CHAPTE R 13 Packets, Frames & Topologies

1. CECS 474 Computer Network Interoperability CHAPTER13 Packets, Frames & Topologies Tracy Bradley Maples, Ph.D. Computer Engineering & Computer Science California State University, Long Beach Notes for Douglas E. Comer, Computer Networks and Internets (5th Edition)

2. Local and Wide Area Packet Networks Recall: Packet switching technologies are commonly classified according to the distances they span.

3. Standards for Packet Format and ID • Each packet sent across a packet network must contain the ID of the intended recipient. • All senders must agree on the exact details thus standards have been created by various organizations. • The most famous: Institute for Electrical and Electronic Engineers (IEEE) • In 1980, IEEE organized the Project 802 LAN/MAN Standards Committee to produce standards for networking.

4. IEEE 802 Model and Standards IEEE 802 divides Layer 2 of the protocol stack into two conceptual sub-layers: • The Logical Link Control (LLC) • The LLC sublayerspecifies addressing and the use of addresses for multiplexing/demultiplexing.  • The Media Access Control (MAC) • The MAC sublayerspecifies how multiple computers share the underlying medium.

5. Figure 13.6: Examples of the identifiers IEEE has assigned to various LAN standards

6. LAN Topologies • Each network is classified into a category according to its topology (or general shape) and MAC protocol. • Examples: • • Ethernet Bus • • Ethernet Star (hub or switch) • • Token Passing Ring • • Token Passing Bus

7. LAN Topologies: Bus • Bus topology usually consists of a single cable to which computers attach • The ends of a bus network must be terminated to prevent electrical signals from reflecting back along the bus • Any computer attached to a bus can send on the cable and all computers receive the signal • The computers attached to a bus network must coordinate to prevent or manage collisions • LAN Topologies: Ring • In a ring topology computers are connected in a closed loop (with possible gateways to other networks). • Ring topologies usually have a direction associated with the transmission (i.e, clockwiseor counterclockwise). • In ring topologies, an acknowledgement of successful transmission can be sent for free.

8. LAN Topologies: Mesh • Mesh networks provide a direct connection between every pair of computers. • Main disadvantage of a mesh: Cost!Mesh networks are not scalable. • The number of connections needed for a mesh network grows faster than the number of computers. • Because connections are expensive, few LANs employ a mesh topology • LAN Topologies: Star • In star topologies, all computers attach to a central point (or hub). • In practice, star networks seldom have a symmetric shape (i.e., the hub is not located an equal distance from all computers). • Instead, a hub often resides in a location separate from the computers attached to it.

9. Packet Identification, Demultiplexing, MAC Addresses • IEEE 802 standards include: packet addressing • Consider packets traversing a shared medium as in the figure: • Each computer is assigned a unique address and each packet contains the address of the intended recipient. • In the IEEE 802 addressing scheme, each address consists of 48 bits. • IEEE 802 uses the term Media Access Control address (also called the MAC address, Ethernet address or physical address). • Each Network Interface Card (NIC) contains a unique IEEE 802 address assigned when the device was manufactured. • IEEE assigns a block of addresses to each vendor and allows the vendor to assign a unique value to each device.

10. Unicast, Broadcast, and Multicast Addresses • The IEEE addressing supports three types of addresses that correspond to three types of packet delivery: • The standard specifies that a MAC broadcast address consists of 48 bits that are all 1s (i.e., FF:FF:FF:FF:FF:FF). • MAC broadcast can be viewed as a special form of multicast. • Each multicast address corresponds to a group of computers. • The Broadcast address corresponds to a group that includes all computers on the network.

11. Unicast, Broadcast, and Multicast Addresses

12. Frames and Framing • Defn: Framing refers to the structure added to a sequence of bits or bytes that allows a sender and receiver to specify and recognize the exact format of the message. • In a packet-switched network, each frame corresponds to a packet at Layer 2. • A frame consists of two conceptual parts: • A headerthat contains information used to process the frame. • A payloadthat contains the message being sent. • Notes: • We say the the message is opaque because the network only examines the frame header. • The payload can contain an arbitrary sequence of bytes that are only meaningful to the sender and receiver. • Some technologies delineate each frame by sending a short prelude before the frame and a short postlude after it.

13. Frames and Framing • Frames depend on the network standard to specify: • a minimum/maximum size • Format for the message • Defn: A hardware packet (or Layer 2 packet) is called a frame. • Case 1:Byte Stuffing with RS-232 as an Example RS-232 is a serial binary standard for transmitting data. It was a tremendously popular standard for sending serial data in computers over short (<50 feet) distances. It was used with the serial ports in PCs to connect keyboards, mice, etc. Although replaced in newer PCs by USB and Bluetooth, it is still used for some equipment. • RS-232 is character-oriented (i.e., RS-232 sends characters not individual bits). • Two special characters are used to delineate the packet: • Start of header (soh) • End of text (eot) Frame Format Used with RS-232

14. Case 1:Byte Stuffing with RS-232 as an Example (Cont’d) Q: What happens when the special characters are used in the message being transmitted? A: We must translate the special characters into alternative form to avoid confusion. This is called byte stuffing. Illustration of Frame with Byte Stuffing: Figure (a) shows the message the sending computer wishes to transmit. Figure (b) shows the Bytes that are actually transmitted. RS-232 Substitution Table

15. Case 2:Bit Stuffing with PPP as an Example • What if the transmission scheme is bit-oriented rather than byte oriented? • Two Real-world examples of bit stuffed networks are: • HDLC (High-Level Data Link Control), ISO standard • PPP (Point-to-Point Protocol) PPP is a popular data link layer protocol for communication between two end-points. It provides authentication, encryption, and compression. PPPoE (Point-to-Point Protocol over Ethernet) is often used to provide a link from a modem to an ISP (Internet Service Provider). Bit-oriented protocols view the a frame as a collection of bits. A special bit sequence 01111110 is used at both the beginning and end of frames. 01111110 111111111 control protocol check 01111110 FLAG Information (variable) FLAG The PPP Frame Format

16. Case 2:Bit Stuffing with PPP as an Example (Cont’d) Problem: This bit sequence (01111110) may occur naturally in the bit stream. Solution: Use bit stuffing. Any time five consecutive 1 bits of the data message need to be transmitted, an additional 0 is “stuffed” in following the five 1s. At the receiving end, after five ones a 0 is always removed. Note: Using this scheme it is not possible to make all frames the same size. Data for transmission (passed down from upper Layer 3 at Source): 010111111011111010111111111010101 Bit-stuffed frame ready to send by Layer 2: 01111110 01111110 01111110 01111110 TRAILER HEADER HEADER TRAILER ---------- DATA SENT OVER NETWORK FROM SOURCE TO DESTINATION ---------- Bit-stuffed frame received at destination Layer 2: Data delivered (ready to passed up to upper Layer 3 at Destination):