Network(4) -- The Network Core

Circuit Switching

There are two fundamental approaches to moving data through a network of links and switches: circuit switching and packet switching.

In circuit-switched networks, the resources needed along a path(buffers, link transmission rate) to provide for communication between the end systems are reserved for the duration of the communication session between the end systems.

When the network establishes the circuit, it reserves a constant transmission rate in the network’s links. Since a given transmission rate has been reserved for this sender-toreceiver connection, the sender can transfer the data to the receiver at the guaranteed constant rate.

When two hosts want to communicate, the network establishes a dedicated end-to-end connection between the two hosts. For each link used by the end-to-end connection, the connection gets one fourth of the link’s total transmission capacity for the duration of the connection.

Multiplexing in Circuit-Switched Networks

A circuit in a link is implemented with either frequency-division multiplexing(FDM) or time-division multiplexing (TDM).

With FDM, the width of the band is called the bandwidth.

With TDM, for example, if the link transmits 8,000 frames per second and each slot consists of 8 bits,then the transmission rate of each circuit is 64 kbps.

Circuit switching is wasteful because the dedicated circuits are idle during silent periods.

IMPORTANT: circuit switching example:

How long does it take to send a file of 640,000 bits from Host A to Host B? Suppose that all links in the network use TDM with 24 slots and have a bit rate of 1.536 Mbps. Also suppose that it takes 500 msec to establish an end-to-end circuit before Host A can begin to transmit the file. How long does it take to send the file?

Each circuit has a transmission rate of (1.536 Mbps)/24=64 kbps, so it takes (640,000 bits)/(64 kbps)=10 seconds to transmit the file. To this 10 seconds we add the circuit establishment time, giving 10.5 seconds to send the file.

Note that the transmission time is independent of the number of links: The transmission time would be 10 seconds if the end-to-end circuit passed through one link or a hundred links. (The actual (1.536 Mbps)/24=64 kbps, (640,000 bits)/(64 kbps)=10

Packet Switching Versus Circuit Switching

packet switching is not suitable for real-time services because of its variable and unpredictable end-to-end delays.

1. Packet switching offers better sharing of transmission capacity than circuit switching and
2. it is simpler, more efficient, and less costly to implement than circuit switching.

A Network of Networks

ISP use technologies including DSL, cable, FTTH, Wi-Fi, and cellular.

The access ISPs themselves must be interconnected. This is done by creating a network of networks.

• Network Structure 1: interconnects all of the access ISPs with a single global transit ISP.
• Network Structure 2: consists of the hundreds of thousands of access ISPs and multiple global transit ISPs. Two-tier hierarchy. In any given region, there may be a regional ISP to which the access ISPs in the region connect. Each regional ISP then connects to tier-1 ISPs.
• Network Structure 3: There may be multiple competing regional ISPs in a region. In some regions, there may be a larger regional ISP (possibly spanning an entire country) to which the smaller regional ISPs in that region connect. Multi-tier hierarchy.

To build a network that more closely resembles today’s Internet, we must add points of presence(PoPs), multi-homing, peering, and Internet exchange points (IXPs) to the hierarchical Network Structure 3.

• A PoP is simply a group of one or more routers in the provider’s network where customer ISPs can connect into the provider ISP.
• multi-home is to connect to two or more provider ISPs.

A pair of nearby ISPs at the same level of the hierarchy can peer, thta is, they can directly connect their networks together so that all the traffic between them passes over the direct connection rather than through upstream intermediaries.

• Internet Exchange Point (IXP): is a meeting point where multiple ISPs can peer together.

• We refer to this ecosystem—consisting of access ISPs, regional ISPs, tier-1 ISPs, PoPs, multi-homing, peering, and IXPs—as Network Structure 4.

• Network Structure 5. On top of Network Structure 4, adding content-provider networks. For example, Google data centers. By creating its own network, a content provider not only reduces its payments to upper-tier ISPs, but also has greater control of how its services are ultimately delivered to end users.