Lec # 22 Data Communication Muhammad Waseem Iqbal

Lecture Objective Internet Service Providers and Internet Backbones ISP Categories POPs and NAPs Delay and Loss in Packet Switched Networks Types of Delay Comparing Transmission and Propagation Delay Queuing Delay and Packet Loss Protocol Layers and Service Models Layered Architecture The Internet Protocol stack History of Computer Networking and Internet

Internet Service Providers What is an ISP? An ISP is an organization that connects business or residential customers to Internet (backbone). An Internet Service Provider (ISP) is a company that provides access to the Internet. Their customers can be businesses, individuals or organizations. The arrival of ISPs has made connecting to the Internet an affordable and convenient option for general people Internet structure is roughly hierarchical In the public Internet, access networks situated at the edge of the Internet are connected to the rest of the Internet through a tiered hierarchy of Internet Service Providers (ISPs)

ISP Categories ISP Categories Backbone Providers / Tier-1 ISPs Tier-1 ISPs (Internet Backbone) Tier-2 ISPs Tier-3 ISPs Backbone Providers / Tier-1 ISPs These ISPs are nationwide or multinational organizations that control Internet routing. They often own significant pieces of backbone itself National Providers / Tier-2 ISPs These ISPs buy capacity (bandwidth) and routing services from backbone providers and run Points Of Presence (POP: location of access points to the Internet) across the country. Local Providers / Tier-3 ISPs These ISPs operate in the same way as the national ISPs, but on a smaller geographical area

Points of Presence (POPs) POPs are private peering points of ISPs Within an ISPs network, the physical location / points at which the ISP connect to other ISPs are known as Points of Presence (POPs) A POP is simply a group of one or more routers in the ISP’s network at which routers in other ISPs can connect. The POP is in the ISP’s switch site or in a colocation space, the contents will always contain “access” equipment and an IP router. At the core of the POP is a router that acts as the central hub for routing within the POP and is also used to terminate high capacity connections.

Network Access Points (NAPs) NAPs are public peering points of ISPs When two ISPs are directly connected to each other, they are said to peer with each other. The NAP can be owned and operated by either some third-party telecommunications company or by an Internet backbone provider. NAPs exchange huge quantities of traffic among many ISPs Often a NAPs uses high speed ATM switching technology, with IP running on the top of ATM

Backbone Providers / Tier-1 ISPs Also known as Internet Backbone Exists at the center of the Internet Architecture Directly connected to each of the other tier-1 ISPs Connected to a large number of tier-2 ISPs and other customer networks International in coverage Two tier-1 ISPs can also peer with each other by connecting together a pair of POPs, one from each of the two ISPs. The trend is for the tier-1 ISPs to interconnect with each other directly at private peering points. Examples (e.g., UUNet, BBN/Genuity, Sprint, AT&T)

Internet structure: Tier-1 ISPs Tier-1 providers interconnect (peer) privately NAP Tier-1 providers also interconnect at public network access points (NAPs)

National Providers / Tier-2 ISPs Provides smaller coverage as compared to tier-1 National Coverage Connect to one or more tier-1 ISPs Connect to other tier-2 ISPs as well. Tier-2 ISPs typically have regional or national coverage and connects only to a few of tier-1 ISPs A tier-2 ISP is said to be a customer of the tier-1 ISP to which it is connected, and the tier-1 ISP is said to be a provider to its customer. The trend for tier-2 ISPs is to interconnect with other tier-2 ISPs and with tier-1 ISPs at NAPs

Internet structure: Tier-2 ISPs NAP Tier-2 ISP Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer of tier-1 provider Tier-2 ISPs also peer privately with each other, interconnect at NAP

Local Providers / Tier-3 ISPs last hop (“access”) network (closest to end systems) Local Coverage Below tier-2 ISPs are the lower-tier ISPs, which connect to the larger Internet via one or more tier-2 ISPs Users and content providers are the customers of lower-tier ISPs and lower-tier ISPs are the customers of higher-tier ISPs

Internet structure: Tier-3 ISPs NAP Tier-2 ISP local ISP Tier 3 Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet

Internet structure: network of networks a packet passes through many networks! Tier 1 ISP NAP Tier-2 ISP local ISP Tier 3

Delay Packet Switched Networks Considering what can happen to a packet as it travels from its source to its destination. As a packet travels from one node to other node (host or end system), it suffers from several types of delays at each node along the path Most important types of delays are: Processing Delay Queuing Delay Transmission Delay Propagation Delay

Types of Delay Processing Delay The time required to process (examine the packet’s header and determine where to direct the packet) is part of the processing delay Processing delay in high-speed routers is typically on the order of microseconds or less. After this nodal processing, the router directs the packet to the queue that precedes the link to the next router. Processing Delay depends on the processing speed of a router.

Types of Delay Queuing Delay At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link. The queuing delay of a packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission across the link If queue is empty, and no other packet is being transmitted, the queuing delay will be zero If traffic is heavy and many other packets are waiting to be transmitted, the queuing delay will be long Thus, queuing delay depends on the intensity and nature of traffic arriving at the queue. Queuing delays can be in the order of microseconds to milliseconds in practice

Types of Delay Transmission Delay It is the amount of time required to push an entire packet into the link The time taken by a transmitter to send out all the bits of a packet onto the medium Also called Store and Forward Delay Node receives complete packet before forwarding Transmission Delay is directly proportional to the length of the packet Transmission delays are typically in the order of microseconds to milliseconds in practice

Types of Delay Transmission Delay Let us denote the length of the packet by L bits. Denote the transmission rate of the link from Router A to B by R bits/sec Transmission Delay (L/R) = Packet Length (L) Transmission Rate (R) Example: It takes 1 sec to transmit a 10,000 bits packet onto a 10Kbps line. (10,000 / 10 x 1000 = 1) R L A B

Types of Delay Propagation Delay Time it takes a bit to propagate from one node to the next. The time required by a bit to propagate from the beginning of the link to the next router is called propagation delay The bit propagates at the propagation speed of the link which depends on the physical medium being used. It is typically in the range of: 2 x 108 meters/sec to 3 x 108 meters/second In wide area networks, propagation delays are on the order of milliseconds

Types of Delay Propagation Delay Propagation delay depends on the distance (d) between the two routers/nodes and the propagation speed (s) of the link. Propagation Delay (d/s) = Distance b/w 2 Routers (d) Propagation Speed (s)

Types of Delay dnodal = dproc + dqueue + dtrans + dprop Total Nodal Delay (the delay at a single router) If we let dproc, dqueue, dtrans and dprop denote the processing, queuing, transmission and propagation delays respectively, then the total nodal delay is given by: dnodal = dproc + dqueue + dtrans + dprop

Queuing Delay Queuing delay is most complicated and interested delay as compared to other components of nodal delay (processing, transmission, propagation) Queuing delay can vary from packet to packet Example: if ten packets arrive at an empty queue, the first packet will suffer no queuing delay while the last packet will suffer large queuing delay

Queuing Delay Queuing delay depends on: Traffic Intensity = La/R Average Rate at which the packets arrives at a queue (a = packets/sec) Transmission Rate of the link (R = bits/sec) Nature of the incoming traffic (bursty/periodic) Assume that all the packets are of equal length say L bits Then the average rate at which the bits arrive at the queue will be La bits/sec Traffic Intensity = La/R This ratio helps in estimating the extent of queuing delay

Traffic Intensity Traffic Intensity If La/R is > 1 It means that the average rate at which the bits arrive at the queue exceeds the rate at which the bits can be transmitted from the queue. In this undesirable situation, the queue will tend to increase without bound and the queuing delay will reach to infinity! A golden rule in traffic engineering “Design your systems so that the traffic intensity is no greater than 1s”

Traffic Intensity Traffic Intensity If La/R is > 1 If the traffic intensity is close to one, there will be intervals of time when the arrival rate exceeds the transmission capacity and a queue will form As the traffic intensity approaches 1, the average queue length gets larger and larger If La/R is < 1 If the traffic intensity is close to zero, then the packets arrivals are few and far between, and it is unlikely that an arriving packet will find another packet in the queue Average queuing delay will be close to zero

Traffic Intensity Average Queuing Delay 1 Traffic Intensity (La/R)

Packet Loss In reality a queue has a finite capacity As the traffic intensity approaches 1, a packet can arrive to find a full queue. With no place to store such a packet, a router will drop that packet; that is the packet will be lost The fraction of lost packets increases as the traffic intensity increases Thus, a node performance also includes the probability of packet loss A lost packet may be retransmitted on an end-to-end basis, either the application or transport layer protocol.

End-to-End Delay The total delay from source to destination is referred to as end-to-end delay Example: Suppose that the queuing delay is negligible as the network is uncongested, then the end-to-end delay between the source and destination having N-1 routers in between will be: dend-end = N (dproc + dtrans + dprop ) R L