Lecture Internet Overview: roadmap 1.1 What is the Internet? 1.2 Network edge  end systems, access networks, links 1.3 Network core  circuit switching, packet switching 1.4 Delay, loss and throughput in Internet 1.5 Protocol layers, service models 1.6 Networks under attack: security

Quick Recap…  Hardware view of Internet  Components of Internet  Structural view  Client-server model  Peer-to-peer model Lecture 2 1-2

Lecture The Network Core  Internet: mesh of interconnected routers  How is data transferred through net?  circuit switching: dedicated circuit per call: telephone net  packet-switching: data sent thru net in discrete “chunks”

Lecture Network Core: Circuit Switching  End-end resources reserved for “call”  dedicated bandwidth resources: no sharing  circuit-like (guaranteed) performance  call setup required

Lecture Network Core: Circuit Switching  Total network resources (e.g., bandwidth) divided into “pieces”  pieces allocated to each call  resource piece idle if not used by owning call (no sharing)  dividing link bandwidth into “pieces”…HOW?  frequency division multiplexing (FDM) Users use different frequency channels  time division multiplexing (TDM) Users use different time slots

Lecture Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example:

Lecture Numerical example 1  You need to send a file of size 640,000 bits to your friend. You are using a circuit-switched network with TDM. Suppose, the circuit-switch network link has a bit rate of Mbps (1Mb = 10 6 bits) and uses TDM with 24 slots. How long does it take you to send the file to your friend? Let’s work it out!

Lecture Packet Switching A B C 100 Mb/s Ethernet 1.5 Mb/s D E queue of packets waiting for output link

Lecture Network Core: Packet Switching each end-end data stream divided into packets  user A, B packets share network resources  each packet uses full link bandwidth  resources used as needed resource contention:  aggregate resource demand can exceed amount available  store and forward: packets move one hop at a time  Node receives complete packet before forwarding  congestion: packets queue, wait for link use Circuit switching Bandwidth division into “pieces” Dedicated allocation Resource reservation

Lecture Packet switching versus circuit switching  Packet switching allows users to use the network dynamically!  resource sharing  simpler, no call setup  With excessive users:  Excessive congestion  packet delay and loss How do delay and loss occur in Internet/network?

Lecture How do delay and loss occur? packets queue in router buffers  store and forward: packets move one hop at a time  Router receives complete packet before forwarding  packets queue, wait for turn…DELAY A B

Lecture Four sources of packet delay  1. nodal processing:  check bit errors  determine output link A B propagation transmission nodal processing queueing  2. queueing  time waiting at output link for transmission  depends on congestion level of router

Lecture Delay in packet-switched networks 3. Transmission delay:  R=link bandwidth (bps)  L=packet length (bits)  time to send bits into link = L/R 4. Propagation delay:  d = length of physical link  s = propagation speed in medium (~2x10 8 m/sec)  propagation delay = d/s A B propagation transmission nodal processing queueing Note: s and R are very different quantities!

Lecture Total delay  d proc = processing delay  typically a few microsecs or less  d queue = queuing delay  depends on congestion  d trans = transmission delay  = L/R, significant for low-speed links  d prop = propagation delay  a few microsecs to hundreds of msecs

Lecture Numerical example 2  Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 6 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero propagation delay. Let’s work it out! R L A B

Lecture Numerical example 3  Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 6 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero propagation delay. Let’s work it out! R R L A B

Lecture Numerical example 4  Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 6 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero propagation delay.  What if there are three packets from A? Let’s work it out! R R R L A B

Lecture Packet loss  queue (aka buffer) preceding link in buffer has finite capacity  packet arriving to full queue dropped (aka lost)  lost packet may be retransmitted by previous node, by source end system, or not at all A B packet being transmitted packet arriving to full buffer is lost buffer (waiting area)

Lecture Throughput  Throughput: rate at which information bits transferred between sender/receiver RsRs RsRs RsRs RcRc RcRc RcRc R

Lecture Numerical example 5: Throughput RsRs RsRs RsRs RcRc RcRc RcRc A B  Example: A has requested for a packet (size 640,000 bits) from server B. The packet will come through an intermediate router C. It takes 0.1 second for the packet from B to C and 0.4 seconds from C to A. (Note: 1Mb=10 6 bits). Assume zero propagation delay.  What is the throughput from B to C?  What is the throughput from C to A?  What is the average throughout from B to A? Let’s work it out! C