Computer connected networks unit 1 and 2

Computer network

It is like a graph consisting of end-systems or hosts connected to one another via communication links and some packet switches

Route

A sequence of packet switches and communication links is called route or path

Internet service provider

ISP (Internet Service Provider) is a business entity or company which provides internet access to the end-systems in return for a subscription fee

Point of presence

The place where end-systems connect to an ISP is referred to as point-of-presence (PoP)
The number of PoPs (typically in 1000s) held by an ISP tells about its outreach in the internet.

PoP consists of routers, link layer switches, MPLS and communication links

Multihoming

Any ISP (except for Tier-1 ISPs) may choose to multi-home, that is, to connect to two or more provider ISPs.
For example, an access ISP may multi-home with two regional ISPs, or it may multi-home with two regional ISPs and also with a tier-1 ISP.

Peering

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

Internet exchange point

A third-party company can create an Internet Exchange Point (IXP) (typically in a stand-alone building with its own switches), which is a meeting point where multiple ISPs can peer together.

Revenue generation in ISPs

• End users pay access ISPs

• Access ISPs pay to regional ISPs

• Regional ISPs pay to Tier 1 ISPs

• Tier 1 ISPs may have several bilateral agreements to share resources such as bandwidth and routers

• Besides, content service providers can enter into bilateral agreements with an ISP at any stage

• ISPs which perform peering or multi-homing share some of their revenue based on equipment and resource utilization

How does internet support distributed applications? What does distributed mean?

Distributed means that applications run independently on the hosts or end systems
• Messages are exchanged by the hosts using the internet socket interfaces of their respective applications

• Protocols define the format and the order of messages exchanged between two or more hosts

• Protocols also define the actions taken on the transmission and/or receipt of a message or other event

• Services (e.g., reliability, guaranteed rate) are provided by hardware or software associated with the devices

Classification of ISPs based on topography

1. Global ISP (Internet Service Provider): Provides internet access to users across the globe.

2. Regional ISP: Provides internet access to users within a specific region or country.

3. Enterprise Networks: Private networks used by organizations to connect their offices, departments, or branches across different locations.

Network edge

Computer networks that sit on the periphery of the internet constitute the network edge or the access network.

End-systems (hosts) can be further classified as clients and servers

Gateway

Router which connect an access network to a regional/access ISP is referred to as gateway

Access network nomenclature

• Based on size: Local area networks, home networks, wide area networks, etc.

• Based on topology: Tree, star, ring, bus, point-to-point.

• Based on physical media: Wired (DSL, Cable, Fiber to the home (FTTH)) or wireless

Home access networks

• The devices in the home are connected to the internet via a LAN or Wifi router

• Different physical media could be provided by different access ISPs to connect the home network with the internet

An infrastructure handled by a telecom or cable or fiber operator

Enterprise access networks

ISP can be telecom operator

• Built using Ethernet cables, switches and hubs

• Ethernet switches are preferred over routers in a LAN

• Routers are used for separating the network into subnets

Wireless networks

• Classified according to radio access technologies

• Spread spectrum, frequency hopping, random access, polling methods, etc.

• More complex compared to wired access networks

• Packet losses and time varying wireless channel characteristics

• Wireless networks can be WiFi-based or cellular-based

• Wireless networks are usually supported by telecom ISPs

• Span of wireless networks can be few meters to several kilometers

• Wireless networks have undergone tremendous evolution especially with the exploding data requirements of the users

Satelite access networks

Remote end systems get access to the internet via satellite links Implemented when other access networks are not feasible Has lowest data rates among access networks The delays are higher. It depends on the distance between the satellite and the users and the type of satellite

Types of satellites: geostationary satellites and low-earth orbiting (LEO) satellites

Network core

Consists of high speed routers and high speed links (Gigabit Ethernet/optical fibers)
Network core is constructed using the interconnection of ISPs
The packet switches (routers) perform store and forward operation

Traffic from access ISPs are aggregated using multiplexers

• Multiplexers are interconnected to more distant switches through a backbone network

• Network core follows mesh topology with lot of redundancy

Design problems in the network core

• Satisfy delay and reliability constraints

• Routing

• Assigning capacity (Flow maximization problem)

• Cost improvement

Switching

Routers in the network core perform switching: Routers have several links on which packets arrive and depart

Switching involves transfer of an incoming packet from one link to an appropriate outgoing link based on IP protocol

The switching operation can be done by hardware and/or software

Circuit switching

• Requires connection establishment before data transfer

• Resources are allocated by every intermediate switch/router between the source and destination hosts

• Resource example: Fixed link bandwidth, internal memory

• In telephony, when a path is established between the source and destination we can say a circuit is formed

• After data transfer, the circuit is closed by releasing the reserved resources at each intermediate router

• No waiting time and no loss of data at intermediate routers

• Throughput reduces with resource sharing

Packet switching

• Data broken into smaller chunk called packets

• No reservation of resources

• Suited for bursty traffic

• Better link utilization

• Packets are stored in buffer and then forwarded one at a time

• Requires protocols for link access and reliable packet delivery

• Packets may suffer queuing delays and get lost at the routers

• This happens when rate of arrivals exceeds the rate of departure

Processing delay

Time taken to inspect (or make) a packet at a packet switch (or source host). (Range: Microseconds)

Queuing delay

Time spent by a packet in the queue before processing. Depends on the number of packets waiting ahead, traffic intensity and distribution of the arrival process. (Range: Microseconds to milliseconds)

Transmission delay

Time taken to push a packet on to the link. Depends on length of the packet (L bits) and link rate (R bits/sec). Expressed as L/R

Propogation delay

Time taken by a bit to travel over a link. Depends on the length of the link and the physical medium propagation speed (e.g., 2×108 to 3×108 m/s)

Traffic intensity vs queuing delay

Suppose arrival rate is a packets per sec and departure rate is L/R seconds per packet, then traffic intensity is given by La/R

Let buffer size be infinite When La/R < 1, every new packet sees an empty queue

When La/R ≥ 1, queue starts to build up and mean queuing delay could approach infinity

End to end delay

The total time spent by a packet to travel from the source to the destination

End-to-end delay is the sum of the delays at the source, delays at each packet switch and the propagation delays on each communication link along the path.

Delay at a packet switch equals the sum of queuing delay, processing delay and transmission delay

Throughput

Throughput is the rate (bits/sec) at which the destination host receives the packets

Instantaneous throughput is the throughput at a given time instant whereas average throughput is throughput over the entire file transfer time (e.g., F/T where F is file size and T is file transfer time).

Layers in a network

Each layer implements a service via its own internal layer actions relying on the services provided by the layer before it

• Each layer performs its own unique sub-task

• On the sender side, each layer waits till the above layer finished its sub-task

• On the receiver side, each layer waits till the below layer finished its sub-task

Why layering?

Explicit structure allows identification, relationship of system’s pieces

Modularization eases maintenance, updating of system

Protocol

The peer layers (i.e., sub-task in sender and its counterpart in the receiver) communicate with one other using formatted blocks of data that obey a set of rules or conventions known as a protocol

Layers implement protocol in hardware or software

Arranged vertically, the layers on the systems collectively constitute the protocol architecture

Basic requirements of a protocol

• Syntax: Concerns the format of the data blocks

• Semantics: Includes control information for coordination and error handling

• Timing : Includes speed matching and sequencing

TCP/IP protocol

• Resulted from protocol research under ARPANET

• Consists of large collection of protocols issued as Internet standards issued by IAB

• It consists of 5 layers namely, Application layer, Transport (host-to-host) layer, Network layer (IP layer), Link layer (network access 8 layer), Physical layer

Application layer (TCP/IP)

Supporting network applications, HTTP, DNS, IMAP, SMTP

• Applications running on hosts generate/receive data

• Data is referred to as message

• A process initiates communication with another by sending a query/request • Message is formatted according to the application layer protocol

• Messages can be big in size

• Applications can have QoS requirements

Transport layer (TCP/IP)

Process process data transfer, TCP, UDP

• Responsible for providing QoS for messages

• Performs multiplexing at the sender

• Performs demultiplexing at the receiver

• Maps each message to a corresponding process

• Appends a new header to each message

• Message plus header is called segment

Network layer (TCP/IP)

Routing of datagrams from source to destination, IP, routing protocols

• Fragments segments into packets

• Moves packets hop-by-hop

• E.g., router to router

• Uses source and destination IP addresses

• Path between source host and destination host is discovered

• Appends a new header to each packet

• Packet plus header is called datagram

Link layer (TCP/IP)

Data transfer between neighboring network elements, Ethernet, Wifi, PPP

• Pushes the packets onto a link

• Using link layer protocols

• Can forward frames using MAC address

• Appends a new header to the packet

• Packet plus header is called frame

• Provides synchronization at receiver

• Checks for errors in frame

Physical layer (TCP/IP)

bits on the wire

• Provides physical interface between the host and the link

• Example: Modem and Ethernet card, wireless adapter

• Converts binary data into signals

• Performs modulation and demodulation

• Performs transmission, reception and filtering of signals

Encapsulation and descapsulation

Encapsulation means the addition of header to the packet at transport, link layer etc. Encapsulation happens before departure Decapsulation happens after arrival