CCM 4300 Lecture 5 Computer Networks, Wireless and Mobile Communications Dr Shahedur Rahman s.rahman@mdx.ac.uk Room: T115 1
Recap of Last Session Described the physical layer Analogue and Digital signal bandwidth and throughput Transmission medium - Twisted pair, Coaxial cabling and optical fiber Described the data link layer Explained Basic definitions Media Access Control CSMA/CD, Token Passing 2
Session Content ALOHA, FDMA/TDMA/CDMA OSI Model/Internetworking Repeater, hub, switches, bridges, router 3
Lesson objectives At the completion of this lesson you should be able to : - describe the operation of ALOHA, FDMA, TDMA, CDMA - understand interconnection devices, mechanisms, and their operations. 4
Pure (unslotted) ALOHA Norman Abramson surfing & packet switching unslotted Aloha: simpler, no synchronization pkt needs transmission: - send without awaiting for beginning of slot collision probability increases: - pkt sent at t 0 collide with other pkts sent in [t 0-1, t 0 +1] If collision occurs transmit after different time intervals 5
Slotted Aloha time is divided into equal size slots (= pkt trans. time) node with new arriving pkt: transmit at beginning of next slot if collision: retransmit pkt in future slots with probability p, until successful. decentralized independently decides when to transmit but needs synchronization Success (S), Collision (C), Empty (E) slots 6
Recap: Pure/Slotted ALOHA Pure ALOHA access protocol - Data from remote terminals to the central computer site share a common transmission medium, a uhf radio channel. - Any terminals with data to transmit simply sends a packet. Hence there are occasional collisions. Slotted ALOHA access protocol - Similar to pure ALOHA except that packet transmission occurs only in agreed time slot - This doubles the maximum throughput compared to pure ALOHA, because a collision occupies no more than one time slot 7
Recap.. Cont. Nodes decision to transmit is made independent of the activity of the other nodes attached to the broadcast channel Nodes neither pays attention to whether another node begins transmitting nor stops transmission if another node begins to interfere with transmission 8
Channel Partitioning MAC protocols What is multiplexing? A mux (acronym for multiplexer) enables data of multiple transmission channels to share a common link. In Its simplest form, multiplexing involves combining data from several relatively low-speed input channels and transmitting these across a single high-speed circuit. Which layer of the OSI does multiplexing occurs? Layer 1 - Physical Layer 9
Network Core: Circuit Switching Capacity of medium exceeds the capacity required for transmission of a single signal How can we improve efficiency? Let s multiplex! Divide link bandwidth into pieces : frequency division - FDMA time division TDMA 10
Channel Partitioning MAC protocols: TDM TDM: time division multiplexing access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 11
Channel Partitioning MAC protocols: FDM FDM: frequency division multiplexing channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle frequency bands 12
Example: FDMA and TDMA FDMA Example: 4 users frequency TDMA time frequency time 13
Advantages / Disadvantages Advantages - eliminates collision - perfectly fair; each node gets dedicated transmission Disadvantages - node is limited to average rate even when it is the only node with frames to send - node must always wait for its turn in transmission sequence even when it is the only node with a frame to send only for TDM 14
Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) unique code assigned to each user; ie, code set partitioning used mostly in wireless broadcast channels (cellular, satellite,etc) all users share same frequency, but each user has own chipping sequence (ie, code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) 15
OSI Reference Model 16
Internetworking? We defined a network as a collection of computers and other devices. Collection of computer networks or network of networks are called an internetwork or internet Individual networks comprising an internetwork are called subnetworks Devices that interconnect subnetworks are called intermediate nodes (or intermediate systems) An internetwork can involve - local networks (e.g. LAN-to-LAN or LAN-tomainframe) - long-distance connections (e.g. LAN-to-WAN) - WAN-to-WAN connections 17
Internetwork (LAN-to-LAN) Intermediate Nodes router Subnetwork 1 Subnetwork 2 Network cloud consisting of many intermediate nodes 18
Network hardware devices Repeaters / Hubs Simplest level of interconnection, operating at the bottom layer of OSI (Layer 1) Suitable if: - both sides of the interconnection are identical - essentially repeaters operating at bit levels - the requirement is simply to repeat and boost all the digital signal transmission across similar media Some repeaters also provide re-timing capabilities A Multiport Repeater is called a Hub 19
Repeaters / Hubs - cont The performance of a repeater does not impact upon the network or its access techniques - e.g. if the network operates at 10Mbps, so does the repeater Repeaters operate at the bit level - typically introduces a few bits delay whilst the signal is boosted Media access techniques operate across the extended network as if it were a single cable - e.g the repeater does not separate CSMA/CD access techniques on either side of it 20
IWUs (Inter Working Unit) Repeaters Simple two-way amplifiers. Distance limitation in local-area networks Electrical signal becomes weaker as it travels Imposes a limit on the length of a LAN Clean up, amplify, and pass on bits. Used to extend the length of LANs. Functionality at the physical layer of the OSI framework. Media dependent and protocol independent Normally confined to a single building. Repeater 21
IWUs Repeaters (2) They provide no traffic isolation. They generally provide no network monitoring tools, you will not want to use repeaters for a link that is likely to fail. Maximum 4 Repeaters between source and destination (Ethernet). 22
Simple Repeater Operates at a very low level. Its primary purpose is to get around limitations in cable length. Passes on individual bits in the signal (even collisions), without doing any processing at the packet level. Note: The basic Ethernet design requires that signals must be able to get from one end of the network to the other within a specified amount of time. This determines a maximum allowable length. 23
Buffered Repeater Operates at the level of whole data packets. It receives an entire packet from one network into an internal buffer and then retransmits it onto the other network. Because such low-level features, as collisions are not repeated, the two networks continue to be separate as far as the Ethernet specifications are concerned. Thus there are no restrictions on the number of buffered repeaters that can be used. 24
Hubs Multi-port repeaters. Generally speaking, the term hub is used instead of repeater when referring to the device that serves as the centre of a star topology network Repeaters and Hubs have the following limitations Aggregate throughput is limited (Each bit is sent everywhere) Cannot support multiple LAN technologies Limitations on maximum nodes and distances 25
Bridges / Switches Link Layer (Layer 2) devices Designed to connect IEEE 802.x LANs together and provide a relay service at the MAC layer Bridge: Store and Forward - nodes connected to bridges share bandwidth Switch (multiport bridge): Store and Forward or Cut Through - Full-duplex (switching matrix / switch port assigned MAC address) - they have private connections Bridges learn which hosts can be reached through which interface: maintains filtering tables - when frame received, bridge learns location of sender by adding sender location in filtering table 26
Bridges Store and transmit packets. Functionality at DLL so media dependent and protocol independent above the DL layer. It is possible to use more repeaters by using switches. 16
Bridges (2) Bridges can determine whether the destination MAC address carried by data is a part of the same network segment as its source. It makes no determination as to what network segmentthedatashouldbesentto. Bridges indiscriminately pass data along to all other segments of the network. This may cause broadcast storms. Switch: Multi-port bridge. 28
Bridge Learning: example Suppose C sends frame to D and D replies back with frame to C C sends frame, bridge has no information about D, so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D C 1 entry added 29
Bridge Learning: example - cont D generates reply to C, sends it bridge sees frame from D bridge notes that D is on interface 2 C 1 D 2 entry added bridge knows C on interface 1, so selectively forwards frame out via interface 1 30
Routers Network Layer (Layer 3) devices - address used to route data (i.e. internet address) WAN is clearly beyond the LAN s domain - used to interconnect two or more administratively separate networks - each network can be set up and operated without knowledge of the other Routers are concerned with addressing 31
Routers - cont Stores information about the whole network, not just about a particular device Default gateway is used to decrease the size of routing table Different from hardware address used in bridging Disadvantages: - high cost - high latency of the device router has to analyse Layer 3 information 32
Summary ALOHA, other random access protocols OSI ref model and Internetworking Interconnection devices Repeater Hub Switches Bridges Router 33