The Network Core
The network core is a mesh of interconnected routers. They use packet-switching to break the application-layer messages into packets:
- Forward packets from one router to the next, across links on path from the source to the destination.
- Each packet is transmitted at full link capacity.
Store and Forward
The entire packet must arrive a a router before it can be transmitted on to the next link.
Including the transmission delay ($\frac L R$) the end-end delay is:
\[\text{E2E}=\frac{2L} R\]this assumes zero propagation delay.
Queuing Delay & Loss
If the arrival rate exceeds the transmission rate of a link then:
- Packets will queue, waiting to be transmitted on the output link.
- Packets can be dropped if the memory buffer in the router fills up.
Network Core Functions
These are the core functions from the perspective of a router.
Forwarding
This is a local action that moves the arriving packets from the router’s input link to the appropriate router output link.
Routing
This is a global action that determines the source-destination paths taken by packets.
Routing algorithms are used to do this.
Circuit Switching
This is an alternative to packet switching. End-end resources are allocated to be reserved for a call between the source and destination.
- Dedicated resources
- There is no sharing and you are guaranteed performance.
- Circuit segment is idle if not used by a call.
- Commonly used in traditional telephone networks.
Packet switching is better suited to the internet as communication is generally is bursts with lots of idle time.
Frequency Division Multiplexing (FDM)
By using a modulation technique, you can split the bandwidth by frequency and transfer data on a circuit at many different frequencies.
Time Division Multiplexing (TDM)
This method splits the entire bandwidth into time slots that can be assigned to certain users.
Circuit Switching Evaluation
- Good for bursty data as we can take advantage of:
- Resource sharing.
- No call setup.
- Excessive congestion is possible:
- Packet delay and loss due to buffer overflow.
- Protocols need to reliable data transfer, congestion control.
- Bandwidth guarantees are still required for video and audio streaming.
Internet Structure
In order to connect all the international ISPs together we use global ISPs. These global ISPs are then joined by internet exchange points (IXPs). Additionally content provider networks may run their own networks to bring their content closer to edge nodes.
flowchart TD
t11[Tier 1 ISP 1]
t12[Tier 1 ISP 2]
cdn[Content Provider]
ixp1[IXP]
ixp2[IXP]
ixp3[IXP]
r1[Regional ISP]
r2[Regional ISP]
a1[Access ISP]
a2[Access ISP]
a3[Access ISP]
a4[Access ISP]
a5[Access ISP]
a6[Access ISP]
a7[Access ISP]
a8[Access ISP]
a9[Access ISP]
t11 --- t12
t11 --- ixp1 & r1
t12 --- cdn
t12 --- ixp2 & ixp3 & r2
cdn --- ixp1 & ixp2 & ixp3
ixp1 --- r1 & a1
ixp2 --- r1 & r2
ixp3 --- r2
r1 --- a1 & a2 & a3 & a4
r2 --- a5 & a6 & a7 & a8 & a9
- Tier 1 - Commercial ISPs
- Have national and international coverage.
- Content Provider Networks - Private networks
- Connects it data centres to the internet, often bypassing tier 1 and regional ISPs.
- IXPs - Internet Exchange Points
- Connect ISPs to one-another.
- Access ISPs
- The people you pay for internet service.