1.3 The netwrok Core Flashcards
Store & forward transmission: definition and delay formula.
It means that the packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.
General case of sending one packet from source to dest. over a path consisting of N link each of rate R:
end-to-end-delay = N L/R, L: size of packet.
Queuing delay and Packet loss.
Each packet switch has multiple links attached to it. For each attached link, the packet switch has an output buffer, which stores packets that the router is about to send into that link.
Thus, in addition to the store-and-forward delays, packets suffer output buffer queuing delays. These delays are variable and depend on the level of congestion in the network.
Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely full with other packets waiting for transmission. In this case, packet loss will occur - either the arriving pacekt or one of the already queued packets will be dropped.
Forwarding Tables and Routing Protocols.
Each router has a forwarding table that maps destination addresses to that router’s outbound links. When a packet arrives at a router, the router examines the address and searches its forwarding table, using this destination address, to find the appropriate outbound link. The router then directs the packet to this outbound link.
How do forwarding tables get set?
The Internet has a number of special routing protocols that are used to automatically set the tables. A routing protocol may, for example, determine the shortest path from each router to each destination and use the shortest path results to config. the fwding. tables in a router.
Circuit switching.
When the network establishes the circuit, it also reserves a constant transmission rate in the network’s links (representing a fraction of each link’s transmission capacity) for the duration of the connection. Since a given transmission rate has been reserved for this sender-to-receiver connection, the sender can transfer the data to the receiver at the guaranteed constant rate.
Multiplexing in Circuit-Switched Networks: how can a circuit link be implemented?
A circuit link is implemented with either:
- frequency-division multiplexing (FDM)
- time-division multiplexing (TDM)
Describe frequency-division multiplexing (FDM).
The req. spectrum of a link is divided up among the connections established across the link.
Describe time-division multiplexing (TDM).
time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots. When the network establishes a connection across a link, the network dedicates one-time slot in every frame to this connection. These slots are dedicated for the sole use of that connection, with one-time slot available for use (in every frame) to transmit the connection’s data.
Packet switching vs. circuit switching.
Packet sw. not good for real-time because of unpredictable delay.
Pros:
- it offers better sharing of transmission capacity than circuit sw.
- it is simpler, more efficient and less costly to implement.
A Network of Networks: Network structure 1.
Interconnects all of the access ISPs with a single global transit ISP. The global transfer ISP is a network of routers and communication links that not only span the globe, but also has at least one router near each of the 100s of 1000s of access ISPs.
A Network of Networks: Network structure 2.
Consists of 100s of 1000s of access ISPs and multiple global transit ISPs.
Access ISPs can choose from competing global ISPs.
Global transit ISPs themselves must interconnect, otherwise, access ISPs connected to one of the global transit providers would not be able to communicate with access ISPs connected to the other global transit providers.
A Network of Networks: Network structure 3.
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; the larger regional ISP then connects to a tier-1 ISP.
A Network of Networks: Network structure 4.
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 Net. struct. 3.
For a customer ISP to connect to a provider’s PoP, it can lease a high-speed link from a third party telecommunications provider to directly connect one of tits routers to a router at the PoP.
Any ISP may choose to multi-home, that is, to connect to two or more provider ISPs.
To reduce costs, a pair of nearby ISPs at the same level of the hierarchy can peer, i.e. they can directly connect their networks together so that all the traffics between them passes over the direct connection rather than through upstream intermediaries.
Along these same lines, a third party company can create an internet Exchange Point (IXP) typically a standalone building with its own switches), which is a meeting point where multiple ISPs can peer together.
Points of Presence (PoPs). Definition, where do they exist?
A PoP is simply a group of one or more routers (at the same location) in the provider’s network where customer ISPs can connect into the provider ISP.
PoPs exist in all levels of the hierarchy, except for the bottom level.
Internet Exchange Points (IXPs).
A third party company can create an internet Exchange Point (IXP) typically a standalone building with its own switches), which is a meeting point where multiple ISPs can peer together.
