Data Communications and Networks

1. Data Communications and Networks Lecture 2 Abdullah Tayyab Abdullah.tayyab@gmail.com

2. Expect thorough TCP/UDP knowledge • SYN flooding attacks? TCP Three-Way Handshake

3. Delay and Loss in Packet-Switched Networks • The packet suffers from several different types of delays at each node along the path

4. Four main sources of Delay • Nodal processing Processing delay is the time it takes routers to process the packet header. • check for bit-level errors in the packet • determine where the packet's next destination Queueing queuing delayisthe time a job waits in a queue until it can be executed. • If packets arrive faster than the router can process them the router puts them into the queue (also called the buffer) until it can get around to transmitting them • depends on congestion level of router

5. transmission A propagation B nodal processing queueing

6. Transmission Delay transmission delay (or store-and-forward delay) is the amount of time required to push all of the packet's bits into the wire. Transmission delay is a function of the packet's lengthhas nothing to do with the distance between the two nodes. This delay is proportional to the packet's length in bits,It is given by the following formula:DT = N / RwhereDT is the transmission delay, N is the number of bits, and R is the rate of transmission (say in bits per second)

7. Propagation delay Propagation delay = d/s  where d is the distance and  s is the wave propagation speed. In wireless communication, s=c, i.e. the speed of light. In copper wires, the speed s is typically about 67% av of speed of light propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver over a medium. It can be computed as the ratio between the link length and the propagation speed over the specific medium.

8. To simplify the analysis of network delay times, the packet delay is broken up into a sequence of nodal delays. Each nodal delay is the time between the arrival of a packet at a node and its arrival at the next node. • dproc = processing delay • typically a few microsecs or less • dqueue = queuing delay • depends on congestion • dtrans = transmission delay • = L/R, significant for low-speed links • dprop = propagation delay • a few microsecs to hundreds of msecs Nodal delay

9. The most complicated and interesting component of nodal delay is the queuing delay • Own Masters research included Weighted Fair Queuing Algorithm – very interesting with even more interesting algorithms. • QoS provisions? Priority queues. Queuing Delay

10. R=link bandwidth (bps) • L=packet length (bits) • a=average packet arrival rate traffic intensity = La/R • La/R ~ 0: average queueing delay small • La/R -> 1: delays become large • La/R > 1: more “work” arriving than can be serviced, average delay infinite

11. Packet loss occurs when one or more packets of data traveling across a computer network fail to reach their destination. • Max-Hops done, time out, TTL! • End-to-End Delay • dendend = Q (dproc + dtrans + dprop) Where dtrans = L/R, where L is the packet size and R is the transmission rate Packet Loss

12. In a computer network, each layer may perform one or more of the following generic set of tasks: • Error control, which makes the logical channel between the layers in two peer network elements more reliable. • Flow control, which avoids overwhelming a slower peer with PDUs. • Segmentation and Reassembly, which at the transmitting side divides large data chunks into smaller pieces; and at the receiving side reassembles the smaller pieces into the original large chunk. • Multiplexing, which allows several higher-level sessions to share a single lower-level connection. • Connection setup, which provides the handshaking with a peer. Protocol layers

13. Protocol stacks Layer 4 layer 4 protocol Layer 4 Layer 3 layer 3 protocol Layer 3 Layer 2 layer 2 protocol Layer 2 Layer 1 layer 1 protocol Layer 1 Physical Medium

14. Protocol stacks • Each layer offers services to layer above • A layering of protocols as used by a system is called a protocol stack • Cross-Layer integration/communiction (good research topic) • Example: In wireless communication the channel information transferred to higher layers • The messages layers communicate between each other are PDUs (Protocol Data Units) at each layer

15. The OSI model does not corresponds to TCP/IP model and has more layers (seven) that are more a conceptual exercise (each layer is reasonably self-contained) then of practical importance.  OSI model, contains seven layers which build on one another. Each layer provides specific services and makes the results available to the next layer. Theoretically each layer should be independent of all others, but this is a simplistic notion  OSI Model

17. Lately the OSI model has been taught using a Mnemonic, to help in understanding the complex model,  • How to remember layer names • All People Seem To Need Data Processing • AajPhir Se Tu Ne Daru (Doodh) Pee.

18. The major functions and services performed by the Physical Layer are: Establishment and termination of a connection to a communicationsmedium. Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control. Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link. Layer 1: Physical Layer

19. The Data Link Layer is the protocol layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area networksegment • Models of communication • Connection-oriented communication • Connectionless communication Layer 2: Data Link Layer

20. The uppermost sublayer is Logical Link Control (LLC). • multiplexes protocols running atop the Data Link Layer • provides flow control • acknowledgment • and error notification • addressing and control of the data link Logical Link Control Sublayer

21. Media Access Control sublayer • The sublayer below it is Media Access Control (MAC). • Sometimes this refers to the sublayer that determines who is allowed to access the media at any one time • Other times it refers to a frame structure with MAC addresses inside. • Examples: • CDMA • TDMA • Slotted ALOHA • CSMA/CD • Token Bus/Ring MAC Sublayer

22. Functions of the Network Layer include: • Routing packets to destination • Connection model • Host addressing • Message forwarding • Ingress/Outgress routers Network Layer

23. Encapsulating application data blocks into data units (datagrams, TCP segments) • abstracting network datagrams and delivering their payload to an application • QoS services • Flow control • Congestion control • Ports • Same order delivery • Timely delivery Transport layer

24. types of messages exchanged, e.g., request messages and response messages; • the syntax of the various message types, i.e., the fields in the message and how the fields are • the semantics of the fields, i.e., the meaning of the information in the fields; • rules for determining when and how a process sends messages and responds to messages. • E.g. Telnet, ftp, http Application layer

25.  Short for network access point, a public network exchange facility where Internet Service Providers (ISPs) can connect with one another in peering arrangements. • The NAPs are a key component of the Internetbackbone • Mostly congested NAP

26. Asynchronous Transfer Mode is a cell-based switching technique that uses asynchronous time division multiplexing. • It encodes data into small fixed-sized cells (cell relay) and provides data link layer services that run over OSILayer 1 physical links. • This differs from other technologies based on packet-switched networks • Whats the difference between cells and frames in IP datagrams? • Uses VC (called Virtual Channels) • Error correction; no retransmission • Congestion-control End to end ATM