routers.
Access Networks & Physical Media
To connect a host to an edge router you would use:
- Residential networks.
- Institutional/enterprise networks.
- Mobile networks.
These connections have the properties:
- Bandwidth/transmission rate.
- Shared or dedicated access.
Cable Internet
This is a single coaxial cable that uses frequency division multiplexing FDM to split the frequency bands for different purposes and users.
graph LR
Host --- cm[Cable Modem] --- Splitter --- od[Other Households] --- CMTS --- ISP
subgraph Cable Headend
CMTS
end
subgraph House
Host
cm
TV
Splitter
end
TV --- Splitter
- CMTS - Cable modem termination system.
- HFC - Hybrid fibre coax
- An asymmetric connection with 40 Mbps - 1.2 Gbps downstream, 30 - 100 Mbps upstream.
Homes share network access to the cable hardened.
Digital Subscriber Line (DSL)
Uses the existing telephone line to central office DSLAM:
- Data over DSL phone line goes to the internet.
- Voice over DSL phone line goes to the telephone network.
graph LR
Host --- cm[DSL Modem] --- Splitter --- DSLAM --- ISP
subgraph Central Office
DSLAM --- tn[Telephone Network]
end
subgraph House
Host
cm
Phone
Splitter
end
Phone --- Splitter
- DSLAM - DSL access multiplexer.
- Voice and data are transmitted at different frequencies over a dedicated line.
- Often asymmetric connection with 24 - 52 Mbps downstream, 3.5 - 16 Mbps upstream.
Home Networks
Often have the following:
- Cable/DSL modem
- Router, firewall, NAT
- Wired Ethernet
- WiFi AP
Several of these functions are often combined into the modem from the provider.
Wireless Networks
A shared wireless access network that connects a host to a router.
- WLANS - Wireless local area networks:
- Typically ~30m
- Often use WiFi IEEE 802.11b/g/n
- Wide area cellular access networks:
- Provide by mobile operators ~10s km
- 10s Mbps
- 3G/4G/5G
Enterprise Networks
Similar home networks but often with separate components and additional hosts.
Sending Packets
For a host to send data on a network it:
- Takes application message.
- Breaks it into smaller chunks, known as packets, of length $L$ bits.
- Transmits packet into access network at a transmission rate $R$.
- $R$ is the link transmission rate or bandwidth.
Physical Media
| Term | Definition |
|---|---|
| Bit | Propagates between transmitter/ receiver. |
| Physical link | What lies between the transmitter and receiver. |
| Guided Media | Signal propagate in solid media: copper, fibre. |
| Unguided Media | Signals propagate freely: radio. |
Twisted Pair (TP)
Two twisted insulated copper wires. This is the basis of the cable used for Ethernet.
- Cat 5 - 100Mbps to 1Gbps
- Cat 6 - 1 to 10Gbps
Coaxial Cable
Two concentric copper conductors. Multiple frequency channels are put on a cable to gain additional bandwidth.
- 100s Mbps per channel
Fibre Optic cable
Glass fibre carrying light pulses, each pulse a bit.
- High speed operation ~10-100s Gbps
- Low error rate as it is immune to electromagnetic interference.
Unguided Media
The signal is carried in the electromagnetic spectrum. There is no physical wire. There are propagation effects from the environment:
- Reflection
- Obstruction
- Interference
There are several types of radio link types:
- Terrestrial microwave:
- 45 Mbps channels.
- WiFi
- Up to 100s Mbps.
- Wide-Area
- 4G ~10 Mbps.
- Satellite
- Up to 45 Mbps per channel.
- 270 ms end to end delay.
- Can gain lower latency by using low orbit vs geosynchronous satellites.